Re: Great, my VT52 is shot.--Progress!

[Chris Zach via cctalk]

So with the op-amp replaced and the unit still whining and putting out 
-15+ volts on the -12 rail I decided to just punt and pull the Q12 
transistor. Removed it, will test but doesn't look happy and replaced it 
with a 7912 -12 volt regulator mounted to the big heat sink with the 
three wires going to both sides of capacitor C17 for ground and input 
voltages and the -12 volt output going off to the top side of capacitor 
C54. Kind of a hack, but I just want to see if this thing works.


Tested the power supply outputs with a bench voltage tester at -20 volts 
and sure enough E2 is reading -11.98 and E3 is reading a solid -5 volts. 
Put the power supply board in the unit, hooked it up, fired up the VT52.


Silence.

Dead silent. No whine, no high pitched sounds, no nothing. Quiet. Then I 
saw the little blinking cursor underline on the screen and found that 
the keys make the clicking and I realized not only is it working, but 
it's better than before.


Turned it off, will wait a few hours for the HV to dissipate, then will 
screw everything down. I'm thinking this -12 volt supply has been a 
problem for some time as it used to make a high pitched TV whine but now 
it's nice and quiet. Possible the root cause is something in the Q12 
power transistor, but to be honest more modern technology in the power 
supply can only be a good thing :-)


Should have it all back together tonight, I'll see if it talks to the 
computer. If so we have a working VT52 and now people know that if the 
screen looks like mine did the problem is most likely in the -12v circuit.


C

On 5/6/2020 9:53 PM, Chris Zach via cctalk wrote:
Well, I pulled the E2 op-amp and replaced it with a NOS one of the same 
model. Put the supply together and now I am getting -17 volts on pins E2 
to ground (E10).


I'm thinking of just replacing the power transistor Q12 with a 7912 -12v 
regulator that I have here and bypassing the whole op amp/transistor 
mess. That should give me a solid -12v on the E2 line and provide power 
for the -5 volt divider circuit.


Thoughts?
C

On 4/22/2020 11:52 AM, Jon Elson via cctalk wrote:

On 04/21/2020 10:09 PM, Brent Hilpert via cctalk wrote:

On 2020-Apr-21, at 5:27 PM, Chris Zach via cctalk wrote:
Meantime reading the manual I found an interesting test: If you 
short emitter to base on Q4 (easiest way is to jumper diode D10) the 
voltage on the -12v supply goes to .4 volts. They're saying it's E2, 
R15,R17,R14.


Is there a way I can test the op-amp in circuit? Maybe it's dead.



Well, if the circuit **IS** regulating, then the voltage on the two 
inputs will be identical.
But, since it might not be regulating, then these voltages would not 
be equal.
But, if you can see that the + input is more positive than the - 
input, yet the output
is pegged negative, for instance, then you know either the op-amp is 
bad, or another circuit is overloading

the output and forcing it that way.

Jon

Re: Great, my VT52 is shot.

[Chris Zach via cctalk]

Well, I pulled the E2 op-amp and replaced it with a NOS one of the same 
model. Put the supply together and now I am getting -17 volts on pins E2 
to ground (E10).


I'm thinking of just replacing the power transistor Q12 with a 7912 -12v 
regulator that I have here and bypassing the whole op amp/transistor 
mess. That should give me a solid -12v on the E2 line and provide power 
for the -5 volt divider circuit.


Thoughts?
C

On 4/22/2020 11:52 AM, Jon Elson via cctalk wrote:

On 04/21/2020 10:09 PM, Brent Hilpert via cctalk wrote:

On 2020-Apr-21, at 5:27 PM, Chris Zach via cctalk wrote:
Meantime reading the manual I found an interesting test: If you short 
emitter to base on Q4 (easiest way is to jumper diode D10) the 
voltage on the -12v supply goes to .4 volts. They're saying it's E2, 
R15,R17,R14.


Is there a way I can test the op-amp in circuit? Maybe it's dead.



Well, if the circuit **IS** regulating, then the voltage on the two 
inputs will be identical.
But, since it might not be regulating, then these voltages would not be 
equal.
But, if you can see that the + input is more positive than the - input, 
yet the output
is pegged negative, for instance, then you know either the op-amp is 
bad, or another circuit is overloading

the output and forcing it that way.

Jon

Re: Great, my VT52 is shot.

[Jon Elson via cctalk]

On 04/21/2020 10:09 PM, Brent Hilpert via cctalk wrote:

On 2020-Apr-21, at 5:27 PM, Chris Zach via cctalk wrote:

Meantime reading the manual I found an interesting test: If you short emitter 
to base on Q4 (easiest way is to jumper diode D10) the voltage on the -12v 
supply goes to .4 volts. They're saying it's E2, R15,R17,R14.

Is there a way I can test the op-amp in circuit? Maybe it's dead.



Well, if the circuit **IS** regulating, then the voltage on 
the two inputs will be identical.
But, since it might not be regulating, then these voltages 
would not be equal.
But, if you can see that the + input is more positive than 
the - input, yet the output
is pegged negative, for instance, then you know either the 
op-amp is bad, or another circuit is overloading

the output and forcing it that way.

Jon

Re: Great, my VT52 is shot.

[Chris Zach via cctalk]

Also what is the V at Q4.E (should be ~ +2.6V), also Q4.B & C.

Q4.E to ground is 3.05
Q4.B to ground is 3.24
Q4.C to ground is -9.18




When Q4.BE shorted, E2.6 should swing well-negative.


Nope, 2.544 on E2.6 assuming E2's pinout is
1 8
2 7
3 6
4 5

C

Re: Great, my VT52 is shot.

[Brent Hilpert via cctalk]

On 2020-Apr-21, at 8:35 PM, Chris Zach wrote:
>> The D8 5V ref and the targetted output V are divided via R15 and R17,R18 to 
>> provide the sense input at E2.2.
>> If one does the R ratio of the three resistors, it comes out, as would be 
>> expected, to ~ 0V.
> 
> Ah hah, that is clever. I wonder if the .8 volts means the output is higher 
> than what should be expected and the op amp isn't amping down or something. 
> However if the op amp was blown it should just allow full voltage through. 
> Maybe (is there a crowbar circuit in there).
> 
>> I'm a little surprised there's that much difference between E2.2 & E2.3 ( 
>> (-0.022) - (-0.8) ) without sending the E2 output off to +V, but not sure 
>> how much device variability to expect normally.
> 
>> You might look for the on-board values of R15,17,18. If they have been 
>> changed from those values specified in the schematic, then the -12 may have 
>> been changed to -15 (could do the ratio calc).
> 
> I tried measuring the resistance in circuit, but that never works. It's 
> possible they are measuring out properly and that .6 volts is the 
> representation of too much voltage at the output side (-15 instead of the 
> expected -12).

Look at the marked (as opposed to measured) values of the (3) installed Rs, to 
see if they have been intentionally changed in a revision to alter the output V.


>> Also what is the V at Q4.E (should be ~ +2.6V), also Q4.B & C.
> 
> I'll check that tomorrow. Connecting Q4.E to B did bring the output voltage 
> down to pretty much zero so that does seem to work.

When Q4.BE shorted, E2.6 should swing well-negative.


> If I can't figure this out I might just pull Q10 and put a 7912 in its place. 
> One chip does the whole job of regulating the output, end of story. Bad me of 
> course, but what the heck and if the display came up I would know where the 
> problem was.
> 
> If it turns out the op amp is dead, would a 741 work as a replacement?

Note comment lower left corner of schematic page.

Re: Great, my VT52 is shot.

[Chris Zach via cctalk]

The D8 5V ref and the targetted output V are divided via R15 and R17,R18 to 
provide the sense input at E2.2.
If one does the R ratio of the three resistors, it comes out, as would be 
expected, to ~ 0V.


Ah hah, that is clever. I wonder if the .8 volts means the output is 
higher than what should be expected and the op amp isn't amping down or 
something. However if the op amp was blown it should just allow full 
voltage through. Maybe (is there a crowbar circuit in there).



I'm a little surprised there's that much difference between E2.2 & E2.3 ( 
(-0.022) - (-0.8) ) without sending the E2 output off to +V, but not sure how much 
device variability to expect normally.



You might look for the on-board values of R15,17,18. If they have been changed 
from those values specified in the schematic, then the -12 may have been 
changed to -15 (could do the ratio calc).


I tried measuring the resistance in circuit, but that never works. It's 
possible they are measuring out properly and that .6 volts is the 
representation of too much voltage at the output side (-15 instead of 
the expected -12).



Also what is the V at Q4.E (should be ~ +2.6V), also Q4.B & C.


I'll check that tomorrow. Connecting Q4.E to B did bring the output 
voltage down to pretty much zero so that does seem to work.


If I can't figure this out I might just pull Q10 and put a 7912 in its 
place. One chip does the whole job of regulating the output, end of 
story. Bad me of course, but what the heck and if the display came up I 
would know where the problem was.


If it turns out the op amp is dead, would a 741 work as a replacement?

Re: Great, my VT52 is shot.

[Brent Hilpert via cctalk]

On 2020-Apr-21, at 5:27 PM, Chris Zach via cctalk wrote:
> Meantime reading the manual I found an interesting test: If you short emitter 
> to base on Q4 (easiest way is to jumper diode D10) the voltage on the -12v 
> supply goes to .4 volts. They're saying it's E2, R15,R17,R14.
> 
> Is there a way I can test the op-amp in circuit? Maybe it's dead.


The three regulators are all referenced via D8 (~ +5V).

Circuit-wise, for the -12, 0V/GND is the reference input at E2.3.
The D8 5V ref and the targetted output V are divided via R15 and R17,R18 to 
provide the sense input at E2.2.
If one does the R ratio of the three resistors, it comes out, as would be 
expected, to ~ 0V.

I'm a little surprised there's that much difference between E2.2 & E2.3 ( 
(-0.022) - (-0.8) ) without sending the E2 output off to +V, but not sure how 
much device variability to expect normally.

You might look for the on-board values of R15,17,18. If they have been changed 
from those values specified in the schematic, then the -12 may have been 
changed to -15 (could do the ratio calc).

Also what is the V at Q4.E (should be ~ +2.6V), also Q4.B & C.


> On 4/21/2020 5:36 PM, Chris Zach via cctalk wrote:
>> Ok, some test results below.
>> 1) Are we sure J2-2 should be -12? It really seems to like being -15.
>> 2) The voltage across the zener diode there is a nice solid -3.8-3.9v. It 
>> appears to be a germanium diode (.1v drop across, both directions) I could 
>> disconnect it and see if that makes things better.
>> 3)

>> E2.2 is -.8 volts referenced to ground.
>> E2.3 is -22mv referenced to ground
>> E2.6 is around .6v. Ramps up from about .58 at startup to .61 then down to 
>> .60something. Note that is a positive voltage wrt ground.
>> Q12 was reading
>> -24.4v emitter
>> -15v collector
>> -23.8v base
>> Q12 does not have a CE short
>> What is this using as a reference to get the -12v from the transistor?

Re: Great, my VT52 is shot.

[Chris Zach via cctalk]

Meantime reading the manual I found an interesting test: If you short 
emitter to base on Q4 (easiest way is to jumper diode D10) the voltage 
on the -12v supply goes to .4 volts. They're saying it's E2, R15,R17,R14.


Is there a way I can test the op-amp in circuit? Maybe it's dead.

C


On 4/21/2020 5:36 PM, Chris Zach via cctalk wrote:

Ok, some test results below.

1) Are we sure J2-2 should be -12? It really seems to like being -15.
2) The voltage across the zener diode there is a nice solid -3.8-3.9v. 
It appears to be a germanium diode (.1v drop across, both directions) I 
could disconnect it and see if that makes things better.

3)


E2.2 and E2.3 should be around 0V.E2.2 is -.8 volts referenced to ground.

E2.3 is -22mv referenced to ground
E2.6 is around .6v. Ramps up from about .58 at startup to .61 then down 
to .60something. Note that is a positive voltage wrt ground.


Q12 was reading
-24.4v emitter
-15v collector
-23.8v base
Q12 does not have a CE short

What is this using as a reference to get the -12v from the transistor?

Maybe the zener is just out of spec. Can someone with a VT52 cross check 
my voltages to see if they are accurate?


Thanks all!



E2.6 I expect would normally be within +/- 2V of GND.
If E2.6 is way-positive (>Q4.E, > ~ +2.6V), then E2 is attempting to 
throttle down the output but failing due to some failure around Q4/Q12.


Measure the voltages on Q4 and Q12.
They could be leaky or low-gain.
It's conceivable Q12 has a CE short, but I'd expect the input to the 
reg to be more than -15.


The low -5 is odd, zener D14 looks to be a 5.1V type.
R45 is probably burning up.
I haven't spotted anything in the schematic that actually uses the -5,
it looks to be provided for some alternative character generator 
(schematic pdf.42),
so that may depend on what character generator chip / daughter board 
is used in your unit.

Re: Great, my VT52 is shot.

[Chris Zach via cctalk]

I wonder if I would go straight to hell if I just pulled the Q12 
transistor and wired in a 12 volt wall wart between ground (J2-2) and 
+12 (ground) to get -12


C

On 4/21/2020 5:36 PM, Chris Zach via cctalk wrote:

Ok, some test results below.

1) Are we sure J2-2 should be -12? It really seems to like being -15.
2) The voltage across the zener diode there is a nice solid -3.8-3.9v. 
It appears to be a germanium diode (.1v drop across, both directions) I 
could disconnect it and see if that makes things better.

3)


E2.2 and E2.3 should be around 0V.E2.2 is -.8 volts referenced to ground.

E2.3 is -22mv referenced to ground
E2.6 is around .6v. Ramps up from about .58 at startup to .61 then down 
to .60something. Note that is a positive voltage wrt ground.


Q12 was reading
-24.4v emitter
-15v collector
-23.8v base
Q12 does not have a CE short

What is this using as a reference to get the -12v from the transistor?

Maybe the zener is just out of spec. Can someone with a VT52 cross check 
my voltages to see if they are accurate?


Thanks all!



E2.6 I expect would normally be within +/- 2V of GND.
If E2.6 is way-positive (>Q4.E, > ~ +2.6V), then E2 is attempting to 
throttle down the output but failing due to some failure around Q4/Q12.


Measure the voltages on Q4 and Q12.
They could be leaky or low-gain.
It's conceivable Q12 has a CE short, but I'd expect the input to the 
reg to be more than -15.


The low -5 is odd, zener D14 looks to be a 5.1V type.
R45 is probably burning up.
I haven't spotted anything in the schematic that actually uses the -5,
it looks to be provided for some alternative character generator 
(schematic pdf.42),
so that may depend on what character generator chip / daughter board 
is used in your unit.

Re: Great, my VT52 is shot.

[Chris Zach via cctalk]

Ok, some test results below.

1) Are we sure J2-2 should be -12? It really seems to like being -15.
2) The voltage across the zener diode there is a nice solid -3.8-3.9v. 
It appears to be a germanium diode (.1v drop across, both directions) I 
could disconnect it and see if that makes things better.

3)


E2.2 and E2.3 should be around 0V.E2.2 is -.8 volts referenced to ground.

E2.3 is -22mv referenced to ground
E2.6 is around .6v. Ramps up from about .58 at startup to .61 then down 
to .60something. Note that is a positive voltage wrt ground.


Q12 was reading
-24.4v emitter
-15v collector
-23.8v base
Q12 does not have a CE short

What is this using as a reference to get the -12v from the transistor?

Maybe the zener is just out of spec. Can someone with a VT52 cross check 
my voltages to see if they are accurate?


Thanks all!



E2.6 I expect would normally be within +/- 2V of GND.
If E2.6 is way-positive (>Q4.E, > ~ +2.6V), then E2 is attempting to throttle 
down the output but failing due to some failure around Q4/Q12.

Measure the voltages on Q4 and Q12.
They could be leaky or low-gain.
It's conceivable Q12 has a CE short, but I'd expect the input to the reg to be 
more than -15.

The low -5 is odd, zener D14 looks to be a 5.1V type.
R45 is probably burning up.
I haven't spotted anything in the schematic that actually uses the -5,
it looks to be provided for some alternative character generator (schematic 
pdf.42),
so that may depend on what character generator chip / daughter board is used in 
your unit.

Re: Great, my VT52 is shot.

[Chris Zach via cctalk]

It's conceivable there was a revision change where the -12V was changed to 
-15V, but it does seem unlikely.

The -12 is used by the vertical deflection circuit, the CRT filament, (and the 
RS-232).


Well, vertical deflection seems to be my problem so this may make sense.

Until I figure this out I'm going to pull that jumper to remove power 
from the HV circuits. I'm guessing the circuit can regulate with no load.



E2.2 and E2.3 should be around 0V.
E2.6 I expect would normally be within +/- 2V of GND.
If E2.6 is way-positive (>Q4.E, > ~ +2.6V), then E2 is attempting to throttle 
down the output but failing due to some failure around Q4/Q12.

Measure the voltages on Q4 and Q12.
They could be leaky or low-gain.
It's conceivable Q12 has a CE short, but I'd expect the input to the reg to be 
more than -15.


*nod* I just took a look at Q12 and it's not shorted which is good. It 
looks to be the regulator power transistor, the other components vary 
the base to keep the voltage at the output constant under load. The 
input voltage to the circuit is around -24v so it looks like the 
rectifier diodes are ok enough.



The low -5 is odd, zener D14 looks to be a 5.1V type.


Yeah, that is really odd, especially if -5v isn't really used. I wonder 
if the zener diode is shorted, that could cause all sorts of weirdness. 
I'll check.


C

Re: Great, my VT52 is shot.

[Hector Peraza via cctalk]

The raster seems folded at the left and at the top, which indicates a 
problem with both the vertical and horizontal sweep.


As Mattis Lind already mentioned, the VT52 uses direct drive of the 
vertical and horizontal circuitry (no oscillators to synchronize) yet 
the character rows in the video are running diagonally totally out of 
sync with the sweep, so I'd check the timing generator (pages 4-17 and 
4-18 of the Maintenance Manual, pages 19-21 of the Schematics PDF). But 
that's after checking that the power voltages are solid and ripple-free. 
I'd also check a few components around the horizontal output stage, e.g. 
if capacitor C44 goes bad (page 10) then the pulses from the horizontal 
output will get into other parts of the circuit via the power supply 
lines and/or the driver stages, affecting the logic's work.


A final note: don't keep the terminal running like that for a long time, 
since the horizontal frequency is out of specs the fly-back can get 
damaged (the core can saturate, increasing the current drawn; that may 
also explain the noise you're hearing).


Hector.


On 4/20/2020 7:40 PM, Mattis Lind via cctalk wrote:

I looked at the video once more and paused around 0:21. It appears to be
some text there on the uppermost row.

Maybe the vertical sweep is highly non-linear. Check all capacitors in the
vertical stage. C43, C41,C40, C42, C35 and C45. Would probably be good to
measure the voltage over the current sensing resistor R73 and see if it is
a proper ramp as it should.

What about the R92 linearity potentiometer. Is it ok? Diode D31? Actually
check all semiconductors.

/Mattis

måndag 20 april 2020 skrev Mattis Lind :






Something else that might help with diagnosis is taking pictures of the
screen with known, simple elements on the screen.
e.g.:
 - clear the screen
 - take pic if it doesn't clear
 - type a simple character like a "-"or "1"
 - pic
 - type a half line or full line of characters
 - pic
 - type different characters on two different lines
 - pic

The idea being to find out, from simple known patterns, where the pixels
end up being displayed,
so it may be possible to figure out what's happening with the scan,
whether it's getting stretched, folding over, etc.




That is a good idea. Loop the terminal and create a bunch of "known
references".

Might add that it would be interesting to see one single character on a
lets say 4 different positions. Column 0, Column 20, Column 40 and end of
line. Approximately since it is hard to be exact.

/Mattis




--
Hector Peraza
BrightSpec NV/SA
Waterfront Researchpark
Galileilaan 15 (Darwin)
2845 Niel
BTW/VAT: BE 0846.102.393
http://www.brightspec.be

Re: Great, my VT52 is shot.

[Brent Hilpert via cctalk]

On 2020-Apr-21, at 7:57 AM, Chris Zach via cctalk wrote:
> Well I'm starting to walk through this. First I took an IR picture of the 
> boards in operation, then started troubleshooting.
> 
> First I started checking voltages. J2 is easily accessible so I put a ground 
> probe on pin 10, and checked voltages as follows:
> 
> J2-3 Should be -5v, reading -3.8v
> J2-2 Should be -12v reading -15v
> J2-4 Should be 5v, reading 5.04v
> J1-10 Should be 15v reading 14.84
> 
> Hm. That's odd. Looks like the negative voltages are a bit off.
> 
> According to the schematic the key transistor on the 5v rail is Q6, which is 
> a 2n3055. The key transistor on +15 is Q10 and that circuit looks ok. However 
> whatever is running the -5 and -12 volts is not working right Looks like 
> Q12 is the key power transistor there with E2 serving as the control.
> 
> Hm

It's conceivable there was a revision change where the -12V was changed to 
-15V, but it does seem unlikely.

The -12 is used by the vertical deflection circuit, the CRT filament, (and the 
RS-232).

I wouldn't immediately anticipate -12 going to -15 alone would produce the 
upset you're seeing on the screen but there could well be interrelated failures 
between the vertical deflection and the over-voltage -12, so the excessive -12 
should probably be addressed.

In the power supply, you could measure some voltages around the -12 regulator.

E2.2 and E2.3 should be around 0V.
E2.6 I expect would normally be within +/- 2V of GND.
If E2.6 is way-positive (>Q4.E, > ~ +2.6V), then E2 is attempting to throttle 
down the output but failing due to some failure around Q4/Q12.

Measure the voltages on Q4 and Q12.
They could be leaky or low-gain.
It's conceivable Q12 has a CE short, but I'd expect the input to the reg to be 
more than -15.

The low -5 is odd, zener D14 looks to be a 5.1V type.
R45 is probably burning up.
I haven't spotted anything in the schematic that actually uses the -5,
it looks to be provided for some alternative character generator (schematic 
pdf.42),
so that may depend on what character generator chip / daughter board is used in 
your unit.

Re: Great, my VT52 is shot.

[Chris Zach via cctalk]

Well I'm starting to walk through this. First I took an IR picture of 
the boards in operation, then started troubleshooting.


First I started checking voltages. J2 is easily accessible so I put a 
ground probe on pin 10, and checked voltages as follows:


J2-3 Should be -5v, reading -3.8v
J2-2 Should be -12v reading -15v
J2-4 Should be 5v, reading 5.04v
J1-10 Should be 15v reading 14.84

Hm. That's odd. Looks like the negative voltages are a bit off.

According to the schematic the key transistor on the 5v rail is Q6, 
which is a 2n3055. The key transistor on +15 is Q10 and that circuit 
looks ok. However whatever is running the -5 and -12 volts is not 
working right Looks like Q12 is the key power transistor there with 
E2 serving as the control.


Hm

C




On 4/20/2020 1:40 PM, Mattis Lind via cctalk wrote:

I looked at the video once more and paused around 0:21. It appears to be
some text there on the uppermost row.

Maybe the vertical sweep is highly non-linear. Check all capacitors in the
vertical stage. C43, C41,C40, C42, C35 and C45. Would probably be good to
measure the voltage over the current sensing resistor R73 and see if it is
a proper ramp as it should.

What about the R92 linearity potentiometer. Is it ok? Diode D31? Actually
check all semiconductors.

/Mattis

måndag 20 april 2020 skrev Mattis Lind :








Something else that might help with diagnosis is taking pictures of the
screen with known, simple elements on the screen.
e.g.:
 - clear the screen
 - take pic if it doesn't clear
 - type a simple character like a "-"or "1"
 - pic
 - type a half line or full line of characters
 - pic
 - type different characters on two different lines
 - pic

The idea being to find out, from simple known patterns, where the pixels
end up being displayed,
so it may be possible to figure out what's happening with the scan,
whether it's getting stretched, folding over, etc.




That is a good idea. Loop the terminal and create a bunch of "known
references".

Might add that it would be interesting to see one single character on a
lets say 4 different positions. Column 0, Column 20, Column 40 and end of
line. Approximately since it is hard to be exact.

/Mattis

Re: Great, my VT52 is shot.

[Mattis Lind via cctalk]

I looked at the video once more and paused around 0:21. It appears to be
some text there on the uppermost row.

Maybe the vertical sweep is highly non-linear. Check all capacitors in the
vertical stage. C43, C41,C40, C42, C35 and C45. Would probably be good to
measure the voltage over the current sensing resistor R73 and see if it is
a proper ramp as it should.

What about the R92 linearity potentiometer. Is it ok? Diode D31? Actually
check all semiconductors.

/Mattis

måndag 20 april 2020 skrev Mattis Lind :

>
>
>
>>
>>
>> Something else that might help with diagnosis is taking pictures of the
>> screen with known, simple elements on the screen.
>> e.g.:
>> - clear the screen
>> - take pic if it doesn't clear
>> - type a simple character like a "-"or "1"
>> - pic
>> - type a half line or full line of characters
>> - pic
>> - type different characters on two different lines
>> - pic
>>
>> The idea being to find out, from simple known patterns, where the pixels
>> end up being displayed,
>> so it may be possible to figure out what's happening with the scan,
>> whether it's getting stretched, folding over, etc.
>>
>>
>>
> That is a good idea. Loop the terminal and create a bunch of "known
> references".
>
> Might add that it would be interesting to see one single character on a
> lets say 4 different positions. Column 0, Column 20, Column 40 and end of
> line. Approximately since it is hard to be exact.
>
> /Mattis
>

Re: Great, my VT52 is shot.

[Mattis Lind via cctalk]

>
>
> Something else that might help with diagnosis is taking pictures of the
> screen with known, simple elements on the screen.
> e.g.:
> - clear the screen
> - take pic if it doesn't clear
> - type a simple character like a "-"or "1"
> - pic
> - type a half line or full line of characters
> - pic
> - type different characters on two different lines
> - pic
>
> The idea being to find out, from simple known patterns, where the pixels
> end up being displayed,
> so it may be possible to figure out what's happening with the scan,
> whether it's getting stretched, folding over, etc.
>
>
>
That is a good idea. Loop the terminal and create a bunch of "known
references".

Might add that it would be interesting to see one single character on a
lets say 4 different positions. Column 0, Column 20, Column 40 and end of
line. Approximately since it is hard to be exact.

/Mattis

Re: Great, my VT52 is shot.

[Mattis Lind via cctalk]

måndag 20 april 2020 skrev Chris Zach via cctalk :

> As I wrote earlier in the thread I think it is a good idea to check the V
>> sync and H sync signals to check that they are right in pulse length and
>> shape.
>>
>
> Good starting point. From the schematics would that be scope probe to J1
> pin 5 for horizontal and j1 pin 9 for vertical? Also which is J1, and how
> are they numbered :-) Also is there a good ground reference point?


I have a picture of the connector on my page. Have a look there. The
connector have the standard color codes to map to numbers. Black for zero,
Brown for one. Etc.


>
> Also is there a document that links the part numbers on the boards with
> what's on the schematics?


No DEC document. When I did my repair I mapped all ICs to the board but not
the connectors, sorry. You can find all here. Including links to schematis
and maintenance manual: http://www.datormuseum.se/home/dec-vt5x


> There are indeed electrolytic capacitors in the vertical deflection circuit
>> but I am not sure if that would make the beam move much faster since for a
>> faster move of the beam also requires a higher voltage over the deflection
>> coil to create a faster ramp up of the current trough it. But it is
>> definitely worth checking. Especially since the scan lines are sloping a
>> bit and not straight as one would expect.
>>
>
> I would believe a capacitor failure due to heat, any ones in particular I
> should check?


C45 and C42 could be useful to check.


>
> So do I. And now we need Chris to do some measurements on it to get
>> further.
>>
>
> More than happy, just haven't worked on TV sets before and would rather
> not blow off my hand. I'm guessing the really dangerous voltage is the one
> on the bottom left that comes off the step up transformer over to the odd
> plug going into the VT52 rear bulkhead (HV to display)


What you could do is what I did. To remove all control board on the bottom
and lay it out flat on the table. Make a small circuit that create a
composite video signal and feed it into a standard composite video monitor
or TV set. If you remove a jumper to th CRT logic there will be no HV at
all generated.

/Mattis


>
> C
>

Re: Great, my VT52 is shot.

[Brent Hilpert via cctalk]

On 2020-Apr-19, at 3:09 PM, Chris Zach via cctalk wrote:

>> As I wrote earlier in the thread I think it is a good idea to check the V
>> sync and H sync signals to check that they are right in pulse length and
>> shape.
> 
> Good starting point. From the schematics would that be scope probe to J1 pin 
> 5 for horizontal and j1 pin 9 for vertical? Also which is J1, and how are 
> they numbered :-) Also is there a good ground reference point?
> 
> Also is there a document that links the part numbers on the boards with 
> what's on the schematics?
> 
>> There are indeed electrolytic capacitors in the vertical deflection circuit
>> but I am not sure if that would make the beam move much faster since for a
>> faster move of the beam also requires a higher voltage over the deflection
>> coil to create a faster ramp up of the current trough it. But it is
>> definitely worth checking. Especially since the scan lines are sloping a
>> bit and not straight as one would expect.
> 
> I would believe a capacitor failure due to heat, any ones in particular I 
> should check?
> 
>> So do I. And now we need Chris to do some measurements on it to get further.
> 
> More than happy, just haven't worked on TV sets before and would rather not 
> blow off my hand. I'm guessing the really dangerous voltage is the one on the 
> bottom left that comes off the step up transformer over to the odd plug going 
> into the VT52 rear bulkhead (HV to display)


Using the schematic and maintenance manual from bitsavers:
http://bitsavers.org/pdf/dec/terminal/vt52/MP00035_VT52schem.pdf

http://bitsavers.org/pdf/dec/terminal/vt52/EK-VT52-MM-002_maint_Jul78.pdf

Monitor schematic: MP00035_VT52schem pdf-page 10
Monitor board component layout: MP00035_VT52schem pdf-page 8

There are *two* horizontal signals from the logic to the monitor (plus the 
vertical & video).

There are some waveform diagrams for what to expect around the monitor board 
presented in the maint manual: 
EK-VT52-MM-002_maint_Jul78 pdf-page 92

Something else that might help with diagnosis is taking pictures of the screen 
with known, simple elements on the screen.
e.g.:
- clear the screen
- take pic if it doesn't clear
- type a simple character like a "-"or "1"
- pic
- type a half line or full line of characters
- pic
- type different characters on two different lines
- pic

The idea being to find out, from simple known patterns, where the pixels end up 
being displayed,
so it may be possible to figure out what's happening with the scan, whether 
it's getting stretched, folding over, etc.

Yes, you want to watch out for HV around the flyback transformer.
The very HV is reasonably insulated, but there's also a few-hundred volts 
around the open componentry around the focus and intensity controls, the stuff 
feeding the neck end of the CRT.

Re: Great, my VT52 is shot.

[Chris Zach via cctalk]

As I wrote earlier in the thread I think it is a good idea to check the V
sync and H sync signals to check that they are right in pulse length and
shape.


Good starting point. From the schematics would that be scope probe to J1 
pin 5 for horizontal and j1 pin 9 for vertical? Also which is J1, and 
how are they numbered :-) Also is there a good ground reference point?


Also is there a document that links the part numbers on the boards with 
what's on the schematics?



There are indeed electrolytic capacitors in the vertical deflection circuit
but I am not sure if that would make the beam move much faster since for a
faster move of the beam also requires a higher voltage over the deflection
coil to create a faster ramp up of the current trough it. But it is
definitely worth checking. Especially since the scan lines are sloping a
bit and not straight as one would expect.


I would believe a capacitor failure due to heat, any ones in particular 
I should check?



So do I. And now we need Chris to do some measurements on it to get further.


More than happy, just haven't worked on TV sets before and would rather 
not blow off my hand. I'm guessing the really dangerous voltage is the 
one on the bottom left that comes off the step up transformer over to 
the odd plug going into the VT52 rear bulkhead (HV to display)


C

Re: Great, my VT52 is shot.

[Jon Elson via cctalk]

After looking at the video, and Mattis' description of the 
sweep method, I'm guessing a counter
in the chain has gone bad.  Assuming this uses a 5 x 7 
character matrix, then there would be probably
a divide by 6 for the scan across each character plus space 
between, then a counter for the number
of characters per line, a counter for the 8 or 9 raster scan 
lines for each row, and then a counter
for the number of rows.  At least one of those counters 
might be counting too fast.  Does the vertical just give one 
reset pulse, or does it give a pulse to bump the trace down 
the screen one scan line at a
time?  Also, a cap in the vertical sweep circuit may have 
gone bad, causing the vertical to move too

fast, as evidenced by sloping lines on the screen.

Just guessing from a distance.

Jon

Re: Great, my VT52 is shot.

[Mattis Lind via cctalk]

söndag 19 april 2020 skrev Chris Zach via cctalk :

> Well, let's see: I took the bottom off the VT52 and checked the
> connections to the high voltage board on the back. Cleaned up both
> connectors, plugged in, same problem.
>
> I do hear a medium to high pitched squeal coming from the back board.
> Can't quite triangulate it (I need a hose and some time) but it might be
> coming from those center transistors. Any idea what these boards are doing,
> and if there is a schematic for these circuits?


The left is the video /CRT part. The right is the power supply part. The
transistors are mostly regulator transistors.

I have linked to both maintenance manual and schematic from the page I
referred to in one of my previous replies to this thread.
http://www.datormuseum.se/home/dec-vt5x

Take a look there!

/Mattis



> Given that it went weird after being powered on for awhile I would suspect
> it has something to do with a component that generates heat. I could also
> take an IR picture of the back board and see if anything is unusually
> hot/not hot.
>
> Audio of the noise: https://i.imgur.com/ps4Ovkj.mp4
> Pic of the back: https://i.imgur.com/HgNoTYK.jpg
>
> C
>

RE: Great, my VT52 is shot.

[Wayne Sudol via cctalk]

Here's a link to the vt52 maintenance manual. Has testing and service info. 
Hope it helps.

 
http://bitsavers.informatik.uni-stuttgart.de/pdf/dec/terminal/vt52/EK-VT52-MM-002_maint_Jul78.pdf

Wayne

-Original Message-
From: cctalk [mailto:cctalk-boun...@classiccmp.org] On Behalf Of Chris Zach via 
cctalk
Sent: Saturday, April 18, 2020 3:56 PM
Cc: General Discussion: On-Topic and Off-Topic Posts
Subject: Re: Great, my VT52 is shot.

Well, let's see: I took the bottom off the VT52 and checked the connections to 
the high voltage board on the back. Cleaned up both connectors, plugged in, 
same problem.

I do hear a medium to high pitched squeal coming from the back board. 
Can't quite triangulate it (I need a hose and some time) but it might be coming 
from those center transistors. Any idea what these boards are doing, and if 
there is a schematic for these circuits?

Given that it went weird after being powered on for awhile I would suspect it 
has something to do with a component that generates heat. I could also take an 
IR picture of the back board and see if anything is unusually hot/not hot.

Audio of the noise: https://i.imgur.com/ps4Ovkj.mp4 Pic of the back: 
https://i.imgur.com/HgNoTYK.jpg

C

Re: Great, my VT52 is shot.

[Chris Zach via cctalk]

Well, let's see: I took the bottom off the VT52 and checked the 
connections to the high voltage board on the back. Cleaned up both 
connectors, plugged in, same problem.


I do hear a medium to high pitched squeal coming from the back board. 
Can't quite triangulate it (I need a hose and some time) but it might be 
coming from those center transistors. Any idea what these boards are 
doing, and if there is a schematic for these circuits?


Given that it went weird after being powered on for awhile I would 
suspect it has something to do with a component that generates heat. I 
could also take an IR picture of the back board and see if anything is 
unusually hot/not hot.


Audio of the noise: https://i.imgur.com/ps4Ovkj.mp4
Pic of the back: https://i.imgur.com/HgNoTYK.jpg

C

Re: Great, my VT52 is shot.

[Mattis Lind via cctalk]

> We had a VT52 do something akin to that once back in the day... The tech
> we had that worked on this stuff said he had to fix something in the
> vertical refresh circuit to achieve vertical hold... My vague memories
> likely won't help you much, but this is quite distinctive and others of the
> group will be able to suggest things :)
>

Vertical hold implies that there is an oscillator that need to get into
hold or lock with an incoming signal.  There is none in a VT52 so that is
not possible. There is simply no way to adjust vertical hold in a VT52.

/Mattis


> Warner
>
>
>>
>> C
>>
>> On 4/18/2020 4:45 AM, Mattis Lind wrote:
>> > Hello Chris!
>> >
>> > Sorry that your trusty VT52 failed. But it shouldn't be too hard to
>> > repair it!
>> >
>> > I have successfully restored a VT52 (actually a VT50 with a brain
>> > transplant): http://www.datormuseum.se/home/dec-vt5x. My experience is
>> > that the capacitors were in good shape. Especially the bigger filter
>> > capacitor. I think that DEC used great quality capacitors.
>> >
>> > It is always a good idea to check the voltages so that they are within
>> > the spec. But it should not be necessary to replace all the capacitors
>> > in my opinion.
>> >
>> > The VT52 is a quite interesting design with a very simplistic CPU built
>> > from TTL components and a microprogram that has 1024 by 8 bits. Most of
>> > the operations inside it is controlled from this program. In mine the
>> > sockets for the mikroprogram were bad which meant that it failed to do
>> > anything. Then I had a couple of gates that were bad.
>> >
>> > Den tors 16 apr. 2020 kl 23:07 skrev Chris Zach via cctalk
>> > mailto:cctalk@classiccmp.org>>:
>> >
>> > Wonderful: A few weeks ago I forgot to turn off my VT52 and left it
>> > running for a day or two. Now the screen is filled with snow and it
>> > looks like the text is all over the place horozontally.
>> >
>> >
>> > It would be great to have a picture since it could give some hints on
>> > what is wrong with it. My understanding is that the scan is working
>> both
>> > horizontally and vertically. Otherwise you would only have a straight
>> > line. Unlike many common CRTs this CRT circuitry has no oscillator in
>> it
>> > that creates a picture without input. And since the horisontal
>> > deflection circuit also creates the high voltage there would be no
>> > picture at all unless the horisontal signal is there.
>> >
>> > This means that the two control boards in the bottom creates some kind
>> > of horisontal and vertical signals. These signals comes from a simple
>> > divider chain. There could be some kind of problem in the divider
>> chain.
>> > But then it would be more likely to not work at all if one of the
>> > counters are bad. But it is a good idea to check the H and V signal som
>> > they are inline with the spec. Around 15kHz and 60 or 50 Hz
>> respectively.
>> >
>> > Since I haven't seen how the output looks like I speculating a bit. It
>> > could be the case that the divider chain is correct but the video
>> signal
>> > is not generated in sync with the divider chain. Then the characters
>> > would end up all over the place on screen.
>> >
>> > There is a flip-flip, made out of a 7400 (E16) and 74H10 (E14) that is
>> > the video flip-flop. It controls when the screen is rendered. It might
>> > be the case it could cause your problem. I think it is a  good idea to
>> > check the signals around this flip-flop.
>> >
>> > I am always a bit skeptical towards 74Hxx. My impression is that they
>> > fail more often than standard 74xx.
>> >
>> > When I repaired my VT52 I made a quick circuit that created a composite
>> > video signal from the control board. The signal was fed into a standard
>> > CRT monitor which I had around.  I could then run the control boards
>> > flat on the bench to take measurements rather than assembling prior to
>> > each test. I thought that it was quite handy.
>> >
>> >
>> > Any tips or thoughts on where to start looking to fix? The keyboard
>> > seems to be working as does the RS232 input (the snow on the screen
>> > changes when the pdp11 talks to it)
>> >
>> >
>> > This indicate that since you are using RS232 you have at least some
>> kind
>> > of voltage level on +12 V and -12 V since otherwise there would be no
>> > communication. And +5V is probably not that bad either. Since not much
>> > would operate in that case. Likewise the -5V is probably ok since the
>> > char rom would not work very well without the bias.
>> >
>> > Good luck with your repair!
>> >
>> > /Mattis
>> >
>> >
>> > Thanks!
>> > Chris
>> >
>>
>

Re: Great, my VT52 is shot.

[Mattis Lind via cctalk]

lördag 18 april 2020 skrev Chris Zach :

> Here's a video of it with sound on.
>
> https://i.imgur.com/X1qVYGP.mp4


You have video even in the horisontal retrace. I still think you have a
problem somewhere around the video flip flop. Check that part of the
circuit.

/Mattis


>
> C
>
> On 4/18/2020 4:45 AM, Mattis Lind wrote:
>
>> Hello Chris!
>>
>> Sorry that your trusty VT52 failed. But it shouldn't be too hard to
>> repair it!
>>
>> I have successfully restored a VT52 (actually a VT50 with a brain
>> transplant): http://www.datormuseum.se/home/dec-vt5x. My experience is
>> that the capacitors were in good shape. Especially the bigger filter
>> capacitor. I think that DEC used great quality capacitors.
>>
>> It is always a good idea to check the voltages so that they are within
>> the spec. But it should not be necessary to replace all the capacitors in
>> my opinion.
>>
>> The VT52 is a quite interesting design with a very simplistic CPU built
>> from TTL components and a microprogram that has 1024 by 8 bits. Most of the
>> operations inside it is controlled from this program. In mine the sockets
>> for the mikroprogram were bad which meant that it failed to do anything.
>> Then I had a couple of gates that were bad.
>>
>> Den tors 16 apr. 2020 kl 23:07 skrev Chris Zach via cctalk <
>> cctalk@classiccmp.org >:
>>
>> Wonderful: A few weeks ago I forgot to turn off my VT52 and left it
>> running for a day or two. Now the screen is filled with snow and it
>> looks like the text is all over the place horozontally.
>>
>>
>> It would be great to have a picture since it could give some hints on
>> what is wrong with it. My understanding is that the scan is working both
>> horizontally and vertically. Otherwise you would only have a straight line.
>> Unlike many common CRTs this CRT circuitry has no oscillator in it that
>> creates a picture without input. And since the horisontal
>> deflection circuit also creates the high voltage there would be no picture
>> at all unless the horisontal signal is there.
>>
>> This means that the two control boards in the bottom creates some kind of
>> horisontal and vertical signals. These signals comes from a simple divider
>> chain. There could be some kind of problem in the divider chain. But then
>> it would be more likely to not work at all if one of the counters are bad.
>> But it is a good idea to check the H and V signal som they are inline with
>> the spec. Around 15kHz and 60 or 50 Hz respectively.
>>
>> Since I haven't seen how the output looks like I speculating a bit. It
>> could be the case that the divider chain is correct but the video signal is
>> not generated in sync with the divider chain. Then the characters would end
>> up all over the place on screen.
>>
>> There is a flip-flip, made out of a 7400 (E16) and 74H10 (E14) that is
>> the video flip-flop. It controls when the screen is rendered. It might be
>> the case it could cause your problem. I think it is a  good idea to check
>> the signals around this flip-flop.
>>
>> I am always a bit skeptical towards 74Hxx. My impression is that they
>> fail more often than standard 74xx.
>>
>> When I repaired my VT52 I made a quick circuit that created a composite
>> video signal from the control board. The signal was fed into a standard CRT
>> monitor which I had around.  I could then run the control boards flat on
>> the bench to take measurements rather than assembling prior to each test. I
>> thought that it was quite handy.
>>
>>
>> Any tips or thoughts on where to start looking to fix? The keyboard
>> seems to be working as does the RS232 input (the snow on the screen
>> changes when the pdp11 talks to it)
>>
>>
>> This indicate that since you are using RS232 you have at least some kind
>> of voltage level on +12 V and -12 V since otherwise there would be no
>> communication. And +5V is probably not that bad either. Since not much
>> would operate in that case. Likewise the -5V is probably ok since the char
>> rom would not work very well without the bias.
>>
>> Good luck with your repair!
>>
>> /Mattis
>>
>>
>> Thanks!
>> Chris
>>
>>

Re: Great, my VT52 is shot.

[Warner Losh via cctalk]

On Sat, Apr 18, 2020 at 10:47 AM Chris Zach via cctalk <
cctalk@classiccmp.org> wrote:

> Here's a video of it with sound on.
>
> https://i.imgur.com/X1qVYGP.mp4


We had a VT52 do something akin to that once back in the day... The tech we
had that worked on this stuff said he had to fix something in the vertical
refresh circuit to achieve vertical hold... My vague memories likely won't
help you much, but this is quite distinctive and others of the group will
be able to suggest things :)

Warner


>
> C
>
> On 4/18/2020 4:45 AM, Mattis Lind wrote:
> > Hello Chris!
> >
> > Sorry that your trusty VT52 failed. But it shouldn't be too hard to
> > repair it!
> >
> > I have successfully restored a VT52 (actually a VT50 with a brain
> > transplant): http://www.datormuseum.se/home/dec-vt5x. My experience is
> > that the capacitors were in good shape. Especially the bigger filter
> > capacitor. I think that DEC used great quality capacitors.
> >
> > It is always a good idea to check the voltages so that they are within
> > the spec. But it should not be necessary to replace all the capacitors
> > in my opinion.
> >
> > The VT52 is a quite interesting design with a very simplistic CPU built
> > from TTL components and a microprogram that has 1024 by 8 bits. Most of
> > the operations inside it is controlled from this program. In mine the
> > sockets for the mikroprogram were bad which meant that it failed to do
> > anything. Then I had a couple of gates that were bad.
> >
> > Den tors 16 apr. 2020 kl 23:07 skrev Chris Zach via cctalk
> > mailto:cctalk@classiccmp.org>>:
> >
> > Wonderful: A few weeks ago I forgot to turn off my VT52 and left it
> > running for a day or two. Now the screen is filled with snow and it
> > looks like the text is all over the place horozontally.
> >
> >
> > It would be great to have a picture since it could give some hints on
> > what is wrong with it. My understanding is that the scan is working both
> > horizontally and vertically. Otherwise you would only have a straight
> > line. Unlike many common CRTs this CRT circuitry has no oscillator in it
> > that creates a picture without input. And since the horisontal
> > deflection circuit also creates the high voltage there would be no
> > picture at all unless the horisontal signal is there.
> >
> > This means that the two control boards in the bottom creates some kind
> > of horisontal and vertical signals. These signals comes from a simple
> > divider chain. There could be some kind of problem in the divider chain.
> > But then it would be more likely to not work at all if one of the
> > counters are bad. But it is a good idea to check the H and V signal som
> > they are inline with the spec. Around 15kHz and 60 or 50 Hz respectively.
> >
> > Since I haven't seen how the output looks like I speculating a bit. It
> > could be the case that the divider chain is correct but the video signal
> > is not generated in sync with the divider chain. Then the characters
> > would end up all over the place on screen.
> >
> > There is a flip-flip, made out of a 7400 (E16) and 74H10 (E14) that is
> > the video flip-flop. It controls when the screen is rendered. It might
> > be the case it could cause your problem. I think it is a  good idea to
> > check the signals around this flip-flop.
> >
> > I am always a bit skeptical towards 74Hxx. My impression is that they
> > fail more often than standard 74xx.
> >
> > When I repaired my VT52 I made a quick circuit that created a composite
> > video signal from the control board. The signal was fed into a standard
> > CRT monitor which I had around.  I could then run the control boards
> > flat on the bench to take measurements rather than assembling prior to
> > each test. I thought that it was quite handy.
> >
> >
> > Any tips or thoughts on where to start looking to fix? The keyboard
> > seems to be working as does the RS232 input (the snow on the screen
> > changes when the pdp11 talks to it)
> >
> >
> > This indicate that since you are using RS232 you have at least some kind
> > of voltage level on +12 V and -12 V since otherwise there would be no
> > communication. And +5V is probably not that bad either. Since not much
> > would operate in that case. Likewise the -5V is probably ok since the
> > char rom would not work very well without the bias.
> >
> > Good luck with your repair!
> >
> > /Mattis
> >
> >
> > Thanks!
> > Chris
> >
>

Re: Great, my VT52 is shot.

[Chris Zach via cctalk]

Here's a video of it with sound on.

https://i.imgur.com/X1qVYGP.mp4

C

On 4/18/2020 4:45 AM, Mattis Lind wrote:

Hello Chris!

Sorry that your trusty VT52 failed. But it shouldn't be too hard to 
repair it!


I have successfully restored a VT52 (actually a VT50 with a brain 
transplant): http://www.datormuseum.se/home/dec-vt5x. My experience is 
that the capacitors were in good shape. Especially the bigger filter 
capacitor. I think that DEC used great quality capacitors.


It is always a good idea to check the voltages so that they are within 
the spec. But it should not be necessary to replace all the capacitors 
in my opinion.


The VT52 is a quite interesting design with a very simplistic CPU built 
from TTL components and a microprogram that has 1024 by 8 bits. Most of 
the operations inside it is controlled from this program. In mine the 
sockets for the mikroprogram were bad which meant that it failed to do 
anything. Then I had a couple of gates that were bad.


Den tors 16 apr. 2020 kl 23:07 skrev Chris Zach via cctalk 
mailto:cctalk@classiccmp.org>>:


Wonderful: A few weeks ago I forgot to turn off my VT52 and left it
running for a day or two. Now the screen is filled with snow and it
looks like the text is all over the place horozontally.


It would be great to have a picture since it could give some hints on 
what is wrong with it. My understanding is that the scan is working both 
horizontally and vertically. Otherwise you would only have a straight 
line. Unlike many common CRTs this CRT circuitry has no oscillator in it 
that creates a picture without input. And since the horisontal 
deflection circuit also creates the high voltage there would be no 
picture at all unless the horisontal signal is there.


This means that the two control boards in the bottom creates some kind 
of horisontal and vertical signals. These signals comes from a simple 
divider chain. There could be some kind of problem in the divider chain. 
But then it would be more likely to not work at all if one of the 
counters are bad. But it is a good idea to check the H and V signal som 
they are inline with the spec. Around 15kHz and 60 or 50 Hz respectively.


Since I haven't seen how the output looks like I speculating a bit. It 
could be the case that the divider chain is correct but the video signal 
is not generated in sync with the divider chain. Then the characters 
would end up all over the place on screen.


There is a flip-flip, made out of a 7400 (E16) and 74H10 (E14) that is 
the video flip-flop. It controls when the screen is rendered. It might 
be the case it could cause your problem. I think it is a  good idea to 
check the signals around this flip-flop.


I am always a bit skeptical towards 74Hxx. My impression is that they 
fail more often than standard 74xx.


When I repaired my VT52 I made a quick circuit that created a composite 
video signal from the control board. The signal was fed into a standard 
CRT monitor which I had around.  I could then run the control boards 
flat on the bench to take measurements rather than assembling prior to 
each test. I thought that it was quite handy.



Any tips or thoughts on where to start looking to fix? The keyboard
seems to be working as does the RS232 input (the snow on the screen
changes when the pdp11 talks to it)


This indicate that since you are using RS232 you have at least some kind 
of voltage level on +12 V and -12 V since otherwise there would be no 
communication. And +5V is probably not that bad either. Since not much 
would operate in that case. Likewise the -5V is probably ok since the 
char rom would not work very well without the bias.


Good luck with your repair!

/Mattis


Thanks!
Chris

Re: Great, my VT52 is shot.

[Mattis Lind via cctalk]

Hello Chris!

Sorry that your trusty VT52 failed. But it shouldn't be too hard to repair
it!

I have successfully restored a VT52 (actually a VT50 with a brain
transplant): http://www.datormuseum.se/home/dec-vt5x. My experience is that
the capacitors were in good shape. Especially the bigger filter capacitor.
I think that DEC used great quality capacitors.

It is always a good idea to check the voltages so that they are within the
spec. But it should not be necessary to replace all the capacitors in my
opinion.

The VT52 is a quite interesting design with a very simplistic CPU built
from TTL components and a microprogram that has 1024 by 8 bits. Most of the
operations inside it is controlled from this program. In mine the sockets
for the mikroprogram were bad which meant that it failed to do anything.
Then I had a couple of gates that were bad.

Den tors 16 apr. 2020 kl 23:07 skrev Chris Zach via cctalk <
cctalk@classiccmp.org>:

> Wonderful: A few weeks ago I forgot to turn off my VT52 and left it
> running for a day or two. Now the screen is filled with snow and it
> looks like the text is all over the place horozontally.
>

It would be great to have a picture since it could give some hints on what
is wrong with it. My understanding is that the scan is working both
horizontally and vertically. Otherwise you would only have a straight line.
Unlike many common CRTs this CRT circuitry has no oscillator in it that
creates a picture without input. And since the horisontal
deflection circuit also creates the high voltage there would be no picture
at all unless the horisontal signal is there.

This means that the two control boards in the bottom creates some kind of
horisontal and vertical signals. These signals comes from a simple divider
chain. There could be some kind of problem in the divider chain. But then
it would be more likely to not work at all if one of the counters are bad.
But it is a good idea to check the H and V signal som they are inline with
the spec. Around 15kHz and 60 or 50 Hz respectively.

Since I haven't seen how the output looks like I speculating a bit. It
could be the case that the divider chain is correct but the video signal is
not generated in sync with the divider chain. Then the characters would end
up all over the place on screen.

There is a flip-flip, made out of a 7400 (E16) and 74H10 (E14) that is the
video flip-flop. It controls when the screen is rendered. It might be
the case it could cause your problem. I think it is a  good idea to check
the signals around this flip-flop.

I am always a bit skeptical towards 74Hxx. My impression is that they fail
more often than standard 74xx.

When I repaired my VT52 I made a quick circuit that created a composite
video signal from the control board. The signal was fed into a standard CRT
monitor which I had around.  I could then run the control boards flat on
the bench to take measurements rather than assembling prior to each test. I
thought that it was quite handy.


> Any tips or thoughts on where to start looking to fix? The keyboard
> seems to be working as does the RS232 input (the snow on the screen
> changes when the pdp11 talks to it)
>

This indicate that since you are using RS232 you have at least some kind of
voltage level on +12 V and -12 V since otherwise there would be no
communication. And +5V is probably not that bad either. Since not much
would operate in that case. Likewise the -5V is probably ok since the char
rom would not work very well without the bias.

Good luck with your repair!

/Mattis


>
> Thanks!
> Chris
>

Re: Great, my VT52 is shot.

[Bill Degnan via cctalk]

On Fri, Apr 17, 2020 at 2:21 AM Brent Hilpert via cctalk <
cctalk@classiccmp.org> wrote:

>
> On 2020-Apr-15, at 5:23 PM, Chris Zach via cctalk wrote:
>
> > Wonderful: A few weeks ago I forgot to turn off my VT52 and left it
> running for a day or two. Now the screen is filled with snow and it looks
> like the text is all over the place horozontally.
> >
> > Any tips or thoughts on where to start looking to fix? The keyboard
> seems to be working as does the RS232 input (the snow on the screen changes
> when the pdp11 talks to it)
>
>
> Try turning the contrast or brightness up till the black level starts
> becoming illuminated. Is the raster (the screen character field / framing
> rectangle) stable?
>
> If no rectangle or stable display field becomes apparent then as Jon
> suggests it may be loss of sync, probably horizontal, and likely to be in
> the monitor circuitry.
>
> (Any affect from twiddling the H or V sync controls?)
>
> If the raster is stable then it might be something in the character
> generation pipeline (the character generator or the row pixel serializer),
> or in the monitor video amp.
>
> ("snow" as a description leaves for a range of possible interpretations.)
>
> There's always checking power supply levels to start with. In addition to
> the +5, there's probably a +/-12 or -5 for the character generator.
>

Ian Primas knows a lot about these.  (snark).but def check the power
first, the large filter caps.  I have a working VT50, which is similar but
not exactly.  If you leave a terminal on for a long time and it dies,
that's a power issue or power related (caps resistor fried diode, etc.)
IMHO.

Re: Great, my VT52 is shot.

[Brent Hilpert via cctalk]

On 2020-Apr-15, at 5:23 PM, Chris Zach via cctalk wrote:

> Wonderful: A few weeks ago I forgot to turn off my VT52 and left it running 
> for a day or two. Now the screen is filled with snow and it looks like the 
> text is all over the place horozontally.
> 
> Any tips or thoughts on where to start looking to fix? The keyboard seems to 
> be working as does the RS232 input (the snow on the screen changes when the 
> pdp11 talks to it)


Try turning the contrast or brightness up till the black level starts becoming 
illuminated. Is the raster (the screen character field / framing rectangle) 
stable?

If no rectangle or stable display field becomes apparent then as Jon suggests 
it may be loss of sync, probably horizontal, and likely to be in the monitor 
circuitry.

(Any affect from twiddling the H or V sync controls?)

If the raster is stable then it might be something in the character generation 
pipeline (the character generator or the row pixel serializer), or in the 
monitor video amp.

("snow" as a description leaves for a range of possible interpretations.)

There's always checking power supply levels to start with. In addition to the 
+5, there's probably a +/-12 or -5 for the character generator.

Re: Great, my VT52 is shot.

[Jon Elson via cctalk]

On 04/15/2020 07:23 PM, Chris Zach via cctalk wrote:
Wonderful: A few weeks ago I forgot to turn off my VT52 
and left it running for a day or two. Now the screen is 
filled with snow and it looks like the text is all over 
the place horozontally.


Any tips or thoughts on where to start looking to fix? The 
keyboard seems to be working as does the RS232 input (the 
snow on the screen changes when the pdp11 talks to it)


Unseat and replug all connectors for a first start.  Your 
description sounds like the monitor may not be getting sync 
from the display generator board.


Jon