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 goi
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/t
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. The
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
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
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 shoul
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
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 4/21/2
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 se
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 bett
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
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 a
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 checke
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 Sh
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 s
>
>
> 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 hal
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
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 J
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, an
Den sön 19 apr. 2020 kl 17:22 skrev Jon Elson :
> 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 cha
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
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
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 connector
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
On Sat, Apr 18, 2020, 1:01 PM Mattis Lind wrote:
>
>
>
>
>> 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 he
> 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
>
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 4/18/202
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
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://
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
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 look
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 s
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 t
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 RS
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