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
