Great, my VT52 is shot.

[Brent Hilpert]

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?