Great, my VT52 is shot.

[Brent Hilpert]

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.