22 Apr
2020
22 Apr
'20
4:15 a.m.
On 2020-Apr-21, at 8:35 PM, Chris Zach wrote:
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.
When Q4.BE shorted, E2.6 should swing well-negative.
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?
Note comment lower left corner of schematic page.