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.
