Whoops said:
As I dont want to "Randomly change" resistors in the PSU circuit, I asked for advise from follow forum members on what resistors in this transistor+opamp voltage regulators could be changed to increase the output voltage of the regulator.
I'll jump back into this. But Whoops, I have to say three things first.
1) You were a bit hard on RadarDoug, he is trying to help you out, as am I. Perhaps it is a language translation thing, but you may owe him an apology. Sure, you don't want to "randomly" change resistors, but it is always best to get to the real problem, without trying to tweak something into working. Changing resistors to compensate for perhaps a leaky transistor, or an op-amp with a wacky output offset, for example, won't fix the problem for long, as the transistor gets more leaky over time, the problem comes back. You need to find the bad, or marginally bad components, even if they previously tested "functional" with basic testing. And who knows, you may have a "dog", something that was worked on before you, and some resistors or whatever were replaced with wrong ones and left that way by some moron, as you seem to suspect. Always look at the trace pads for reheated joints that can clue you in to whatever was messed with in the past, since the original wave bath and cleaning.
2) Make sure any vent holes in the outer housing rings are not clogged or covered with tape on that U67 capsule from another post. That will kill the LF response.
3) I will run through a few more things for you to try on this scope, and you need to do them in order, even if you feel you have already done so.
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We'll concentrate on the +5 volt rail first. Board powered up from the transformer, no loads connected.
1) The latest schematic shows no anti-squeal caps on the 78/7912 regulators, if you don't already have them, add them. 0.1 or 0.22uF discs from supply to the ground, as close as you can to the regulators. I have been known to tack them right to the regulator pins, above the circuit board, if there is no room to tack them to the pin pads under the circuit board.
Test the no-load voltages again, this may have actually fix the +5 voltage, if the +12V was squealing RF before adding the caps
2) Still +3.3V? Remove U-901 and 902 to eliminate most +/- 12V loads and measure the (+/- 18V test points feeding the 12V regulators. With no load on the PS, I would expect these test points to be just shy of +/- 24V, this is expected. The + and - should be really close to each other. If the + is lower, lift the collector of Q-904 or the resistor feeding it, and retest.
With no drive on the base of Q-904 and U-901 missing, there should be no load on the +12V, unless Q-904 is leaking collector current, assuming your schematic is complete.
3) If we are not there yet, annotate the schematic with the four resistor values in the +5 section, along with measured voltages around U-901 and Q-904, and we'll go from there tomorrow, I'm spent, one of those nights where I am trying to keep up with my avatar.
Gene
