Oh, what fun is that? Assuming one can do a decent job soldering and desoldering parts without damaging boards, use a meter without inadvertently shorting pins together, and resist the urge to put one's tongue across the AC mains to check if it is plugged in, then why not a little debugging?
OK, one ought to have an oscilloscope to look at this circuit since the voltages are all moving around when the circuit is oscillating. Seems to not be doing that right now, so debugging with the meter, as you are doing, should work OK. (You could check the signals with the meter on AC volts, which should be near zero on all pins if this thing is not oscillating. And one would see the signals on AC volts if it was indeed oscillating I have shown below.)
pin 1 = -13.58 Output pin, this will go between + and - 13 or 14V when oscillating as shown on the schematic. (AC voltage would be around 10V if running)
Assuming we are stuck and not oscillating, see the notes next to the pins:
Here are voltages of IC801:
pin 1 = -13.58 Output pin, this will go between + and - 13 or 14V when oscillating as shown on the schematic. (AC voltage would be around 10V if running)
pin 2 = .589 This is a problem; with negative output it ought to be discharging the capacitor through the 22K fixed resistor and 2.5M rate potentiometer, pulling that pin negative until it crosses the pos input's voltage when the thing flips around and starts charging the capacitor, etc. The pin should not be sitting there positive with the output negative. (AC about 1V if it were operating OK)
3 = -1.508 correct at (10/(10+75))*(pin 1 voltage) or -13.6/8.5=-1.6 (also AC around 1V)
4 = -15.02 Neg supply, ok
5 = .589 + input op amp b
6 = .590 - input op amp b
7=.592 output op amp b This amp is in a voltage follower configuration where the output is a copy of the pos input pin, just stronger, and it appears fine. [I have to comment that a capacitor to ground directly on the output of an op amp is not good design practice, leading to potential instability/oscillations where one doesn't want them. Would be a lot better with a 50 or 100 ohm resistor in series with the output first, or do the filtering before the op-amp.]
6 = .590 - input op amp b
7=.592 output op amp b This amp is in a voltage follower configuration where the output is a copy of the pos input pin, just stronger, and it appears fine. [I have to comment that a capacitor to ground directly on the output of an op amp is not good design practice, leading to potential instability/oscillations where one doesn't want them. Would be a lot better with a 50 or 100 ohm resistor in series with the output first, or do the filtering before the op-amp.]
8= 15.02 positive supply, ok
1st hypothesis is that either R801 (the pot) or R802 (the 22K resistor) or the wiring between them is open circuit. Turn the power off, put the meter on ohms and measure the resistance between pins 1 and 2 on the op amp. The reading should go between 22K and 2.522 Meg ohm (+/-10% anyway, but ballpark). Try swapping the meter leads if the readings are off. If the resistance doesn't move try the 22K resistor by itself, the pot on its leads and the interconnections.
2nd thought is the C801 capacitor, could be open circuit, but the numbers don't really support this notion. 2.2uf Tantalum, 25V part, probably not a terrible idea to change it anyway.
3rd thought, continuing #2, The voltage at the other side of C801 should be checked to see if it is indeed -1.5V or so. That's a new line of debugging if that voltage is somewhere else, capacitor might be shorted or otherwise unhappy if the polarity across it is reversed. We'll then look at the -1.5V source if that voltage is off.
For now, the vote is #1.
Good luck,
Hugh
On Sat, May 25, 2013 at 6:01 PM, scott frye <painintheamp@...> wrote:
I can recommend someone in the states depending on where you areIf not, you should seriously look at finding someone professional to deal with the OP AMP problemDo you have real world electronics exp?Hi...To the guy trying to fix this
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Scott Frye
Audio-fixation.net
Vermont