alltrax wrote: ↑12 Oct 2020, 07:23
Yes, the link is there and the voltages look good
If all the links, cuts, solder joints and components look ok then it's time to start checking components.
I have no idea why there are 2 caps and 2 resistors in series (C1, R10, C17, R11). This indicates to me that whoever "designed" this didn't really know what they were doing and probably cobbled together stages from other circuits.
The combined resistance of the 2 series resistors R1 + R17 is 33k + 10k = 43k so why not use a single resistor around that value.
The combined capacitance of 2 capacitors in series is a bit more complicated but when they are a couple of orders of magnitude difference in value you could replace the 10uF capacitor with a link and you wouldn't hear the difference.
Back to debugging. Try the easy things first.
If you socketed the ICs (I prefer to) then try another 072 if you have one or if you don't then swap IC1 and IC2 and see if the fault changes to the clipping amplifier section.
If you didn't socket the ICs then check the resistors and capacitors before you try changing the 072. If you do need to unsolder it I would advise fitting a socket (personal preference).
The gain of an inverting opamp = R feedback (R18) / R input (R1 + R17)*
If R10 or R11 are high resistance or C1 or C17 are open circuit then the gain will be reduced according to the formula above.
You can check the resistance of each without desoldering them because the 2 capacitors will block DC (which ohmmeters use to measure resistance with so non of the other circuitry will affect the readings).
If either C18 is leaky (possible) or R18 is a low value (unlikely) then the gain of the opamp will be lowered accordingly.
I'm not sure how much the internal circuitry of the opamp will affect the reading if you put an ohmmeter across C18/R18. Try it and see what it reads but if it does read less than 47k don't assume that either R18 or C18 is faulty. Unsolder one end of each and check them out of circuit.
*In the middle of the pass band, the lower end is of which is determined by C17 and R10+R11, the high end by C18 and R18.
Hope this helps.
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