Replaced U503 with another from a pre card, same symptoms. One input to the 4427 has high duty cycle, the other low. Is it possible something in the preamp got damaged when the outputs blew, causing an OPAMP to short and rail or something along those lines?
If that IC hast positive and negative supply (so that it's in operation/functioning), the input impedance to the pin should be very nearly infinite so the only load should be whatever it was when you lifted pin 5. What is the voltage on the other end of the resistor (going to pin 5) you have the jumper soldered to?
Oops, realized I missed a step back on #28. I never tried lifting pin 5, which is a bit difficult because of the tight working area. I'll redo the steps on #28 and also measure the other end of the resistor for voltage when I get home today.
Pin 5 of u503 is in fact being driven by the offset pot, pin 7 (output) is following the pot as I turn it. Seems to be arranged in unity gain configuration (buffer), so what goes in is what I get out more or less. What pin on u513 does u503 connect to, 5?
So I discovered a 1k resistor right near pin 1 of U513 that measured in megohms, but after I reflowed the solder it measured OK so I left it in. After that I thought I should try and look at the gate drive on the outputs to see if anything had changed, and low and behold the offset pot was now controlling duty cycle. So what’s the next step, remove the jumpers and see what it does?
Rectifier. If I had a nickel for every time I've done that...I generally catch it but was in a rush.
You can leave the resistor, or not. Some amps need them, some don't. I don't think you need the jumpers if you install the outputs.
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