Hi dmx2020,
Q513 is a constant current source, don't worry. Replace those parts and bring it up slowly on a variac. If you aren't drawing current at 20VAC in, stop and measure DC offsets and bias currents. Compare the two channels. Do the same at 40, 60 and 80 VAC in. If things are running okay, increase the AC supply to a normal line voltage. Watch the bias current, no load through this. The relay clicking in will tell you the offset is within a few volts worse case. If the relay kicks out, measure the offset after confirming the bias current is close to the other channel. You may need to adjust this.
-Chris
Q513 is a constant current source, don't worry. Replace those parts and bring it up slowly on a variac. If you aren't drawing current at 20VAC in, stop and measure DC offsets and bias currents. Compare the two channels. Do the same at 40, 60 and 80 VAC in. If things are running okay, increase the AC supply to a normal line voltage. Watch the bias current, no load through this. The relay clicking in will tell you the offset is within a few volts worse case. If the relay kicks out, measure the offset after confirming the bias current is close to the other channel. You may need to adjust this.
-Chris
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