Step 38 and beyond

[craigmorris74]

Having some trouble finishing my build (Rev. 3.3, 990c's). Got the voltages to +-24V at U1 and U2. U3 is +-12V.

MY question is, do I still follow the directions for step 38 or proceed to the directions on the sticky? If I need to to step 38, what should I do with J6 and J8?

I'm asking because my voltages at U2 pins 4 and 6 would start at more than 1V and then fall to about .5 volts. Seems quite a bit off to me based on the directions.

Possible in error, I connected the J6 and J8 jumpers to ADJ, shorted pins 2 and 3 on CONN 2, and tried to adjust the input bias. Now I'm getting something like 15V on the output. I'm afraid I blew one or both of the opamps in this process. Any insight?

EDIT: Tim, went back and followed your instructions via the "Did I smoke my 99oC?" thread, and got 9V between pins 4 and 6 on one of the opamps.

I replaced that part and went back to step 38. Now I have a little over 100 mV between 4 and 6 on U2.

Craig

[tpryan]

The procedure in the sticky essentially replaces step 38. If you have difficulty getting stable voltages, try adjusting bias current compensation for each op-amp separately, starting with U1.

A steady 9V offset probably indicates a problem with the op-amp.

[craigmorris74]

Did the steps in the sticky. Got the voltages within a few mV of one another. When I moved my jumpers back to the normal position the voltage went to about 3 mV on the output of each opamp. Is this acceptable?

Thanks,

Craig

[tpryan]

Is this acceptable?

Yes.

[craigmorris74]

Great advice Tim! It's up and running.