Adcom GFA-535II problem

Here's an easy check of ADCOM 3A/ LT1006 health:

The opamp input bias current is about 10nA (25nA max), PNP input stage, so bias current will pull the 4.7M bias resistor (eg. R689) positive to about +47mV, but you can't measure easily with a DVM because of the meter's ~10M input resistance. But short pin 6 to pin 2 to form a unity gain follower. The output will track the input bias and you'll see about +47mV.

This tip is especially easy on dual opamp mini-dip when output and inverting pins are adjacent: you can angle your probe to bridge the two pins and form the follower connection, and the meter displays the voltage on the +input. :)
 
  • Like
Reactions: 1 user
The schematic is pdf page 16, from the file in post 4.

The tests I described above are done in situ so easy to preform--- you don't have to remove any components, just tack in a jumper. Of course, being able to apply this technique is very situation dependent, but this circuit is especially convenient.

During troubleshooting of this Adcom amp, I anticipate it may be desirable to force the opamp the amp to the ground rail, and also to its 12V rail. Easy to do, I think. To get nearly 0V, tie output to inverting input, as already described. To get near +12V, ground the inverting input; since +input is about +47mV, the opamp will be driven to the positive rail.
 
  • Like
Reactions: 1 user
but you can't measure easily with a DVM because of the meter's ~10M input resistance.

I noticed with the bad channel the voltage on pins 2 and 3 remained stable, but on the better channel when I put the meter lead on 2 and then on 3, I saw the voltage constantly change.

Could that indicate that the OP-AMP in the bad channel is bad?

That said the Fluke 8840A meter I used to measure the voltage has the following specs.

So if I'm reading that right, it would have minimal loading.

11.png
 
Agreed, 10G ohm is negligible load, so I tend to believe your measurements. But you didn't say what voltage you observed, or if you measured between pin 2 and 3. Assuming your readings are consistent with the readings in post 3, the opamp sure looks suspicious.

The "pin 6 to pin 2 test" I mention in post 21 is a easy test to confirm failure.

Good luck!
 
Wow! Those are big offsets.

I suppose the opamp may be providing some benefit, as the better output was at least in the linear range. You should examine the 12V supply rail--- maybe bypassing is inadequate, leading the opamp to oscillate. Integrator usage does give full feedback to the opamp, so less forgiving of bypassing. Long shot, though.