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.
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.
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.
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.
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.
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!
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.
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.
I removed the OP-AMP from the bad channel and I get +2.94Vdc across the speaker terminals.
Used a chip tester and it identifies the OP-AMP as a LM-741 so does that mean I have a good OP-AMP?
Also is it better to troubleshoot without the OP-AMP in circuit or should I put it back in?
Next I'll try replacing Q609. I don't have a KSA1381. Are there any others that will be good enough for at least a temporary sub?
EDIT:
I can get a 2N5416 which has the same voltage rating as the KSA1381. May try that, however the Ft is only rated at 15MHz whereas the original is 150MHz.
Used a chip tester and it identifies the OP-AMP as a LM-741 so does that mean I have a good OP-AMP?
Also is it better to troubleshoot without the OP-AMP in circuit or should I put it back in?
Next I'll try replacing Q609. I don't have a KSA1381. Are there any others that will be good enough for at least a temporary sub?
EDIT:
I can get a 2N5416 which has the same voltage rating as the KSA1381. May try that, however the Ft is only rated at 15MHz whereas the original is 150MHz.
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The voltage could have been negative, however as I forgot to note which DMM lead was connected to what speaker terminal.
About to fire the amp up with the 2N5416.
EDIT:
That's not it as the voltage is the same as it was with the stock Q609.
Is it ok to leave the 2N5416 in or will it cause issues due to the lower Ft?
EDIT:
I removed R605 and made sure the leads were connected properly + to + speaker terminal - to ground. I got -2.796Vdc so the problem is elsewheres and I suspect whatever it is also could be the issue in the better channel unless the other channel just has a bad Q610.
About to fire the amp up with the 2N5416.
EDIT:
That's not it as the voltage is the same as it was with the stock Q609.
Is it ok to leave the 2N5416 in or will it cause issues due to the lower Ft?
EDIT:
I removed R605 and made sure the leads were connected properly + to + speaker terminal - to ground. I got -2.796Vdc so the problem is elsewheres and I suspect whatever it is also could be the issue in the better channel unless the other channel just has a bad Q610.
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With R605 removed, -2.796V may be reasonable. The gain of the power section is about +23V/V. Refferred-to-input, that's about -120mV. This design relies upon an initial negative offset, as the servo opamp generates only positive output. Plug in the new opamp and see what you get!
P.S. I estimate the servo would need to deliver about +2V to null the PA output.
P.P.S. Opamp may still be in question. With opamp removed, you should observe negative PA output. Tack R605 to +12V; you should then get several volts of positive PA offset. If so, a good opamp may resolve.
P.S. I estimate the servo would need to deliver about +2V to null the PA output.
P.P.S. Opamp may still be in question. With opamp removed, you should observe negative PA output. Tack R605 to +12V; you should then get several volts of positive PA offset. If so, a good opamp may resolve.
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The OP-AMP output pin 6 is at +11.46Vdc.
That feeds through the 1.5 meg resistor R605 which feeds R607 a 100k resistor which has one side grounded. Calculating things, I should be getting +716mVdc on Q601 B, but I measured -1.822Vdc that is if I measured it right initially.
Will retake the measurement on Q610 B to see what it is.
That feeds through the 1.5 meg resistor R605 which feeds R607 a 100k resistor which has one side grounded. Calculating things, I should be getting +716mVdc on Q601 B, but I measured -1.822Vdc that is if I measured it right initially.
Will retake the measurement on Q610 B to see what it is.
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