Thanks so much @BSST. I've done as you suggested here is the results I got.
I disconnected LK100 & 101, gain pot at zero (actually I soldered a jumper) -
There is no noise from the mixer output, I don't measure any offset at the op amp pin 1 and don't see noise/buzz when I probe that pin.
I probed LK100 and 101 on the preamp side and I can't see any noise/pops on the scope. The DC voltage I measure there is now -12V it was -10V before.
Also I measure 2.9V across R106 & R103.
So then I did as you suggested in the photo.
I added the 1.2k resistor and 10uF cap. I clipped the ground clip to the floating cap wire and then I probed LK100 and 101 on the preamp side. The result was not very conclusive however I did sort of see some more jumping of the trace when I touch LK101 than LK100.
I really would like to ask you about the techniques you suggested, could you please explain what's the purpose of adding that cap and using it while taking measurements. What's it actually doing and why did you suggest adding it on the LK100 side? I couldn't understand the bit you mention about reversing the polarity of the cap to measure the +15V rail also.
Also what's the purpose of adding the 1.2k ohm resistor? I should add I didn't see any difference on the output of the opamp. Is that whats suppose to happen?
Thanks so much again for your help.
I disconnected LK100 & 101, gain pot at zero (actually I soldered a jumper) -
There is no noise from the mixer output, I don't measure any offset at the op amp pin 1 and don't see noise/buzz when I probe that pin.
I probed LK100 and 101 on the preamp side and I can't see any noise/pops on the scope. The DC voltage I measure there is now -12V it was -10V before.
Also I measure 2.9V across R106 & R103.
So then I did as you suggested in the photo.
I added the 1.2k resistor and 10uF cap. I clipped the ground clip to the floating cap wire and then I probed LK100 and 101 on the preamp side. The result was not very conclusive however I did sort of see some more jumping of the trace when I touch LK101 than LK100.
I really would like to ask you about the techniques you suggested, could you please explain what's the purpose of adding that cap and using it while taking measurements. What's it actually doing and why did you suggest adding it on the LK100 side? I couldn't understand the bit you mention about reversing the polarity of the cap to measure the +15V rail also.
Also what's the purpose of adding the 1.2k ohm resistor? I should add I didn't see any difference on the output of the opamp. Is that whats suppose to happen?
Thanks so much again for your help.
I'll try to answer your questions, but the comment below leads me guess you may have installed a short across the gain pot. If so, that's actually equivalent to setting gain to maximum. The gain is minimized when pot resistance is at full 10k resistance. (Reducing gain to 0 would entail opening the path, eg. lifting R104. If the noise problem vanished without R104, problem would mostly likely be a noisy pot.) Try removing the short and setting control to mechanical minimum. Then try looking again at LK100 vs. LK101 terminals with your scope for differing noise behavior.
Please also report actual voltages at V1 and V2.
With the added resistor at the opamp, the opamp output should still be about 0. The gain from the opamp seen from the + input rises t0 (1+6.8k/1.2k) = 6.666. It's being adapted as a signal-tracing amplifier. More details later.
Thanks.
Please also report actual voltages at V1 and V2.
With the added resistor at the opamp, the opamp output should still be about 0. The gain from the opamp seen from the + input rises t0 (1+6.8k/1.2k) = 6.666. It's being adapted as a signal-tracing amplifier. More details later.
Thanks.
Also, please confirm that installed values agree with schematic at R106, R102, and R107. I can't reconcile my calculations with your voltage measurements.
Thanks again.
Thanks again.
Oh yes silly me, I have got the pot out at the moment since I'm installing a new one. I have ruled out a noisy pot when I tested it out with a new pot that was the same value but different type of casing. I have got the correct one now and was getting ready to install it. Shall I repeat the test without the pot installed it would same as removing R104 right?
Its suppose to be +/- 15V. But I'll measure the actual voltages, I haven't got it noted down.
I'll double check these but as far as I can tell they are what the schematic says.
Very much looking forward to learn about the capacitor trick, and also I wanted to ask about how you got the 0.8V at the resistors
Yes, you might as well test with gain pot removed--- fewer suspects in play.
A properly working mixer channel accommodates the biased signals in the input preamps by added them as differential signals, and their equal bias voltages are subtracted and yield about 0V bias at the opamp output.
My debugging approach was to use a signal-tracing amplifier to investigate each of the two input stages independently to localize the defect. So the opamp stage was reconfigured to serve this function. As noted earlier, the patched-in resistor raises opamp gain to about 6.6 but it's DC coupled and we need an AC coupled amp for debugging. Hence an added blocking cap to reject DC. So the added 10uF has to be either non-polar or oriented to accommodate bias on the applied signal--- in this case the negative bias on R103 or R106.
A properly working mixer channel accommodates the biased signals in the input preamps by added them as differential signals, and their equal bias voltages are subtracted and yield about 0V bias at the opamp output.
My debugging approach was to use a signal-tracing amplifier to investigate each of the two input stages independently to localize the defect. So the opamp stage was reconfigured to serve this function. As noted earlier, the patched-in resistor raises opamp gain to about 6.6 but it's DC coupled and we need an AC coupled amp for debugging. Hence an added blocking cap to reject DC. So the added 10uF has to be either non-polar or oriented to accommodate bias on the applied signal--- in this case the negative bias on R103 or R106.
@BSST, I was able to conduct the test as you suggested before I had to give the mixer back.
I have the preamp section and the opamp section disconnected. Had the gain pot out and used the op amp as a signal generator. The result was not conclusive but wanted to run something by you. I was using a small length of wire one end connected to the capacitor, at first I thought I figured the path of the noise as LK101 as it was noiser compared to LK100. Checked it couple of time and the result was same. But since I was holding the wire I sort of got 50Hz buzz type of noise (why is this particular area of the circuit so prone to this kind of noise when touched?). I took a break turned everything off and checked after an hour. This time both LK100 and 101 didn't sound noisy only thing I could hear was the 50hz buzz, so the result is not conclusive. I wanted to troubleshoot further but simply didn't have the time. Hopefully I get this back to investigate further.
I also wanted to ask just for completion, how can the "signal tracer" be used to measure the voltage rails and ground- should I be measuring the output of via DMM?
Thanks for the help
I have the preamp section and the opamp section disconnected. Had the gain pot out and used the op amp as a signal generator. The result was not conclusive but wanted to run something by you. I was using a small length of wire one end connected to the capacitor, at first I thought I figured the path of the noise as LK101 as it was noiser compared to LK100. Checked it couple of time and the result was same. But since I was holding the wire I sort of got 50Hz buzz type of noise (why is this particular area of the circuit so prone to this kind of noise when touched?). I took a break turned everything off and checked after an hour. This time both LK100 and 101 didn't sound noisy only thing I could hear was the 50hz buzz, so the result is not conclusive. I wanted to troubleshoot further but simply didn't have the time. Hopefully I get this back to investigate further.
I also wanted to ask just for completion, how can the "signal tracer" be used to measure the voltage rails and ground- should I be measuring the output of via DMM?
Thanks for the help
When you pinch the probe wire to investigate, your body is acting as an antenna and coupling power line noise into the circuit. Use your ingenuity to attach the probe wire to the end of wood craft stick or similar improv. Then you can probe with the wire without injecting so much interference.
I assume the power supplies are shared amongst the many working channels, so I doubt they are culprits. But in principle you can probe supply points or grounds. Of course, you have to appropriate polarization of the blocking cap and a 1k resistor in series with the cap might be a good precaution against transient charging currents damaging the opamp.
I want to comment a bit about the circuit, so I'll get back to you with more details later.
Best,
I assume the power supplies are shared amongst the many working channels, so I doubt they are culprits. But in principle you can probe supply points or grounds. Of course, you have to appropriate polarization of the blocking cap and a 1k resistor in series with the cap might be a good precaution against transient charging currents damaging the opamp.
I want to comment a bit about the circuit, so I'll get back to you with more details later.
Best,