One stage of a preamp I'm working on is oscillating. Anyone know if the type of oscillation shown below indicates what could be causing it? It's starting in an amplification stage with the volume control. Touching the volume control makes it worse. Signal is 1kHz @ 1.5v, 1st pic not touching vol control, 2nd touching. third is touching at .2 uSec on scope. Have replaced all caps and volume pot. All trans in the preamp are biased ok. Unit is a Pilot integrated receiver. Preamp output actually looks ok but I'd like to get rid of the oscillation before driving the amp board with it.
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Ok, here's a photo of the preamp stage. The T205/105 trans are the ones oscillating(only 205 channel is in the photo.) The vol pot acts like an antenna when touched. Swapped out one trans and it also oscillates. Touching other pots and switches does nothing -balance, treble, bass, loudness.
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Well, I hate to **** with things that were working ( unless this was a design fault) but you could try a small cap (a few 10s of pF) between the base and collector of the oscillating transistors. But, before doing that, can you say what you changed before that besides the caps and pots - eg did you replace any of the trannies?
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Ok, will try the small cap fix. Yes, well a bit of history with this problem: Purchased the receiver for a few $ without turning it on. Seller said it only needed the volume pot cleaned. One channel was actually dead. Sound from the good one was excellent so decided to attempt a fix. Checked the supplies -ok. Pulled the amp board and put a signal on it -ok. Signal getting into preamp but disappears after second stage. Found two wires fused open on the Tape Monitor Switch. Reconnected them, signal now passed by preamp but is glitchy/clipping when turning the vol pot quickly and T105/205 oscillate. Re-capped the preamp and clipping gone, oscillation still there. I'd guess someone connected a power amp to the tape input, can't see a line spike fusing open those wires. One trans stage before the monitor sw doesn't show any oscillation so I assume the high current didn't come from there. Yes, I replaced T105 and it made no difference.
Well, that was interesting. Just jumped the output of T204 to input of T206. Oscillation was feeding back from the base of 204 into 206. So, pulled R239 from 206's collector to see if the osc. was further down. And: pffft, powered up and threw a 110v breaker. Reconnected R239 and everything's as before.
Checked oscillation with vol down and it still oscillates. On the scope it looks like 100kHz modulated by 60Hz sitting on top of the 1kHz injected signal.
Checked oscillation with vol down and it still oscillates. On the scope it looks like 100kHz modulated by 60Hz sitting on top of the 1kHz injected signal.
Obviously and eventhough you have findings on what is wrong your method seems to be wrong .
There is no chance that you touch the pot and that will effect the sound ...That means that from some point a ground is missing or signal is leaking from any place of the amplifier to ground / to another circuit / to the potentiometer itself .
so for starts check to see if any signal from the faulty ch is leaking any ware then check tape monitor switches or generally focus on hardware issues .
one option will be to isolate the circuit input to output and feed signal directly to input while observing the output ..that will be easy to do
@ Bonsai
never expected an answer like that from such a guy like you !! Never treat electronics with cortisone there is no point on trying to suppress the problem cause it will never be self cured ...You need to find out whats is going on!
Kindest regards
Sakis
There is no chance that you touch the pot and that will effect the sound ...That means that from some point a ground is missing or signal is leaking from any place of the amplifier to ground / to another circuit / to the potentiometer itself .
so for starts check to see if any signal from the faulty ch is leaking any ware then check tape monitor switches or generally focus on hardware issues .
one option will be to isolate the circuit input to output and feed signal directly to input while observing the output ..that will be easy to do
@ Bonsai
never expected an answer like that from such a guy like you !! Never treat electronics with cortisone there is no point on trying to suppress the problem cause it will never be self cured ...You need to find out whats is going on!
Kindest regards
Sakis
Yes thanks, posted here to find a better method... I just jumped the tape monitor sw out of the signal path: jumped 1st stage to last and found oscillation on emitter of 1st stage. Then found that the schematic calls for a .0047uf in series with 3.9k from emitter to gnd and they are not on the board. They also are not on the board layout that came with the schematic. Bonsai, you mentioned earlier that it may be the design. You could be right, it may work with the oscillation. I'll connect everything up and see. But I agree with Sakis that there is no way touching the vol pot should change the signal. I'll check the switches again. Thanks.
Mooly ??? how about the other ch working correctly ? guys come on !!!!
that's the fun of it ...chasing the ghost !
since i am trying to help you can i threadjack for a second ?
Posted this ghost and got no answer ... help guys !!!
http://www.diyaudio.com/forums/solid-state/240532-rotel-ra985-bx-problems.html
that's the fun of it ...chasing the ghost !
since i am trying to help you can i threadjack for a second ?
Posted this ghost and got no answer ... help guys !!!
http://www.diyaudio.com/forums/solid-state/240532-rotel-ra985-bx-problems.html
Old timers in the 70's when they had a relatively big amplifier to repair and of course never had the luxury of expensive instruments they looked for oscillation with a trick
cleaned the bench from anything that might produce noise like transformers ( smps didnt exist anyway at the time ) , fluo lamps and things like then switched on an AM transistor radio over the amplifier and try to actually listen or tune to oscillation if any existed ...
neat eee ?
cleaned the bench from anything that might produce noise like transformers ( smps didnt exist anyway at the time ) , fluo lamps and things like then switched on an AM transistor radio over the amplifier and try to actually listen or tune to oscillation if any existed ...
neat eee ?
Sakis, those darned old timers new a thing or two! Turned off all the lights and no oscillation. I'd just bought this magnifying glass lamp with a florescent bulb. Ha, that was it. Below are pics of the oscillating stage with the light on and off. That stage is still sensitive to touching the vol pot. The fourth pic is with light off and touching the vol. You can see the signal gets dispersed with some junk on it but by the last stage of the preamp it's clean. I'll go ahead and hook everything up and try some music with it. And there's a pic of the darned light.
Thanks everyone for the help.
Thanks everyone for the help.
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fine with me ... still you need to trace down what causes the effect of touching the pot and that disturbs the wave form and then again you need to also find out why this is happening to one ch only
you are not done yet you are only half the way there ...I still think you have some hardware issue and to my understanding this is related to ground
Kind regards
sakis
you are not done yet you are only half the way there ...I still think you have some hardware issue and to my understanding this is related to ground
Kind regards
sakis
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