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Old 25th July 2012, 04:18 AM   #1
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Default Diagnosis please

I Have an older valve amplifier. I have replaced all components and valves. Recently one channel will be loud and distorted whilst the other is thin and weak. Always the same channel.
Plugging and unplugging the RCA leads will induce the problem and, after a while, correct it. removing and replacing the input and PI valves will also correct the problem. Its not the speakers nor i believe the OT. It "feels" like it is something to do with the input earth's.
I have on oscilloscope on its way but in the mean time, this is driving me insane.
Help
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Old 25th July 2012, 07:35 AM   #2
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Sounds like there is somewhere a DC path that needs time to charge/dischare/bleed off after plugging in a source.
Do you have coupling caps on the inputs?
Did you check power supply voltages - do they vary with the 'event'?
Can you fix it permanently by replacing those tubes?

jan
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Old 25th July 2012, 08:57 AM   #3
DF96 is offline DF96  England
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Could be RF instability, caused by components no longer being in exactly the same place as they were. Try grid stoppers?
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Old 26th July 2012, 08:00 AM   #4
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Hate to labour u with this but
Listening for half an hour, one channel very suddenly very loud ( my son said what the ...) the other very weak. Unplug some leads, swear, put finger on signal wire, loud buzzing, plug, unplug, swear.....
And it's back to normal.
What gives
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Old 26th July 2012, 08:35 AM   #5
chrish is offline chrish  Australia
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Hmmm, top of my head - maybe grid leak resistor faulty?
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Old 26th July 2012, 08:35 AM   #6
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Going very loud sounds like it should be a clue. Failure in feedback circuit?
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Old 26th July 2012, 08:38 AM   #7
DF96 is offline DF96  England
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Why not tell us what it is, and maybe even put up a circuit diagram? We are clever, but we can't do miracles by remotely diagnosing a fault on an unknown unit with no test measurements.
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Old 26th July 2012, 11:53 AM   #8
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http://www.oocities.org/stereo20/dwnloads/stereo20.gif
Hope this helps. my CRO was delivered today, hope this will help me to find the problem, I dont mind spending some money if it helps find the problem. it is ******* me off
Grid leak resistors replaced(my second thought) its something like that.
Take one of the output valves from the loud side (when IT happens) and it makes no difference,
feed back circuit also came to mind
If i new what it was I wouldnt take up your time, I am baffled
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Old 26th July 2012, 12:58 PM   #9
MelB is offline MelB  Canada
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If it is not the feedback loop then check R9??
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Old 26th July 2012, 01:01 PM   #10
DF96 is offline DF96  England
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OK, Leak Stereo 20. Two suggestions:
1. gently tap with an insulator (piece of wood?) around the circuit while it is working to see if you can make it happen - the fault may be near where you were tapping.
2. monitor the voltage across C12; it will vary a little as your mains varies. See what happens to this voltage when the fault occurs.

Look out for other symptoms: red valves, hum, whistles etc. The PSU and grounding are the only connections between channels so these is the only ways one channel can affect the other. Could the fault be in the source or preamp?

Last edited by DF96; 26th July 2012 at 01:01 PM. Reason: typo
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