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VFET amp Bourns trimmer exchange
VFET amp Bourns trimmer exchange
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Old 15th June 2021, 08:15 PM   #41
Zen Mod is offline Zen Mod  Serbia
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VFET amp Bourns trimmer exchange
process is simple - swap VFets between channels, re-set drain voltage

if problem moves, following now suspicious VFet, you know the culprit
if problem stays, change opto and all thingies on problematic pcb

you'll solve it , one way or another, only working on amp, not here in thread

If I know that it would help, I'll write (it'll be gone in 36hrs"

but I don't know that

so - just dismantle it .......
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Old 15th June 2021, 08:25 PM   #42
MEPER is offline MEPER  Denmark
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Swapping VFETs is a major task. Opto and or zeners a minor task.
Think I will need a de-solder station to swap VFETs to be sure not to damage them (could be a good excuse to buy a de-solder station.....but which?).

But these spikes I see......if I measure on VFET gate.....and spikes are still there.....or not there.....could that make a conclusion? .....or is there another "secret" place to measure to be able to conclude something? .....measurements so far are all on speaker out.
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Old 15th June 2021, 09:34 PM   #43
ClaudeG is offline ClaudeG
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Prob that comes and goes... hmm, hot weather currently, could it be temp related?

Thinking of an average connection, or not perfect solder joint that seems to ensure continuity "most of the time" but not really properly and when conditions change the noise comes and goes? Jumps are quite odd otherwise...

Just thinking out loud, having had a similar intermittent small failure, in from of random drops "sometimes", on a DVD player last week... your case could be of course very different

Good luck either way

Claude
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Old 15th June 2021, 10:14 PM   #44
MEPER is offline MEPER  Denmark
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I have ruled out bad solder joints. It seems not to be sensitive to mechanical knocks. What I don't understand is the quite low frequency content of the noise. The largest spikes can be +-8 to -+ 10 mV pp. There can be periods of several seconds where there is silence. Then some seconds with many spikes.

I can get some liquid nitrogene and cool VFET down to -196C and see what happens.....and give "him" a shock..... :-) .....it may reveal the bad component....something I would never do with tubes......
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Old 16th June 2021, 05:52 AM   #45
MEPER is offline MEPER  Denmark
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Next step is to trace the "funny" noise signals inside the amp.
First to be 110% sure it is not from FE-board and then trace it forward in signal path.....
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Old 16th June 2021, 06:54 AM   #46
Zen Mod is offline Zen Mod  Serbia
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did you tried crossing signal routes between FE and OS boards?

as far as I remember, I wrote that once
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Old 16th June 2021, 09:33 AM   #47
MEPER is offline MEPER  Denmark
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No, I did not try that but I remember I did consider to be brutal and short the Edcor output. But I could not "hear" any noise at the FE-output / VFET board input when using my special amplifying device (battery driven amp with built-in speaker). After this I concluded that it was not the FE-board. But scope will show what happens at a more "technical" level......and it will hopeful add something to my "education".....
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Old 16th June 2021, 10:18 AM   #48
Dennis Hui is offline Dennis Hui  Canada
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VFET amp Bourns trimmer exchange
Too bad the noise is back. Hopefully scoping at various points in the circuit will help you find the problem.
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Old 16th June 2021, 06:05 PM   #49
MEPER is offline MEPER  Denmark
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I have performed some additional scope measurements inside the amp.
I found that -D input (output from FE-board) is "clean" but both sides of R4 (2SJ28 gate) is noisy (same type of "spikes" as on speaker out).

Then C2+ side of R7 is noisy but other side of R7 is "clean".

Probably difficult to conclude much......other than maybe VFET is ok?
If gate change then ok that output change......the question is who comes first?
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Old 16th June 2021, 06:29 PM   #50
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VFET amp Bourns trimmer exchange
I'm still saying that only way is changing VFets between channels, or to change everything on pcb but VFet

49 posts, and you practically changed just trimpot
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