"N_Cook" wrote in message
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Mark Zacharias wrote in message
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"N_Cook" wrote in message
...
Mark Zacharias wrote in message
...
"N_Cook" wrote in message
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My VN-900 has low level noise problems I have so far been unable to
resolve.
Coupling caps, power supply, signal transistors, no luck. Had to set it
aside for some future date...

Mark Z.


As far as intrusive noise.
Anyone know why +74V DC rail to a preamp ? producing low current shot
noise?
Particularly the third transistor in,
250V rating 2SC1103 and associated Rs would be my guess at noise source.
Assuming its not bleed over from the pink noise generator





Definitely not bleed over, the noise varies too much in nature and
intensity. I suspect the internal wiring to the back panel could be an
issue - they used individual (cheap) shielded phono cable and if you look
you'll see the grounds don't tie together at the rear panel (not all of them
anyway).
I am also suspicious of an oily residue on the tone board. Perhaps someone
tried to clean the controls with WD40 or the like and made a mess of it.
It's definitely a preamp issue, as the main amps work fine when divorced
from the preamp section.
I noticed the higher than usual voltages. Not being a old radio guy, the
idea of "noisy resistors" still bothers me a bit (even though I acknowledge
it is a known phenomenon), although if it's due to their resistance
shifting, I can definitely see them causing noise in a given circuit.

Mark Z.