JVC VN 900, 1973 amp

[N_Cook]

followup for someone here with a "static" noisy one.

I got back to this one last week, after rebuilding both power amps
with modern functional replacement components.
Then one channel was obviously noisey. Swapping predriver
boards around, same ch problem. Temp rewiring, crossed over, the
outputs of the preamp board , same channel noisey.
Shorting the base of the final third SEA filter transistor to ground
stopped the noise but the first one was the problem
Replaced X301 and X302 of each channel with leg-twisted BC547

 

[Mark Zacharias]

followup for someone here with a "static" noisy one.

I got back to this one last week, after rebuilding both power amps
with modern functional replacement components.
Then one channel was obviously noisey. Swapping predriver
boards around, same ch problem. Temp rewiring, crossed over, the
outputs of the preamp board , same channel noisey.
Shorting the base of the final third SEA filter transistor to ground
stopped the noise but the first one was the problem
Replaced X301 and X302 of each channel with leg-twisted BC547

Yeah - I had replaced those already. I'm still leaning towards that oily
contamination on the tone board - just haven't gotten back to it yet.

I have noticed with all these old JVC's that they are not tolerant of ground
loops etc, even less than other ordinary amps. They don't seem to work out
well as bench amps or when paired with my computer. Pick up all kinds of
noise, hum, etc. The JVC's in question are the VN-900 (in addition to the
noise problems I've been talking about), a VN-700, and a 4VR-5456 receiver.

Haven't figured them out yet. In a plain system setup, they're fine. But
when I get a mass of wiring involved, which is pretty typical for me, it
always seems to wind up that I can't use the JVC in question. Sticking
another unit in there, a Kenwood or Onkyo etc resolves the problem every
time.

Weirdness.


Mark Z.