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From: Chuck on 28 Jan 2010 11:33
On Thu, 28 Jan 2010 05:46:00 -0600, "Mark Zacharias"
<mark_zacharias(a)sbclobal.net> wrote:

>"N_Cook" <diverse(a)tcp.co.uk> wrote in message
>news:hjrieg$vft$1(a)news.eternal-september.org...
>> Mark Zacharias <mark_zacharias(a)sbclobal.net> wrote in message
>> news:003e02f5$0$28396$c3e8da3(a)news.astraweb.com...
>>> "N_Cook" <diverse(a)tcp.co.uk> wrote in message
>>> news:hjmgpk$rdm$2(a)news.eternal-september.org...
>>> > Mark Zacharias <mark_zacharias(a)sbclobal.net> wrote in message
>>> > news:00cec390$0$8066$c3e8da3(a)news.astraweb.com...
>>> >> "N_Cook" <diverse(a)tcp.co.uk> wrote in message
>>> >> news:hjjk1l$64m$1(a)news.eternal-september.org...
>>
>>
>>> 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.


The noisy resistor problem in the preamps of Kenwood receivers of the
early 1970s was fairly common (If I remember correctly, they were
emitter resistors). The company I worked for didn't carry JVC
receivers so I don't know if they had this problem. Chuck
From: isw on 28 Jan 2010 23:38
In article <hjrieg$vft$1(a)news.eternal-september.org>,
"N_Cook" <diverse(a)tcp.co.uk> wrote:

> Mark Zacharias <mark_zacharias(a)sbclobal.net> wrote in message
> news:003e02f5$0$28396$c3e8da3(a)news.astraweb.com...
> > "N_Cook" <diverse(a)tcp.co.uk> wrote in message
> > news:hjmgpk$rdm$2(a)news.eternal-september.org...
> > > Mark Zacharias <mark_zacharias(a)sbclobal.net> wrote in message
> > > news:00cec390$0$8066$c3e8da3(a)news.astraweb.com...
> > >> "N_Cook" <diverse(a)tcp.co.uk> wrote in message
> > >> news:hjjk1l$64m$1(a)news.eternal-september.org...
>
>
> > 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 ?

With bipolar transistors, the way to get low noise is to run low emitter
current. Low emitter current means high Rbe (intrinsic base resistance).
High Rbe means low gain (for gain determination, Rbe in in series with
the external emitter resistance). In that environment, one way to
recover some stage gain is to run a high collector resistor, and that
calls for a high rail voltage.

Isaac
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