68C diode from 1963 [Repair]

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From: N_Cook on 27 Jan 2010 03:43

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...

>
> As a teenager in the '70's I used to like these models, (the VN-900 and
> VN-700) but now as a technician I find they are really kind of junky as
> regards the amp section and overall quality, compared to the Pioneer,
> Kenwood, Marantz etc of the time.
> Seems like they put a little too much of the manufacturing cost into the
SEA
> tone section.
> Also, I find one can clean the controls until you are blue in the face,
yet
> within a couple months they are acting up again. I seem to have much
better
> luck on this with other brands.
> 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.
>

Placed switch pivot in a plastic bag and glue passed 1lb load on end of pin.
Next the vol control, quad ganged pot with bass comp CTs, not many of them
around no doubt. To disassemble requires grinding off the swaging on the end
of the shaft, presumably drill + maybe tap for screw and pad/washer to
refit.
A grey small electro on the main board has leaked at some time

SEA (punningly in blue print on front) stands for Sound Effects Amplifier ,
from the user manual, me thinking Selective Equalisation Adjustment

From: N_Cook on 27 Jan 2010 10:20

so far not broken an age-hardened hookup wire.
Easier to work on than a lot of 70s stuff, can remove sw and pots etc with
room to manoeuvre, wired in place. And the edge-connectored multiple cards
makes it a lot easier, presumably can swap between channels for diagnostic
purposes if required.

From: N_Cook on 28 Jan 2010 03:40

From: Mark Zacharias on 28 Jan 2010 06:46

"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.

From: N_Cook on 28 Jan 2010 07:54

Mark Zacharias <mark_zacharias(a)sbclobal.net> wrote in message
news:01a9f783$0$6837$c3e8da3(a)news.astraweb.com...
> "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.
>

I put some manky oily residue (not all over) down to a leaky nearby cap ,
replaced , but tested ok.
Perhaps a breakdown product of that dark phenolic 60s type pcb, like "dolls
disease" in soft plastics. I'm wondering if the SEA board , populated with
70s Si Tr and Rs &cs ,along with the main caps, is a hang over from earlier
60s model.

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