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Humming/distored phono channel on vintage Kenwood amp?

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Anonymous
April 19, 2004 4:08:41 AM

Archived from groups: rec.audio.tech (More info?)

I recently picked up a vintage Kenwood KA-5002 integrated amp. It's from about
1970, all transistor but built like a tube set (open metal chassis, few PCBs,
lots of wiring and bare parts), lots of unusual settings and inputs/outputs,
very high quality and sounds great, but...

The right channel of both phono inputs (it has two) has a strange distortion.
High frequencies sound thin and warbly. Any loud bass notes trigger what I can
only describe as a cross bewteen a less-than-60cps-hum and "farty" distortion.
It's not pure distortion like you get with something being overdriven--there's
a hum component.

It's open and easy to work so I can fix whatever the problem is, but my
knowlege of preamp theory is lacking.

Thanks

GTO(John)
Anonymous
April 22, 2004 10:40:50 AM

Archived from groups: rec.audio.tech (More info?)

In article <20040418200841.10395.00000023@mb-m16.aol.com>, gto69ra4
@aol.com says...
> I recently picked up a vintage Kenwood KA-5002 integrated amp. It's from about
> 1970, all transistor but built like a tube set (open metal chassis, few PCBs,
> lots of wiring and bare parts), lots of unusual settings and inputs/outputs,
> very high quality and sounds great, but...
>
> The right channel of both phono inputs (it has two) has a strange distortion.
> High frequencies sound thin and warbly. Any loud bass notes trigger what I can
> only describe as a cross bewteen a less-than-60cps-hum and "farty" distortion.
> It's not pure distortion like you get with something being overdriven--there's
> a hum component.
>
> It's open and easy to work so I can fix whatever the problem is, but my
> knowlege of preamp theory is lacking.
>
> Thanks
>
> GTO(John)
>
John,
You will need a good DVM at the very least, a scope & a signal generator
would be a big plus. The good news is that you have one good phono
channel to use for comparison. Here's how I would proceed:
First thing I would do is clean everything and do a careful visual
inspection. Look for loose connections, cracked solder joints, cracked
traces, broken components, bits of conductive material bridging traces,
etc. Chances are good you will find the problem this way. It's possible
that an electrolytic cap has gone bad. These things can leak electrolyte
when they get old. It should be visible. Many early transistor amps used
electrolytic caps in the signal path (yuck). Get a schematic. If you
can't find one, trace the schematic from the PCB. Measure the collector
voltage on each transistor in the good channel. Then do the same for the
bad channel. When you find a significant difference, you're close to the
problem. It sounds to me from your description that the preamp could be
oscillating. This could be caused by a bad ground, contamination on the
PCB or a component that has changed value (aging or damaged). Poking
around with a scope will confirm whether the preamp is oscillating. If
the preamp has feedback (and I'm 99% sure it does), the problem could be
anywhere inside the feedback loop and you'd see the effects everywhere.
I could be of more help if I had the schematic to look at.

HTH,

-- CB
Anonymous
April 22, 2004 10:40:51 AM

Archived from groups: rec.audio.tech (More info?)

They xtill do.

mz


>Many early transistor amps used
>electrolytic caps in the signal path (yuck).

--
Please reply only to Group. I regret this is necessary. Viruses and spam
have rendered my regular e-mail address useless.


"Chuck Bones" <cb@fever.in.the.funkhouse> wrote in message
news:MPG.1af108c44585ca16989682@news.west.earthlink.net...
> In article <20040418200841.10395.00000023@mb-m16.aol.com>, gto69ra4
> @aol.com says...
> > I recently picked up a vintage Kenwood KA-5002 integrated amp. It's from
about
> > 1970, all transistor but built like a tube set (open metal chassis, few
PCBs,
> > lots of wiring and bare parts), lots of unusual settings and
inputs/outputs,

<snip>
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April 23, 2004 12:52:41 AM

Archived from groups: rec.audio.tech (More info?)

gto69ra4@aol.com (GTO69RA4) wrote in message news:<20040418200841.10395.00000023@mb-m16.aol.com>...
> I recently picked up a vintage Kenwood KA-5002 integrated amp. It's from about
> 1970, all transistor but built like a tube set (open metal chassis, few PCBs,
> lots of wiring and bare parts), lots of unusual settings and inputs/outputs,
> very high quality and sounds great, but...
>
> The right channel of both phono inputs (it has two) has a strange distortion.
> High frequencies sound thin and warbly. Any loud bass notes trigger what I can
> only describe as a cross bewteen a less-than-60cps-hum and "farty" distortion.
> It's not pure distortion like you get with something being overdriven--there's
> a hum component.
>
> It's open and easy to work so I can fix whatever the problem is, but my
> knowlege of preamp theory is lacking.
>
> Thanks
>
> GTO(John)


Was this thing designed to let you record onto tape one phonograph
record on one turntable while listening to a second turntable at the
same time? Is it possible that there is only one stereo phone pre-amp
and that its input is switched between the two sets of RCA jacks on
the back? On something built back then, especially the Japanese
brands, especially the ones intended for their domestic market and not
export to the U.S. but bought there and brought back to the US anyway
by miltary personnel or dependents, all sorts of what we would now
consider unusual features are very possible.
Anonymous
April 24, 2004 1:27:52 PM

Archived from groups: rec.audio.tech (More info?)

"Mark D. Zacharias" wrote:

[fixing topposting]

> >Many early transistor amps used
> >electrolytic caps in the signal path (yuck).

> They xtill do.

Yes, but it is a different issue with a single rail supply and half the
PSU voltage as bias across the cap. It is when they are used unbiased
they distort. To use electrolytics with a split supply design if
required, and it can so be for reasons of real estate or cost or both,
all the designer has to do is to is to use a series pair and supply the
proper side of the supply voltage to their center. It will at the very
least minimize the issues by removing the known theoretical mechanism of
distortion in an electrolytic cap.

> mz


Kind regards

Peter Larsen

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