[Fred[_12_]]

"Zack" wrote in message ...
Fred wrote:

If replacing these caps doesn't eliminate the hum, or if all this sounds
way over your head, it's either off to a warranty station with the unit,
or off to the
electronics store in search of a replacement. I would expect repair to
cost
between $100 & $200. This is a pretty old unit; the service manual was
printed
on paper. These days everything is in PDF on the computer.

Fred,
if you think that is old, I have an AX-630 which drops one or both channels
intermittently. During the dropout, the audio is low and distorted with
mostly just buzzing. Increasing the volume sometimes breaks through the
distortion and the problem goes away for a time. The unit has an AST pack,
so replacing just the amp (which otherwise sounds great when it is working)
would require new speakers.

Any ideas what might be wrong with it? Reseating the wires external and
internal does not seem to result in a fix. Mostly happens while using the
tuner, a TX-400U - which had a bad solder joint on the output that was
fixed a few years ago.


Hey Zack,

You say it mostly happens while using the tuner. I assume that's because
you mostly listen to the radio. But if it also happens when listening to other
sources like CD, the problem has to be in the amp. So here's a plan:

First thing to do is attack the switches & controls with Caig De-Oxit D5.
Push button switches often require removal of the front panel so you can
spray de-oxit into the switch where the shaft from the button enters the
switch. Spray and then cycle the switch 8 or 10 times.

Pots only need de-oxit if they scratch or cut out when rotated from limit to
limit. If they're doing that, they need to have the de-oxit sprayed in where
the terminals come out. It often helps to put a 90 degree bend in the last
inch or two of the spray tube so you can aim the spray up or down and into
the pot where it's connected to the pc board. After spraying the pot, turn it
back and forth, limit to limit, at least a half dozen times. Sometimes you
have to pull the pc board out of the unit to get access to the pots. ;-(

De-oxit is powerful stuff. You don't have to flood the area for it to work - a
quick blast will do ya if it's aimed right. Given that both channels are cutting
out individually, it's about a 98% chance the de-oxit is going to solve your
problem if you do it right.

But if that doesn't do it, then it's a matter of finding the bad connection by
visual inspection, mechanical shock (a tap here, a tap there), thermal shock
(freeze spray, heat gun), and/or good old fashioned signal tracing with a
scope or signal tracer while the unit's misbehaving. Comparing the same
point in the bad channel with the working channel will tell you if what you're
seeing or hearing is OK or not.

Keep in mind it's possible the bad connection is inside a transistor or IC -
freeze spray is good for finding that kind of problem.

It really helps to have a schematic so you can see where the signal goes
next if de-oxit doesn't fix it. Given its age, you can probably find the service
manual for your amp on the internet in pdf form, possibly for a few $ from
one of the many service manual purveyors out there these days. Google is
your friend.

Good luck,

Fred