[Readily Visible]

Dave wrote:
"Readily Visible" wrote in message
...
If I turn the volume up high enough the dropped channel can be heard
faintly and if I turn it up even higher it will kick back in with a
crackle and play okay for a while before dropping out again. To avoid
blasting the volume out of the speakers when doing this I used the
speaker button on the front to disconnect the speakers. This technique
resurrects the channel as well as when I leave the speakers connected.
This seems significant as turning the volume up past a certain point
resurrects the channel whether or not current is flowing through the
circuit. Apparently, the higher voltage applied to the circuit is enough
to do the trick.


The most common affliction older gear like yours gets is oxidized switches
and pots in the signal path. Over time oxidation builds up on the contact
surfaces of switches and pots (and relays), especially switches and pots
which are rarely used (hence the tip about the tape monitor switch(es)).

I understand. But doesn't the fact that the left channel continues to be
the one that drops out when I switch the right channel preamp output to
the left channel main amp input and same for the other two ins and outs
at the back of the unit... doesn't this isolate the problem to the main
amp section? The left channel is out at the moment and operating the
tape monitor switches does nothing to bring it back. But turning up the
volume to just past halfway *always* brings it back.


The oxidation is an insulative layer which resists the flow of current.
When the contacts stop conducting, a voltage differential is set up across
the contacts. By increasing the volume, you are increasing the voltage
differential. When it gets big enough, the signal will arc and blast a tiny
hole through the oxidation and you'll hear sound again... for awhile. If
you'd like to clean it up yourself, it ain't rocket science. For tools all
you'll need is a screwdriver to open up the receiver. You'll also need an
aerosol can of contact cleaner such as Caig DeOxit and an aerosol can of
cleaner/lubricant such as Faderlube (might be called "tuner spray").

I've done that in spades in the past, except for the Faderlube, with no
improvement. And again, I hate to belabor the point, but it seems
certain that the volume pot is not the problem since the problem channel
does not switch from left to right when I switch the left and right
preamp outputs and main amp inputs in the back of the unit so that the
right preamp circuit drives the left main amp circuit and the left
preamp circuit drives the right main amp circuit. If a faulty volume pot
were the problem, the faulty channel would change from left to right
when I do this.

But it doesn't.

You
can buy these, or something like them, at any electronics store including
Radio Shack. Unplug the unit and take off the cover. I haven't done a lot
of work on Pioneers so I'm not sure how accessible the front-panel switches
and knobs are, but you may need to take off the knobs and faceplate...
hopefully not as sometimes these are a real pain to get apart/together.

Not a problem. I've done it already. I have also hot soldered all the
cold soldered joints just to rule that out. Must have been 150-200 of
them, if not more. Got them all.

Use the DeOxit first. Use the thin straw that comes with the can to apply the
cleaner INSIDE each switch and pot (knob)... you need to do ALL of them.
Volume/balance/bass/treble/loudness/high-cut filter/low-cut filter/tape
monitors/speaker selector/tone defeat/turnover frequency switches and any
others you find. Each control should have one or more small holes through
which you can spray the cleaner. Remember, soaking the outside will do no
good, it's gotta get inside the control. After a good squirt, operate each
control 50 times. Don't cheat. You're scrubbing away the oxidation. Allow
the unit to dry out overnight. Now repeat the process with the Faderlube.
Faderlube containes a solvent and a lubricant which coats the innards of the
controls and keeps them from immediately re-oxidizing. After letting it dry
out overnight again, give it a try. If the problem persists, you may try
cleaning the contacts in your speaker protection relay, or just replacing
the relay... they're cheap and available. Most relays of that era have a
plastic removable cover... it may take some fiddling to figure out how it
comes off. The relay will be located near the back of the unit and will
likely be about 1" x 3/4" in size. The relay contains gossamer thin copper
arms with a contact at the ends... they're very easy to bend or break so be
very careful. You can use a fine emery board or a points file to clean up
the contact surfaces. I find that if I put the emery board between the
contacts, I can GENTLY push them closed while pulling the board out.

If none of this works, get a bright light and a magnifying glass and start
looking at solder joints... it's tedious but must be done. Start around the
speaker terminals and pay close attention to any joints which are subjected
to any mechanical forces at all.

If you happen to have an oscilloscope and a schematic for the unit, you can
probably save yourself some time as this appears to be a very reproducible
problem. Apply a sine wave to the input and start probing at the output
working backwards until you find the problem component(s).

I cannot stress enough two things:

1. Switching the right and left preamp outputs with the right and left
main amp inputs does *not* change the problem channel. It remains the
left channel that drops out.

2. The dropout is not always instantaneous. Sometimes, it fades out and
fades in. Sometimes in goes out instantaneously and does not come back.
It occurs to me as I type this that this could very well be two
different problems.

I will pop the top and check the relay. If it proves difficult to open
up the speaker relay, I could easily shunt around it for the left
channel just to rule it in or out before busting in to it.