mech986 said:
It isn't just the pots that can cause problems. It can be anywhere in the signal path from where the source comes from to the speaker output terminals (and occasionally in the speakers themselves!). Switch contacts and solder joints, even the component solder joints, are all potential sources of poor or intermmittent contact.
So, if your source is external (phono, Tuner, Tape, Aux (CD/MP3/video sound/processor input here), then you must start at:
1. the input RCA jacks, then follow the path - usually to
2. a buffer or amplifier board and their solder joints/contacts/connections
3. then to the input source selector switch contacts/wafer - remember there are 2 channels/sections at least to the switch. Don't forget the tape in/out or tape monitor switches that divert signals too.
4. Then to the tone controls - pots, turnover switches (chooses what frequencies a specific tone control may use), tone controls in/out/bypass switches
5. Sometimes there will be filter switches (Hi, low, rumble, FM MPX, etc.) or processor switches (Dolby FM switch on Sansui 9090B are notorius)
All of the above contact areas are low level (voltage) and low power (current) so corrosion is easy and common and there's not enough punch (voltage or current) to consistently burn through any contact corrosion. Exercise and cleaning of these areas will give better, longer lasting contacts.
6. Then the line preamp stage with any controls (volume), selector switches, etc. to get at. The board connections to the next area (main power amplifier) may also go through a pre-amp out/in switch or back panel jacks.
7. The Power amp has a lot of connections including those that run from the board to the power output transistors/jacks if they are not directly mounted on the board in question. These should be checked and cleaned carefully.
8. Finally, there is the power amp output to speaker selector switches (discrete or rotary) then on to the speaker output connectors.
9. Somewhere in all this there are muting / protection relays as well that should be checked since they act as power switches for the signal.
And we haven't even talked about bad components in the signal path, just the path and physical connections. Assuming everything works ok for awhile usually indicates try cleaning contacts first. However, flaky active devices (tubes or transistors) or the circuits that support/supply them can act funny after they warm up or get full voltage to them. So they can also cause your same problem.
Hope this helps.
Bart
Sorry,
I'm basically thinking out loud about the whole signal path as seen (visualized) on a schematic. If you follow where the signal comes into the receiver from a source, you start to see all kinds of contact (jacks, pins, switches, pots, etc.) and connections (solder joints of components, board to board contacts and wiring, wire wrap for some older Pioneers stuff), and protection relays and circuits.
Anything in the signal path, wire, solder, connections, contacts, active or passive components is a potential source of interruption or intermittent signal. So being paranoid about it, you could clean any or all of it. With Vintage stuff, that is really the best way to deal with the whole unit, can get it back to a very good state with all the signal path, assuming the parts will not fail due to age or drift.
Another good way to deal with this is to divide and conquer troubleshooting. That is, try to isolate it down to a specific input area (tuner, Aux, phono, tape, onboard tuner or outboard source) first. If switching them makes the problem go away, its in one of those.
If switching doesn't change it, then its a common pathway where all the signals converge - namely the line/tone amps and buffers and the power amps and their protection circuits and outputs. You can even get down further if you have a preamp in/out. Send the offending channels out of the pre into another amp, if it persists, then its the preamp/line amp circuits. If it gets better, then the amp is suspect. To confirm, use another preamp to inject signal into the amplifier alone.
Hope this helps to explain what I meant. Sorry for any confusion.
Regards,
Bart
