Right side works, left side not, unless volume is set way high, then horrible staticy low volume. All output transistors removed & checked, all good. Any ideas? Otherwise it's gonna get tossed, or maybe Ebayed for parts.
-- Steve Walker Fusion640@frontierbrain.com (remove brain when replying)
Hmmm... that doesn't sound like a fried-output-transistor sort of behavior.
Given the age and the symptoms, I'd suspect the usual few culprits:
- Dried-out capacitors in the signal path (although one web page I see on this amp suggests that it's a direct-coupled design with no such 'lytics).
- Bad switches, potentiometers, or relays - either dirty, or with worn contacts.
- Blown fuse in the supply to the output or driver transistors.
The amp seems to use some sort of active servo and biasing circuit... if this has failed you might not be getting any signal drive into the output transistors.
Seems to me it'd be worth the usual "inject a signal at the left input, follow it through the circuit, and see where it goes away" diagnostic technique. Would be a shame to bin it, if it's repairable.
--
Dave Platt AE6EO
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I did find a manual with adjustments, & schematics online. Unfortunately my electronics level is not good enough to read a schematic & figure out where the problem could be. I am good at R&R parts, I do have a decent Protek digital VMM. I'll try cleaning & checking the balance & volume pot. I didn't see any bulging caps, & all fuses that I could find were good. Then feed the inputs & try to trace where it goes bad. Thank you.
-- Steve Walker Fusion640@frontierbrain.com (remove brain when replying)
the bad caps are not always bulky ... you need an esr meter to check them really... see my web site below for lots of schematics diy. regards,
-- ---------- Kripton the ESR Repository @ http://kripton2035.free.fr/esr-repository.html the Geiger Repository @ http://kripton2035.free.fr/geiger-repositor.html
Check the tracking of vol and balance pots , compacted grease under a wiper or bent wiper
er
Since the left and right channels are identical, use the working channel to diagnose the failing one. Check for the simple stuff first. Check the signal at the tape out jacks. This should verify that the signal is getting through the source switch. I would verify that the +/- 15 supplies are there for the left side. If you have a signal generator, feed the same signal into botht he left and right and trace both sides. There is an IC inline that may be defective. Check the pins on that IC for signal (either the generator signal or just a CD.)
Update this with any progress you have made.
Dan
Check the emitter resistors and that the speaker relay is working correctly - it could have bad or damaged contacts, even just on the one channel.
For that matter, check for any other possibly burned resistors. Could be loss of voltage to the driver transistors on that channel.
Mark Z.
all
It could be an FET in an interstage portion of the channel that's breaking down. The older amplifiers that used FET's sometimes suffered from this. We used to touch them with a soldering iron and that would temporarily restore operation. Or perhaps an oxidized switch contact. My 1974 Sansui receiver does this and I have to operate the tone cut switch to get it working again. Disassembly is a nightmare and I've been putting it off. It might be something very simple but with all due respect this may be beyond your abilities to find it. I sincerely wish you luck however if as you say you cannot read a schematic then I would hold little hope of your success. Lenny
An earlier poster said what I should have said. As the right channel is working, you have a perfect reference. Find out what's different between the channels, and you should be close to fixing it.
Of course, it helps to have a schematic, so you know where you're poking your probes.
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