Technics SU-V2 Protection Relay Question

Picked this up over the weekend for $20 from GW. Immediately noticed an issue with the main speaker left channel connection: left speaker would play then cut out until the positive binding post was wiggled, at which point it would play again. Taking the lazy way out I moved the cable to the remote speaker connection, checked the DC offset (low numbers) hooked the amp up to a pair of EPI 100s and let it run for about an hour.

Pulled the top cover to have a look at the speaker connections. I fiddled a little with the left positive post (pretty much just more wiggling) and got called away to dinner. Left the amp running for another hour or so then shut it down for the night. Went to turn it on yesterday and ... nada. I noticed that the relay that had been clicking on after a couple of seconds no longer was.

Important disclaimer; Repairs of audio equipment is new to me. I'm investigating whether the move from rebuilding and restoring automobiles can be replaced with audio gear. I have quite a lot of experience with the former, I'm a tyro with the latter. So, not certain where to begin I studied the schematic (lots of Greek there to me at this point) and grabbed the multimeter. Truth is, I didn't make any sort of determination but I did get to poke around a bit. Although I didn't figure the relay itself to be the problem I desoldered it anyway and applied 12vdc to the contacts. As expected, it activated properly so I soldered it back in.

I did notice that the relay activates when pin no. 6 of IC601 is grounded. If I then turn the amp off I can still cycle the relay for a couple of seconds. I'm guessing this is because there's a capacitor I haven't identified yet that's powering the relay until it's discharged to the point where it no longer can.

Incidentally, I double checked the DC offset with the relay activated. Again, low numbers (didn't write them down but they were all south of 5mv).

Anyway, I'm hoping someone can help this greenhorn solve this (to me, anyway) mystery and get this old amp up and running again.

Disconnect the power pins of one or both of the stk power paks and see if the relay will engage after that. If it does, your amp is 100% beyond repair and you can instantly go on with your life.

I'd say take a careful look at the PCB solder. Some Technics units are known to have bad soldering. (EDITED: not this series. "SU-VZ2" in what I was thinking about, those black units)

Check with a magnifying glass, specially at the speakers connector, the Relay itself, the power supply, and the soldering from the transformer,

Don't by-pass the Relay. It's a protection circuit. Always assume the protection is working because there is a problem somewhere. Once you discard all the options, start suspecting from the protection circuit.

Grounding random parts of the circuit is not a good idea. Don't ask why I know, but you can damage a very expensive and hard to find component doing this. Please ask before doing that.

ZOOM said:

Disconnect the power pins of one or both of the stk power paks and see if the relay will engage after that. If it does, your amp is 100% beyond repair and you can instantly go on with your life.

Click to expand...

That'll be the first thing I try this evening and if it's 100% beyond repair I can instantly go get my money back.