Hello all, I hope I'm in the right place of the internet

Here's my issue: I've purchased an old Technics SU-700 amplifier to match my equally old JBL speakers. All works fine except that the Safety function kicks in past a certain level, shutting down the sound until power is re-established. The manual tells this is supposed to happen in case of a short with the positive and negative speaker wires, but that's not my case. In fact, the same issue happens even without the speakers being connected. I guess we can establish something is happening in the box, so I've opened it but saw nothing apparently damaged upon first look. The one fuse I've spotted near the power plug is healthy. Dust is present but fairly minimal, considering the age. No signs of prior tweaking.

The level past which the Safety function engages is around 3 o'clock on the Volume knob, with music playing from the headphones mini-jack out of a tablet (at 100% volume), connected to the Aux/CD RCA input. I've tried different combinations of level settings between the tablet and the amplifier, and no matter what, past a certain 'master' loudness at the end of the chain, the Safety shuts down everything. I've also tried with the Tape input, which according to the manual have the same specs as the Aux/CD input — same thing. Just to clarify, this threshold level is nothing extreme, or I wouldn't consider it a problem.

Funnily, just about before shutting down, I can hear something in the circuitboard resonating/buzzing mutually with the signal, in a way that makes the music intelligible!

I hope someone could point me in the right direction. I'm equipped with a tester and able to perform soldering. Many thanks for any suggestions!

 

Without a schematic it is very hard to guess. Generally speaking, output protection trips at a current level, voltage level, temperature level, or some weighted combination of everything. Nice job trying different combinations of things to narrow the suspects. This type of issue is almost always something in the power amp stages, and you pretty much confirmed this. Your power amps are so highly integrated that component-level maintenance will be difficult.

Does this happen when the system is cold, or only after the amps have been working for a while? The large fin assembly is a heat pipe. The copper tube has a fluid in it that moves heat from the two power amp devices out to the array of fins. First time I've seen one in a commercial power amp, but my days of climbing around in this stuff are mostly behind me. Anyway, If the amp runs at full blast when cold (first 30 seconds), then the protection kicks in, it could be that the heat pipe assembly no longer is tightly coupled to the power amp hybrids, and their internal temperature sensors are signalling an alarm.

ak

 

Hey AnalogKid, thanks for the message!
Edit: to answer your question, the problem happened within seconds from switching on the amp.
Good idea to check the dissipation. I just finished running a test: I diy cooled the black plates with an iced thermal brick (wrapped to prevent humidity) and run the amp as usual with the whole dissipation contraption slightly cooler than room temperature to the touch. Safety punctually kicked in as soon as I went past 3 on the volume knob. At that point the heat was barely perceived, even the copper tube.
I understand that this test doesn't exclude that the suspected overheating happens at earlier stages, namely on the circuit board, so I'll now proceed and disassemble to look into that.
Thanks again!

 

Even highly integrated hybrid power amp devices take more than a few seconds to overheat, so this looks more like a circuit component failure or value shift.

ak