Hi-fi stereo amplifier issue

[Vince-Noir-99]

Hello. Before registering to some hi-fi forum I thought I'd try here first.
I found an old Technics SU-700 amplifier to drive an equally old pair of JBLs and am running into this problem: past a certain level, whether achieved via master volume, MP3 player's level, or the two combined, the Safety kicks in and switches off everything. The manual says this is supposed to happen in case there's a short with the speaker wires, but it's not my case. The problem happens even with speakers unplugged completely. I'm sending signal through Aux/CD with an RCA>minijack tested elsewhere without problems. The one fuse I found near the power entrance soldering is healthy. Dust is present but minimal, considering the age. No other first impact anomalies found thus far.
I'm equipped with a multimeter and able to perform soldering, I'm just needing some guidance to know where to look at, before I disassemble everything...
Thanks a bunch for any advice!

[Carly(Poohbear)]

I'm no expert however have you tried sending in the signal via another source and is the result the same?

Have you tried another pair of speakers? (my thinking here is maybe that safety feature is also kicking in when no speaker is connected).

[normen]

Did the JBLs run with that amp before? Maybe they're too low impedance and the amp switches off because of that, what happens if you connect the two speakers in series (should double their impedance). Otherwise check all components that are connected to heat sinks and maybe clean them and apply new heat conducting paste.

[Vince-Noir-99]

Hey, thanks all for the feedback. To answer your questions, the issue happened with or without the speakers connected, with different line-level gear as the source, tested on CD/Aux and Tape inputs (the manual says their specs are identical). JBLs impedance is around 6 and the amp's impedance switch was set accordingly- besides, as I said the safety acts the same regardless of the speakers being plugged in: as soon as the output level goes past ~3 o'clock the amp shuts down.

A few days ago I asked on allaboutcircuits forum and it's been suggested the dissipation system might be faulty. I'm a bit stuck here trying to understand at which exact point the issue is happening though... There's no strange overheating but if something's wrong, it must surely be happening before the actual cooling plates even get hot (thanks to the safety).

Here's the experts assessment:

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.

[normen]

There's no strange overheating but if something's wrong, it must surely be happening before the actual cooling plates even get hot (thanks to the safety).

Well the point of the cooling elements is to get hot - instead of the chip getting hot. If the cooling elements don't get hot that doesn't mean the chip doesn't get hot. That said none of the components before the actual power amp should really get too hot anyway..

[Vince-Noir-99]

There's no strange overheating but if something's wrong, it must surely be happening before the actual cooling plates even get hot (thanks to the safety).

Well the point of the cooling elements is to get hot - instead of the chip getting hot. If the cooling elements don't get hot that doesn't mean the chip doesn't get hot. That said none of the components before the actual power amp should really get too hot anyway..

Right. I hope I'll have some time to stick my nose back into it soon.