Marantz SR4600mgoes into Protect as soon as I change the volume

[kkrmkr]

Hi
I started repairing audio amplifiers as a hobby about 2 years ago. I am not a very experienced self taught but I know most basics on audio gear. Currently I have a Marantz SR4600 that jumps into protection as soon as the volume knob is moved. After a few seconds in comes out of Protect and is fine unless the volume gets adjusted again. First time I started it, it went straight into Protect and it stayed there. One thing that I found was the 10 ohm resistors connected in series with the caps at the speaker terminals showed signs of overheating and one cap was damaged by the resistor heat and was in short. The resistance for that channel was about 30ohms so the unit was detecting that and was going into protect. I assume the amp was loaded with some low impedance speakers and it worked quite hard for a while. I don't know for sure, I bought it from local internet market as faulty no additional info available but the fact that the resistors run that hot to burn the color bands and the caps that were close makes me think of this scenario.
- I replaced that cap, but this new behaviour started.
- I then removed all the resistors to make sure there was no other one shorting to ground but no change
- I measured the voltage drop across the 5w, 0.5ohm resistors and they all seem fine at no volume (12-15mV), therefore bias is fine. However as soon as I have the one unit of volume up the voltage drop jumps to 200mV and the amp gets slowly hot, too hot to keep my fingers on it after 20 min or so. the heat seems to be building up from the area where the voltage regulators are mounted on the heat sink. To me this is not normal
- I have all voltages as pe nominal values, no issues there
- I tried to measure the DC voltage for each channel, but I am not sure on my findings, there is some DC that I can spot very briefly, 0.5V but the amp is also cutting out at the same time, so it could be the effect of the amp cutting out rather than the cause.
- I measured the base voltage and it is fine, 1.2V.
- I measured the signal coming out of the preamp for DC and there isn't any
So, I am after ideas or previous experience other people may have, a bit of a guide on steps to do next

[Bud]

It could have been another way around- the capacitor leaked or developed a short , which caused high current through the resistor.
I'd check if the amplifier gets into oscillation. This will cause high current consumption from the power supply, and it will heat up. Or the power supply itself may oscillate. You'd need an oscilloscope for that, or at least a good multimeter capable of low frequency AC measurements (20-100КHz).

[kkrmkr]

I just bought an oscilloscope and studied all Youtube videos on how not to blow it so this will be a real test. I'll come back with more info after the test, I have some other ideas too. I wonder if one transistor is not blown, but completely out of specs and it switches fully rather than amplifying so the unit senses this huge load. I could disconnect all of them but one channel and see if that changes anything, right? quite a bit of work but I am learning anyways, and I don't mind the extra effort if I can isolate the issue better.
I didn't mention that all this behaviour happens with no audio being input or connected to any loads (well the speaker terminals are disconnected now anyways)

[Bud]

I would not poke around the amp with no load. Connect resistors to the output as dummy load.
Also check and clean the volume pot. It may be scratchy and losing contact as you turn it which may cause momentary overload and the amp's protection kicks in.

[kkrmkr]

yes, make sense to have some load, in this case I just wanted to see if a short develops under load so I removed those resistors. Regarding the volume pot, it doesn't make sense to worry about it because it is not an actual pot, it's just an encoder, the volume is controlled by an IC based on the encoder increments (could call it electronic volume control)