Problem: Power on, Tune to a station, low volume, sounds great

Turn volume up past a certain level, cuts out until you back the volume level down then it comes back on.

Heres what I've done so far:

Downloaded service manual from Hi-Fi Engine, and singled out the Protection Circuit board.

Checked all transistors and replaced 3 that were bad, including Q1 and Q2 which were
mentioned in threads before as always being suspect when this problem occurred before,
recapped all capacitors with new, checked all resistors and diodes.

Now I'm kinda stuck as I figured that would be where the problem was.

Any help would be appreciated

No time right now, but one easy check: monitor relevant voltages and see if the protection circuit is shutting it down.
....or temporarily bypass the protection circuit

First option preferred.

I assume both channels are quitting.....if so, that points to the protection circuit.....or any other element that is common to both channels...like a power supply.

I would check bias adjustments.

But if the bias is off, why? I agree but you might want to find out why (IF) the bias is off. Electrolytic capacitors etc...

Poster has already said he replaced/checked resistors, capacitors, transistors and diodes. What else is left? The bias adjustments.

Nuff said?

You're right. Somehow I missed that... Sorry.

Thank you for the replies. Upon closer inspection it looks like the power amp board has been removed
some time before. With the concensus that there could be a problem with the bias I have decided to remove
the power amp board and replace the (4) 470 uf orig electrolytics and check/and or replace transistors.

Is there a way to clean the bias controls while accessible?

I'm missing the logic for the bias theory.....is the bias adjusted separately for the two channels? If so, then we would need two failures for both channels to quit.
Also, how would a bias problem cause the observed symptoms?

According to the service manual each channels bias is adjusted by (2) bias controls on the power amplifier board.
The (4) 470uf caps all checked about 595uf so they will be replaced but I don't think this is the area of my problem, I don't know where to look,really. just for the record both channels do shut down.

Did you check the value of the 470 ufd capacitors with a digital meter? Or were they tested with a capacitor tester, something like a Heathkit C-3 or EICO 950B? Reading high with a digital meter is sometimes suspect, as in they are actually going bad.

Caps were checked with a digital
Pretty sure they are the orig so I'm going to replace with new
wish I knew with a little certainty I am in the right area before I start pulling transistors
but it's good I pulled that board cause the last removal they just cut the wires and then resoldered
Took me 1/2 hour to unwind the old off the 20 pins

I would certainly suspect capacitors before transistors.

If the bias settings are separate for each channel, it does not seem likely that this would cause the issue. BUT--why not try it?

I would be looking for things that are common to both channels.

there was discussion of the protection circuit---can someone summarize how it works?

Set tone controls to flat, filters and loudness turned off.

Scope both channels across load resistors. Make sure scope ground on
probe doesn't short a channel.

Feed 200 Hz into an aux input, with amp connected to two 8 ohm load resistors.

Plug power cord into a kill-a-watt meter. Slowly advance level watching
input wattage.

Try test with balance control, to one side only, or feed only
one channel.

Then try test with 33 ohm load on one channel, 8 on the other.

What will this prove? Never heard of such a test for an audio amp.

OK, I tried to adj the bias and the best I could get was 001.0 to 003.0, supposed to be "0" once in a while goes to -001.0 but the controls are so sensitive and like a dirty volume control but when I got it to the best I could when I turned up the volume it cut out. It took me about 10 min for each one to get to the lowest because of having to make micro adjustments and then letting it settle down.
When turning adj the voltage was all over the place hitting over 100 and then settling down.

Those controls need to be cleaned or replaced.

If you cannot achieve the result expected then there is still something wrong in the circuit; your bias diodes may be at fault. They may be mounted to the heat sink (usually brown or maroon).

The protection circuit has only one relay that cuts off both channels when it detects a problem on either channel. On my SX 950 which is similar the problem of over sensitive protection tripping was due to one of the power supply negative voltages being way out of regulation. That voltage was supposed to be something like -38 or so and it had gone up to -50. The receiver worked but it tripped the protection very easily. There were a couple of cooked regulator transistors caused by bad electrolytics.

How else could you find which channel may be causing the trip without taking
off the cover ?

Also, the kill-a-watt is a fairly good dim bulb indicator without input
voltage starvation.

Ive seen lots of them incorporated into bench systems.

Radio power monitoring has been possible for 70 years with
the wattage eye function on signal tracers.

Did you look on page 20 Fig. 16 and check for potential difference at the diff amp (Q2 & Q3)?

I see you checked DC balance, but what does your meter say for idle current? Are you able to get a stable reading?