Now the error in picture
Number 10 - Have a look at C615 in the upper left corner - see anything wrong ?
It is installed according to the silk screening on the PCB...... But this is wrong !!!
I took it out and put it back in the correct way round (but wrong way round according to the PCB marking).
Lastly The 'Buzzing' Fault
I had tried without success to even actually hear it - and was beginning to think I wouldn't ever hear it..... how wrong could I be. During one of the post re-cap listening tests, I was standing next to the amplifier - when I heard a rustling sound in the left speaker, I hit the -20dB mute switch and it didn't get any quieter - it got louder - then the protection activated !!! I powered off the amplifier - FAST !
So, at last I had seen and heard the fault. Armed with the knowledge that I had already replaced many of the components that might cause this, I set about finding what was wrong.
Back to the bench - right channel driver board removed - tone and phono power disconnected. I started measuring voltages - 10v of DC offset !!! - tapping various parts of the amplifier, I could not get the fault to correct itself even temporarily. I hooked the scope up and monitored the jittery voltage on the output. I substituted the MV103 diodes in the input section of the driver board - same. I was turning over in my mind whether I would swap the 2 dual FET's and risk damaging them OR.... should I shotgun the driver board small signal transistors...... I have never liked shot-gunning a problem - so I decided to gently prod some of the driver board components - when I got to TR04 - there was a familiar click and the protection light was steady !!! - further checks eliminated everything except TR04 - which I then changed - and the problem was gone. I was especially pleased to find and resolve this fault as another tech had tried and could not fix it.
It appears that over the years one leg of the transistor had become 'socketed' and the tiniest bit of movement would disconnect the leg concerned. I was especially pleased to find this as when DC coupled circuits fail - everything seems wrong - so its hard to find out where the problem is.
After re-assembly, a reset of bias and offset - extensive soak and listening tests there has been no return of the problem.
You could not meet a nicer person than Simon (Des Ibels) - I was very pleased to have worked on his amplifier and resolved the problem to his satisfaction.
That's all folks.