GK 400RB misbehaviour

I've got an earlish GK 400RB, same one as in this thread at TalkBass. See image from that thread below. Q1 - anyone know which version this is?

GK 400rb troubleshooting help needed

Now mine is a whole lot scruffier; had the transformer changed (now torodial) to run off local voltage (230VAC vs 110VAC); has been blown up at some time; but mostly works.

Except that notes are a little distorted as they fade away. So there I am thinking that maybe bias might be out & its running slighlty in class B. It sort of almost sounds like that.

Put it on the bench today and I find there's c. 250kHz at about 20mV pp on the output. And when I feed in 600Hz sine wave with about 100mV pp output, the signal both gets the noise added and the signal itself is not stable. The effect out is clean as a whistle, but the main out has this noisy 40us period signal. No surprise: none of the front panel controls affect this in the slightest.

So, I popped the top to have a look. The repairs are OK, looks like 1/4W resistors went BANG. Lots of scorch marks on the PCB.

But: it looks like there is a space for another 5W resistor to the left of R30 (5R zobel?) and the four emitter resistors. Noting that the image linked above also only has 5 big resistors.

I've checked the earthing of the case components and that looks OK (that's known to cause hum problems)

Anyway, before I do any more poking around I thought I'd ask if anyone else has seen a GK have this sort of oscillation?


Edit - a view under the hood of mine.
I think the scorch marks are under the resistors in the bias setting ladder. How on earth do you cook them?
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To start with: Remove the resistors and the carbon from the burnt sections of the pcb. Spray some plastic over it after clean-up. Solder the resistors back in place.
Check the needed components for the missing ones. Look at the schematic here attached.
Now do the tests again.


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Tarzan, thanks - I'd not seen the whole manual before (I had some of the schematics). The resistors are "new" (i.e. unburnt) but I'll certainly be cleaning up the PCB.

I need to find the source of the oscillation, I'm wondering whether the op amp has (partially) died?
Try to think and act in steps. First this, write it down. Then that and write it down. Continue it this way. If you ever have to go bacj a few steps you know what to find.
Check the components if they are installed and of the right value according the schematic.
Only then fire up the amp via a lightbulb circuit. Lamp should dim after a few seconds.
Measure if any voltage is present on the output.
And then, We will see