Misterg
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Thinking out loud a bit:
Just to confirm that the parallel resistors in the emitters (R424, 425, 430,431) are definitely present and all 0.47ohm?
Biasing instructions say to set test point (e.g. TP401) pin 1 to 8mV? Therefore the emitters should also be at 8mV ± 0.5mV when adjusted correctly?
I can't see the the specific transistor numbers that you quote above, but assume that the channels are all similar (low Q# is npn, higher Q# is pnp)
Q434 (pnp) Vbe 1.15V Ic 200mA
Q428 (npn) Vbe 1.07V Ic 115mA
Q422 (pnp) Vbe 1.0V Ic 850mA
Q416 (npn) Vbe 0.86V Ic 800mA
npn is 2SD2390 darlington (datasheet)
pnp is 2SB1560 darlington (datasheet)
From the datasheets, an Ic of ~200mA is consistent with a Vbe of ~1-1.2 V (the pnp being slightly higher Vbe for a given Ic) so the Q434/Q428 pair seem to be functioning as expected.
Unless they are very hot, one would expect Vbe >1.2 V for Ic of ~~1A
So (unless hot), the Q422/Q416 pair shouldn't be passing that amount of current with that Vbe. If the emitter resistors are OK, then I suspect that the transitors may be faulty? There's a 70 ohm internal resistor between the output device base and emitter. If the input device had failed short circuit, this would allow ~850mA through the devices.
The specified bias voltage of 8.0mV ±5mV corresponds to Ic of ~32 to 36mA, so (it would appear to me that) all channels are running excessive bias?
I'm suspicious as to why both might be faulty, but can't think of another explanation yet. I'd try and test Q422/Q416 out of circuit.
Late thought - Maybe if *one* of the devices had failed as above, it could send the other into violent oscillation?
[ETA: I mean one of the npn / pnp pair, not one of the devices within the darlington pair - The good device could appear to pass the same average current as the 70 ohm internal resistor in the other.]
My 2p, anyway.
