Hi Mark,
Thanks very much for the explanation. I'd found the circuit for the A1 (on your site) and noted the similarity of the two designs.
Incidentally, as I found the B1 schematics on the 'net (I can't remember where, but it took a
lot of searching) and they were freely available, I don't see any reason why you shouldn't add them to your site if you like. At least they'd be easier to find for anyone else attempting to service a B1.
I've now come up with a way of adjusting it. The biggest difficulty was attempting to watch two measurements while adjusting the two interdependent 'set bias' presets.
This is much easier to perform using a 'scope in X/Y mode, X connected to the top end of R64 and Y to the output.
I start with both inputs grounded and centre the dot. Then I shift it left (negative) by the desired voltage on R64. If I now adjust both presets to bring the dot back to the centre,
both measurements will be correct.
It's still tricky and the dot tends to drift, but it's the easiest method I've come up with so far.
I'm still waiting for the postman to deliver some bits; at the moment, one channel's fitted with two 'Pecor' 2N3055s in the lower half, two marked 'A1000N' in the upper half and somewhat fried-looking 0R47 resistors. (The other channel's missing all four transistors now...)
I've also got around the problem of the dodgy oscillator in my ST1700B with the aid of a computer and high-quality sound card. A 24-bit 1000Hz sine wave generated with a sample rate of 96kHz measures 0.004% when the card's connected directly to the analyser; I think that'll suffice.
Using the 'scope, I can fairly quickly set the voltage at the top end of R64 to anything I like while keeping the DC offset at zero (with no signal), then measure distortion.
So I think it'll be possible to figure out a reasonable bias setting without it taking forever
A few distortion measurements made with different bias settings suggests ~25mV (~50mA through R64) may be "about right" but I'd like to fit new parts (and have both both channels working) before reporting any further findings.
Cheers, Kat