Hi,
Just to clarify, this is a stereo integrated amplifier made in the early nineties by Musical Fidelity, a British hi-fi company founded in 1982. There's no connection that I know of between this company and the consumer electronics company which shares half its name.
There's some information on the
company's archive page for the B1.
Searching for information about this amplifier turns up a few results, including forum posts by people who've already contacted Musical Fidelity seeking this information to no avail. I may try contacting them myself, just on the offchance.
The two presets marked 'set bias' on the diagram do, indeed, adjust the bias. What I need to know is what to measure, where to measure it and what measurement to aim for when adjusting.
So far, I've obtained the best results by feeding a 1kHz sine wave into the unit, listening to the output and adjusting each preset alternately for minimum audible distortion, with a final tweak of one preset to reduce the DC offset to zero. At each extreme of each preset, distortion is obvious, reducing over a range in the middle. Both adjustments are interdependent.
However, I'm aware that I can't hear distortion below a certain level. I can't measure it either, as my Sound Technology ST1700B's oscillator isn't currently working in 'low distortion' mode.
Alternately adjusting each preset to the middle of the range which produces least audible distortion also leaves the output stage running quite hot. This may be correct; Musical Fidelity's class A amplifiers had a reputation for running too hot to touch. It's possible that this class AB design is intentionally biased a little more towards class A than many.
After adjusting one channel by ear, I've taken voltage measurements across the 0R47 load resistors R63 to R66 and calculated the current through them. The voltages are approximate and drift as the transistors warm up.
80mV across R63 - current 170mA
50mV across R64 - current 106mA
10mV across R65 - current 21mA
30mV across R66 - current 64mA
One observation is that these measurements are all over the place; I'm beginning to wonder if Musical Fidelity used matched transistors. Due to lack of transistors I only replaced the dead ones; which isn't what I usually do. Replacing all four in each channel with ones from the same batch is one of the next things I'll try.
I'm very inclined to match pairs up, then use each pair in parallel.
It's also possible that the 0R47 (5%) resistors have drifted in value. A few have discoloured a bit though they all measure ok.
I should soon have more than enough 2N3055s to replace all eight and replacements for the resistors though.
In the absence of any definitive procedure, my intuition suggests adjusting by ear while aiming for somewhere around 75mA. The two transistors passing over 100mA are getting hotter than I'd like. The other two are warm, closer to the temperature I'd expect under quiescent conditions.
Unless anyone has any better ideas, that is
(I'll also try, yet again, to get hold of a service manual for my ST1700B.)
Cheers, Kat