Listed on eBay as 'parts or not working' for having what was described as a volume control issue. Didn't sound too fatal, so I took a risk and bought it.
There are a lot of different versions of this amplifier - different marks, then different versions of those marks. This is the A1 mk3, the version that uses a single LM4766 chip for the power amp, rather than dual TDA1514 chips used in the Special Edition. You can tell what you're getting by the ventilation holes on the top. The dual chip version has these running front to back.
This particular version cheaps out on parts. Carbon film resistors throughout and cheapo capacitors, plus an EI transformer (the Special Edition version has a toroidal transformer instead). But I don't really care. I'm not convinced alternative components would make that vast of a difference to the sound, and I'm not willing to spend the money to find out. This was a fairly well respected amplifier and will serve my needs nicely.
The input board is full of lies though. Obviously a board that drops into a number of different amplifiers. There's not much on this list that's true. I can only dream of having pre-amp modules that are Hyper-Dynamic.
The first problem to tackle is the brown crust all over the place. This is glue that they added to the base of large capacitors to keep them from doing a runner. Applied carefully with a sawn-off shotgun it appears. In reality I don't expect it's really necessary (the cheapo capacitors are more likely to leak to death before they work themselves loose from the board) and it has a habit of becoming corrosive over time. In my case, it had started to eat through the leg of one of the voltage regulator diodes. I think it still worked, but the whole thing needs a good clean, so any components tainted with glue have to come off the board.
The board cleaned up nicely with some scraping and a bit of IPA. You can find a schematic for this particular version here, but note that the zener diodes that feed the pre-amp circuit are in fact 15v, not 10v. I replaced both zener diodes with BZX85C15 15v, 1W zeners and they work fine.
Next, the volume control issue. It would play audio even with the volume right down, but only came through the right channel once the knob was turned past a certain point. I cleaned up all of the pots with Servisol and even desoldered the volume pot to check its resistance was measuring correctly. No problems there.
After literally poking around at every component while playing some music it became clear that the problem was the wire link J20, which had a bad solder joint. Resoldering this took care of the problem and everything works fine.
Seems like a decent little amplifier to me. Its 2 x 25w of power is plenty for my little speakers in my little room. It is lacking a phono-preamp though, which was an optional extra that cost £20 back in the day and more like £60 second hand now. With all the infrastructure already in place on the input board, I have a plan to build my own pre-amp to fit inside.
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