Work, rest and play

Once again, the astute reader of this blog will has have noticed that nothing seems to have happened for ages.  Of course, it's not true - I've just been to engaged with all the myriad things which retirement unexpectedly throws one's way.  There's been a bit of work (e.g. erecting a 12m x 6m marquee in a field in the pouring rain, and building an enclosure for two compost loos out of ex-military tent canvas, scaffold tubes, ratchet straps and rope) as well as quite a bit of rest (it is still the tennis-watching season, after all) and quite a bit of play, including a few classic tasks.  My favourite at the moment is fixing my Tannoy SFX sub-woofer, which started making an impressively loud 50Hz buzz whenever it was switched on, about a year ago.  It has taken this long to even look inside.  I started with the internet, which turned up one chap showing off his fix for one with no sound at all, due to a PSU fault.  He replaced an open-circuit transistor Q152, which (he says) controls the -12V line.

Here's my PSU PCB:

Ha!  Evidently there has been overheating in the region of Q152 (actually removed before I took this picture)...  Obviously mine has gone short-circuit, and the 12V line has been whanging up and down like a whangy thing.  Methinks Q152 may be a bit under-specified.

Q152 is an A1015GR, which cost about £0.001 each if you buy a few thousand, 2p each if you only want 50, 1p each if you want 10 (all from China) or a spectacular £3.50 for 5 from Germany.  I went for the latter and got them within a few days.  I suspect that I would still be waiting for my delivery of 10 units for 99p (free postage) from Shenzen (but I dare say they would arrive eventually).

Anyway, I stuck one in, and lo - it works perfectly.  All done with no reference to circuit diagrams, and with precious little understanding of anything really.  Hoorah!