"Alex Bird" wrote in message
oups.com...
Arfa Daily wrote:

www.grandata.co.uk have all three. Good company.

Great, cheap too!

I've just been reading a few threads about counterfeit transistors. Is
this a problem in the UK?

If your output TRs have failed, make sure that the low value emitter
resistors have not gone open circuit also

I've tidied it away for now, but would that be one 'designed' to blow,
about 1cm off the board ? I can't remember the actual circuit.
Runs fine with transistors from the other channel. I've a feeling I'll
need to adjust the bias, any FAQs on that ? They didn't seem to get
warm when idling, does this mean bias is 'ok' ?

Thanks!
Alex


I personally, have never had a problem here with counterfeits, as far as I
know, although I did have a problem just this week, with some 7915's. which
came from a very large reputable supplier that I deal with regularly. I had
ordered in 5 to replenish my stock, and one of them was needed for a repair
in hand. When I fitted the new one, it didn't work, nor did the next, or the
next ... In fact none of the 5 had output, despite a good voltage level
going in. I finally fitted one from another source, and that was fine. All
faulty ? Marked up wrong ? Counterfeits ? I don't know. I wouldn't have
thought that this particular company would have bought from anyone but the
manufacturers, or their agents, but you just never know.

The emitter resistors that I referred to, are usually white ceramic square
bodied types, rated about 4 watts, and may have a value anywhere from about
0.01 ohms to 0.47 ohms. It's very common for them to fail when the output
transistors do, but if the amp works with transistors from the other
channel, they are probably ok. I say probably, because even though they are
usually very stable wirewound types, I have known them to go up in value, as
a result of the overload imposed on them, by short circuit output
transistors.

As far as the bias goes, I would not expect there to be much difference
between the original failed transistors, and the ones that you have cross
substituted from the other channel, as they will probably have originally
all come from the same production batch. However, when you fit the new ones,
they may have significantly different characteristics, and require the bias
to be set. In general, if the heatsink temperatures remain about the same to
the touch between the channel with original transistors in, and the new
transistors, after the amp has been left to idle for 10 mins, then assume
the bias is ok, and leave it alone. If the renewed channel is significantly
warmer, then the bias needs turning down, and if cooler, turning up.

Rule of thumb value for the quiescent current in an ' average ' domestic
stereo amp, is about 20mA, and this is a good starting point, if you don't
know exactly what it should be. Some amps are equipped with linked
testpoints, which can be used to break an ammeter into the circuit to
measure this current directly. Others, however, use an indirect measurement
method, where the voltage across a resistor is measured, usually with a
millivolt meter, or 'scope set to DC input. You can always compare with the
good channel, for a valid setting.

A word of caution before altering the bias. Because the bias presets are
generally not altered during their lifetime, they can get ' touchy '. Before
adjusting, mark the current position, then add a few drops of switch cleaner
to the pot. Making sure that the amp is off, give the pot a vigourous scrub
back and forth, then reset back to the original position. If you don't do
this, and the pot goes open circuit as you move it, the output stage can go
into thermal runaway, and ultimate meltdown. Also, only adjust the pots in
VERY small increments, as some are really quite violent in their operation,
and the difference between a virtually cut off stage, and one pulling 2
amps, might only be a few degrees of rotation.

Hope all this is helpful to you

Arfa