Re: Another proposal


"Eeyore" <rabbitsfriendsandrelations@xxxxxxxxxxx> wrote in message
news:478D570A.7BE7855F@xxxxxxxxxxxxxx


Trevor Wilson wrote:

[ASIDE] Many years ago, I worked on a horrible design from Tapco (US).
Each
output stage was constructed on a large PCB, with each transistor mounted
with it's own individual 'pin-fin' heat sink. A fan blew across the PCBs
to
cool the thing. It was rated at 250 Watts/ch and was a BJT design. When I
saw the first one on the bench, I informed the importer that he had a
disaster in the making. I then listed a whole host of mods, which might
make
the thing reliable (larger value Emitter resistors, matching output
devices
and linking heat sinks together). Tapco informed the importer that there
was
no problem and my mods were not necessary. Six months passed and several
dozen blown up amps later, the importer called me in. He presented me
with a
modification sheet and the necessary components to stop the amps failing.
You guessed it: Larger value Emitter resistors, selecting transistors for
specific gains, to be placed strategically on the PCB, but no heat sink
linking.

Dickheads.

Yes.

It never fails to amaze me how the same mistakes get repeated time and
time
again by 'newbies' who are too proud to accept that may have got some
details
wrong.

Got a 'clone' of a QSC style amplifier made by Sekaku for evalution some
years
back. It was almost an exact copy of an RMX2450. My colleague put it on
soak
test into only a 4 ohm load and it 'went bang' after about only 25-30
mins. Why
? They'd reversed the cooling airflow direction and the temp compensating
parts
were now at the COLD end of the heatsink ! Idiots !

As for the emitter resistor thing I even once wrote to Sinclair about
their
crazy practice of NOT FITTING ANY AT ALL in the Z30 and Z50 amplifiers.
I'd
ended up repairing so many I'd got fed up with it. I did receive a reply
IIRC
but they pooh-poohed my suggestion. About a year later they started
fitting
them. It's NUTS, any competent designer KNOWS what's going to happen if
you do
stupid stuff like that.

Graham



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