"Phil Allison" wrote in message
...

N_Cook wrote:


The schematic for seemingly all these amps , minor variations, is on
Electrotanya as Parsek Mark Bass Little
not all the fan current goes through this R67 and without measuring some
actual circuit nodes on this amp and modelling the schematic I don't
know how much current is actually now or should be taken by the MJE340,
but something is not hunkey dorey.


** I had a look at that schem and could see no sign of current sensing for
the fan. The MJE340 is there to increase fan speed as the heatsink gets
hotter.

IIRC, the 2W resistors are carbon composition types, which are notorious for
increasing their value over time - but I have seen them do the reverse when
subjected to excessive temps.

Famously in the ridiculously heavy Bose 1800 amplifer, where two 3kohm 2W
composition types supplied current to the op-amp input stage.

http://i.imgur.com/ETpss.jpg

These resistors ran hot ( 1.6watts each) and the amp had no internal fan, so
the insides got hot too. They steadily dropped value until one of the 16V
zeners fried and went short. The amp then went full rail DC sending 80V into
the speakers.








Back in the day I had an unfortunate experience buying an HH S500D amp cheap
as it was "faulty".
http://medias.audiofanzine.com/image...0-d-970073.jpg

Plugged it into a friends 2 x 12 cab to have a listen, and both speaker
cones immediately vomited out of the cabinet.

Because of one shorted Zener on the op-amp supply.


That taught me a lot that I still use every time I have an amp to repair.


Gareth.