I assume this will become a generic problem as they age. A dropper in the fan supply line is R67 of value 150R, barely 2W size perhaps 1.5W. On the schematic it is marked as 150R 5W. The relay dropper R66 of value 150R , same physical size, has good green middle band. The middle band of R67 is something like grey. Measures 176R, which means that as it increases resistance with age , the protect circuit , monitoring both too low or too high fan current,will engage and amp will drop out.
Its not a tolerance thing, gold fourth band. R67 and R66 are exactly the same resistor batch, the green of the heat aged one is now not green, its value is now not 150R
You miss my point. Resistors drift, true. And the resistors in place seem to be undersized (operative word "seem") You intend to to replace them with higher-watt devices that are at the correct value. All good, so far.
But, I am asking whether the circuit itself may be tolerant of that drift - hence the question on band 4 - if any.
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. Note the refs to 2W and 5W on that schematic,R67 and R67, in reality exactly the same size , colour body , shape on this amp
** 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.
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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.
Fitting new zeners and changing the resistors to 4.7k, 5W wire wound made sure it never happened again.
In the examples I saw, the speakers were Bose 802s which had 4 amp fuses fitted on the back which saved the drivers.
** 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.
formatting link
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".
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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.
** I remember the HH S500D - it used a driver module for the output stage which in the early versions was fully potted in black resin, fitted into a plastic tray.
Since the was no way to access the module without wrecking it, I was forced to buy a new one from the local HH agent even for the simplest fault.
The new one came as a normal PCB assembly fitted in a similar tray but with no potting. What on earth were HH thinking??
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