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N_Cook

On 20/08/2015 21:24, wrote:
Are you sure it is PTC ? Kinda seems like it ought to be NTC.

No ptc or ntc in there. If I'd put a thermistor in there, it would be
low R cold and high R warm.
Worked out the etiology of failure.
Dust and grime stalling the front fan. Amp still runs on rear fan. But
stall currrent must overheat this 27R 2W dropper , before the TIP
bypasses the current , on high demand. 27R goes high with the excess heat.
Eventually the rear fan would not have enough drive, as presumably only
2 or 3 V when the front fan is stalled and passing thru 38R.
At that stage the amp would cut out , well before any large power use,
so not enough drive to kick the fans into operation.
As I received it the front fan would operate fitfully, not actually stalled.
To check for 2 fans operating , as a test, in cold drive of 6V it is
possible to stall the rear fan with a rod and then listen for the front
fan. Plus a torch shone thru from rear t front should show up the level
of dust in the mesh.