One fan fails to start. Presumably 12V but only 3.8 to 4V over both fans at start up. Recognised problem /solution.? Assuming its like the 2500 (sm out there) then 39R/3W dropper, is 27R/2W in here. I can see it being replaced with 10R + PTC thermistor of 15R hot . It would seem that , when operating at power, a TO220 transistor bypasses that resistor, proportionally.
The 4V over the fans was due to the dropper going high to 38R, 27R 5W vitreous went in there instead. 2200uF/25V and 1K between base and 12V now gives the more reliable fans full-on at switch-on then drops down over about 5 seconds to low noise 6V drive, waiting for high power , on demand.
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.I originally red that 2W as 27R. Looking closer to it , orange/brown/black. Not E24 value but orange is midway orange and red,more orange than red , the background is light grey. Central section is slightly darker grey and coating is a bit flakey in a spiral pattern, so overheated , loosing MO material and going up in resistance. Definitely brown ring, but I assume the colours were red/purple/black originally. Going from 27R to 38R
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