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.

12V .37A fans. Same circuit structure as the J2500. Could not find a
suitable PTC. May as well use the TIP41 and put an R and C between 12V
and its base for switch-on full-on confirmation and 47R over the
existing 27R 2W, to raise the "cold" fan voltage a bit, 4V I consider
too low, 1/2 normal rating is waht I'd expect for slow running, plus
initial kicker voltage. Fans are fine, the front one clogged with dust
and hair.

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.

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.

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