Tapco Juice J.1400 slave amp

[N_Cook]

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.

[N_Cook]

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.

[N_Cook]

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.

[jurb...@gmail.com]

Are you sure it is PTC ? Kinda seems like it ought to be NTC.

[N_Cook]

On 20/08/2015 21:24, jurb...@gmail.com 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.

[N_Cook]

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