On 08/11/2016 12:21, N_Cook wrote:
Regular 2 wire 12V , 0.14A brushless fan. At some point in its history
something has been rubbing against the interior part of the impellor.
Not these days, perhaps dislodged and blown out through the grill.
Black plastic impellor with whitish cicular streaks, the body of the
impellor is black throughout , so distressed plastic going white, like
flexing a bit of black plastic and goes white at the stressing, not
white paint or such.
Anyway that or just normal aging/grime etc it now makes a nasty bearing
noise when warm , so I replaced it.
This is a Mark Bass amp with low speed start and on-demand speeding up
circuit. I replaced it with 12V 2A and that was sufficient to
(presumably) overload the 18V, or so, line off the SMPS and amp goes
into protect. Not even a slight kick of the impellor
Tried the original again, and that is fine.
Tried a 12V ,.15A one and again protect mode, same sort of start up
current draw as the original, but goes into protect.
Tried a 12V ,0.08A half size one and that is ok.
A resistor in the fan control circuit should be 150R and is now 176R ,
so something obviously wrong there, something also wrong in the ps as
well, if it can drop out just from increase of load by 0.01 amp ,
something is awry surely. Something special/odd about Chinese XinFans ?
The fan drive starts in slow speed mode, no confirmation full speed for
2 seconds at startup, which I like to see on such amps , but I don't
think is built-in to these amps

Curiouser and curioser. I found a pretty knackered , high impedance, 12V
0.16A fan and that worked fine.
Removed fan and placed a pot across and dropped down to 10K ,
eventuallly, and no protect mode. Tried fixed 8K2 and no protect mode.
Put an impedance "break" as far as DVM-R is concerned, of a Darlington
BDX47 and 10K bias R, so DVM-R across the new low impedance fan I was
hoping to place in there now measuring 2M . Dropping 0.9V on bench ps.
But still triggered protect mode of SMPS.
So not simply resistance problem.
SMPS is perfectly happy with 78 &79 V regs off those SMPS Tx taps, but
the fan supply is taken off before a choke, significant?