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From: N_Cook on 12 Jul 2010 05:24
Seems to be a notorious problem with fan system on these, so many I cannot
find a recognised solution in amongst the complaints.
I've not taken apart yet,all wrapped up under the h/s, is it active or
thermistor monitoring ? and then a cut out mechanism as well.
Monitoring V to the 12V fan takes ages getting from 1.2V to 3V and stops
there as fan starts taking current at 3V. Already the h/s is too hot too
keep fingers on. I never liked the idea of slowly ramped V to fans but will
ask the owner
if they want the temp monitoring changed or fan on all the time.




From: N_Cook on 12 Jul 2010 06:56
Got in under there. Anyone know what the temp setting of the S1 thermal
switch is ? I'm reluctant to bend it back to reveal the other face as pins
look semi rigid and probably not marked anyway.
Not so straightforward a work around. Active monitoring with a TO220 device
LM35DT, linear output, and perhaps JM65RC means selected for 65 deg C.
Plenty of data on them but will have to sketch out the LM35 and 2
transistors to see a way around dropping 65 to 40 deg C in effect


From: N_Cook on 12 Jul 2010 10:14
Going to do a talk tonight on PbF and what do I find in this 2007 PbF amp?
The self-tap screws supposedly holding the IEC socket to the chassis have no
bite and so each insertion/pulling of the line cord plug, stresses the PbF
solder joints on the pcb carying mains voltage . Solder OK at the moment but
looking a bit suspect already and so a year or two on ,a conflagration in
the making. So remedial solder and bolts and captive nuts replacing the
self-taps.

The cut out sw is nearest the hottest part of the h/s so cuts out before the
fan drive has really got going, it still has to cool down the bulk of the
heatsink, all the while the underside is still going up. I can see a diode
going in "under" the LM35 to raise the 0V output for 0 deg C to a voltage
that will at least turn on the pass transistor to the fan, at room
temperature, and then increase it with use. As it is the heatsink gets too
hot to keep fingers on (50 deg C?) before the fan gets any voltage at all ,
surprisingly 24V but 3V will start to turn it.


From: N_Cook on 12 Jul 2010 10:40
I tested the cut out sw in situ. I can just see make out markings through
the insulation wrap.
028 A 20 161
perhaps 161 deg F
Sw goes o/c about 80 deg C and s/c again 65 deg C


From: Meat Plow on 12 Jul 2010 11:18
On Mon, 12 Jul 2010 10:24:03 +0100, N_Cook wrote:

> Seems to be a notorious problem with fan system on these, so many I
> cannot find a recognised solution in amongst the complaints. I've not
> taken apart yet,all wrapped up under the h/s, is it active or thermistor
> monitoring ? and then a cut out mechanism as well. Monitoring V to the
> 12V fan takes ages getting from 1.2V to 3V and stops there as fan starts
> taking current at 3V. Already the h/s is too hot too keep fingers on. I
> never liked the idea of slowly ramped V to fans but will ask the owner
> if they want the temp monitoring changed or fan on all the time.

Ask the owner, chances are they will want it on all the time. I used a
Crown Microtech 1000 for a bass head for many years and it's still part
of my rig along with a BBE 383 preamp. The fan was loud and ran full
speed but the din of the audience nulled any noise it made between songs/
sets and of course when we played all you could hear during silent
periods were your ears ringing.
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