Electronics Repair (sci.electronics.repair) Discussion of repairing electronic equipment. Topics include requests for assistance, where to obtain servicing information and parts, techniques for diagnosis and repair, and annecdotes about success, failures and problems.

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Old November 10th 16, 02:27 AM posted to sci.electronics.repair
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Default Fan problem

On Wednesday, November 9, 2016 at 2:46:10 PM UTC-5, Jon Elson wrote:

On Tuesday, November 8, 2016 at 3:42:12 PM UTC-5, Jon Elson wrote:

On Tuesday, November 8, 2016 at 7:21:17 AM UTC-5, N_Cook wrote:

Much Snippage


a) What is the actual resistance in ohms of the OEM fan and the
replacement fan if measured across the contacts.

Meaningless. These fans have a commutation circuit and some FETs in
them, so you will not read anything that resembles a resistance on them.


No. The two that work ohm out at ~10 meg. The two that do not ohm out at
~20K. So, yes, you are seeing 'resistance' on them. Clearly "these fans"
are not created equal.

But, clearly, if it was TRULY a 10 Meg resistor, the fans would not produce
any airflow. When you get enough voltage on them, the circuitry turns on
and starts drawing current to spin the rotor.

The 20K ones might have a popped component, or just a different control


You miss the point - no surprise there - what the "protection mode" sees is the initial state of the fan, which is around ~10 megs. Which does not trip it. When it sees 5% of that, it trips. When the fan is running, clearly the protection circuit is OK with what must be a much lower operating (spinning) impedance.

Yes, perhaps a different circuit, no sh*t. Point being, yet again, is that the fans are not created equal. And the point of measuring initial resistance is to be able to separate the ones that might work from the ones that will, clearly, not work without having to test-by-substitution, a not very efficient method.

Peter Wieck
Melrose Park, PA

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Old November 10th 16, 02:42 AM posted to sci.electronics.repair
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Default Fan problem

Nutcase Kook wrote:

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.

** The above drivel makes no sense whatever.

The switching PSU in the Mark Bass "Parsec" series is unregulated and has no remote shutdown system. The DC supply for the fan is also unregulated and not sensed.

"Protect" mode, when triggered, mutes ( labelled DIS on the schem) the input signal the power amp and detects overtemp or excess current in the output MOSFETS.

The speaker relay operates independently and senses only large DC offsets.


..... Phil

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Old November 10th 16, 11:06 AM posted to sci.electronics.repair
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Default Fan problem

Enough of this flaffing around, should have been just a change of fan
job. Found a "high impedance" 12V ,0.16A fan in a parts mule and that
works fine, incidently from a linear ps amp.
I'll assume its something to do with the 0.1sec or so full tilt, fan
start up function of the control circuit. Instead of initial 2 seconds
or so to audibly confirm to the owner that the fan is working, just a
short kick start before dropping into low temp speed . But potentially
18V over 12V fan for 0.1sec and perhaps some added monitoring variant
somewhere in the SMPS , in this 2013, Mini CMD 121 flavour of mark Bass
, different to the only representative Mark Bass schematic that is out
Anyway no nasty ticking of the SMPS going into protect at switch on and
normal running fan noise and increase speed on warming up.
At least anyone else coming across this problem has now some clues as to
what is going on.
Incidently the usual scratched off ident to the SMPS supervisor chip,
dicussed and collectively identified previously on this board.

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