Thermistors! Ah!!
 

The fan on the power supply in my Kenwood TS-940 section stopped coming
on. I replaced the motor with a new one. The thermistor would kick on
the new fan when I placed a soldering iron against it. I put thin layer
of thermal grease between the thermistor case and the heatsink and
screwed it firmly to the sink. The fan refuses to come on with this
configuration. Just can't get enough heat into the thermistor I guess.

Do thermistors go bad or what? I'd like this thing to come on at a
lower temp anyway. Does anybody know of a replacement which would do
that? I have zero data on the factory thermistor...I don't have a temp
probe to check it myself either.

-Scott.

Thermistors! Ah!!
 

Thermistors! Ah!!
 

Michael A. Terrell wrote:
wrote:


Do thermistors go bad or what? I'd like this thing to come on at a
lower temp anyway. Does anybody know of a replacement which would do
that? I have zero data on the factory thermistor...I don't have a temp
probe to check it myself either.

-Scott.


Yes they go bad. Ask any old TV repairman how many hundred
thermistors they replaced in the degaussing circuits.


Michael A. Terrell
Central Florida

Mm, yes - TV set PTC's do go crook, but thats because they get full
mains voltage at many amps across them. And it takes a few years to do
it. And its usually the heat generated dry joints on the main board
anyway that winds the set up in the shop.

Easy method - repalce the thermistor, in the meantime stick a fan at
the back of the radio so it doesnt melt down. Or wire the fan hard on
always....(Hint - put your hand on the heatsink/case, if its too hot to
touch, its too hot.)

If you want to get technical, then its time for logical fault finding.
There are 3 things that can go wrong. _ the fan, the thermistor, the
switching cct. You have replaced the fan. The thermistor DOES switch
when it gets heaps of heat. Leads me to think the switching cct is
faulty.

BUT to prove - disconnect the thermistor from cct, stick your ohmeter
across it, apply heat. Should be fairly rapid change in dc resistance -
and use a hairdryer, not your soldering iron. If the change is only a
few ohms, then its faulty - replace. Otherwise, on to next step -
check out switching circuit of controller.

This is a "bread and butter" fault, shouldnt be too hard to fix.....

Andrew VK3BFA.

Thermistors! Ah!!
 

Thermistors can certainly go bad. Pull it out and test it with an Ohm
meter while heating and cooling.

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