I don't know about you, but I don't think solid state equipment needs
to run hot enough to fry an egg on. The fan on the 440 is set to come
on at 50 deg C. That makes the heatsink hot enough that you can only
put your hand on it for about 3 seconds without being in pain.
Looking at the schematic and pondering the situation, I decided that it
would be quite easy to change the "trigger" point for fan operation.
This mod will do just that and not effect the "high-temp-shutdown" mode
that is provided in case the fan dies. Actually, it wouldn't be a bad
idea to make the same sort of mod to that circuit too, but let's just
do this one for now. I'll put out another bulletin on this if I go into
the failsafe circuit.
The fan is controlled by two sensing circuits on the FINAL UNIT. On the
schematic, Q9(1/2) is the failsafe trigger that activates the powerdown
circuit in case the final reaches 80 deg C. It won't normally do this,
even under continuous keydown conditions, unless the fan has failed.
Q9(2/2) is the stage that controls operation of the fan. It is a simple
voltage comparator and therefore can be made to trigger wherever you want.
TH1 is a thermistor with a negative coefficient. That is, when the temp
rises, the resistance goes down. This pulls the - input to Q9(2/2) lower
and lower until it is at or below the 2.26v reference that is present on
the + input of the same stage. At that point, the output flips high and
turns on Q8, thus turning on the fan. By raising the reference voltage
at the + input, the TH1 voltage will fall to the reference voltage sooner
and turn on the fan at a cooler temperature. By clipping the top loop of
R27 on the final unit (2.2k res. that provides reference voltage) and
temporarily inserting a 5k precision multi-turn pot in series with it, I
determined that I liked the way it operated with about 800 ohms extra
resistance in the circuit.
I then installed an 820 ohm fixed resistor in
series with the cut loop. I also put a very small dab of silicon seal
between the new "hung" resistor and the ferrite transformer right next to
it so that there would be no added strain on the remains of R27. The
alternative would be to remove the final unit completely and do the mod
the right way, replacing the complete R27 with a 3k resistor. This was
a step that I didn't really want to perform on a two week old rig. When
you choose your series resistance, remember that the more resistance you
add to it, the sooner the fan will turn on. If you go too high, it will
be on all the time.
This modification brings the fan on at least 10 deg. earlier. By the way,
my only reservation about this mod is the added wear and tear on the fan
unit itself. I have been buying parts from Kenwood for about 10 years now
and they have always seemed remarkably inexpensive. Ordering a spare fan
to have on hand seems like a very reasonable thing to do if you are worried
about it.
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