TS440 Fan Temperature Modification

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
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.

GW1NGL NA7KR Kevin Roberts Ham Radio

Page last updated on 09/10/2012 by Kevin Roberts NA7KR a colection of Ham Radio and Electronic Information