Two problems plagued my 1KD. The high voltage to the finals (30V) shut down and the fan for the finals quit running. Both problems ended up on the same circuit board but were unrelated. Unfortunately for me, the HV would worked sometimes while the fan was inoperative. I really cooked the finals a couple of times that way by not staying off the air when I knew better. Luck has been good to me and nothing was damaged by the heat.

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3YThe regulator problem turned out to be the single transistor that drives the high power series regulator transistors (three in parallel).


An equivalent part had to be ordered from Mouser as Yaesu was out of stock and wasn't going to get a fresh batch for a couple of months.

5YI trimmed the leads to the proper length and formed them to match those of the original transistor.

The new transistor mounted and soldered right in place.


The HV works fine now. This step had to be completed first as the fan runs off the regulated 30V supply.

7YThe fan problem turned out to be two problems. A low speed control circuit is triggered by a thermistor that rests on the regulator heat sink. This device normally becomes more conductive when it heats up causing an imbalance  in an op-amp comparator which in turn turns on the fan. The faulty thermistor doesn't loose enough resistance to trigger the fan. I am waiting for the parts to be delivered from Yaesu, in the mean time, I tack soldered a resistor in parallel with the thermistor to trick it into turning the fan on (low speed) all of the time.

The control circuit has a transistor that pulse starts the fan at full voltage, then drops it down to a slower speed using a second transistor and a series resistor. I was worried that the pulse timing would end before the 30V supply could come up to enough voltage to start the fan. I lucked out and have no start-up problems.

A second device is used as an emergency trigger to put the fan in high speed if it can't keep up with the heating. This device is a electro-mechanical N.O. switch that is mounted on the final amplifier heat sink. It also failed (it should have saved me from cooking the finals). I will have to wait for it to be delivered from Yaesu. This part is going to require me to rip the final circuit board out of the rig and is a much more involved job. In the mean time, I'm doing fine on the low speed fan work-around.

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