KPA 1500 Fan Speed not locking in at transition points

wb6bee
 

I have a KPA 1500.   According to Elecraft notes, the fans are to accelerate at 70C to level 2, returning to level 1 at 55C.  They are to accelerate to level 3 at 80C and return to level 2 at 75C.

When I got the amp,  the fans would not lock in.   For example, if you had the amp on a dummy load and held key down to 70C, the fan would go to level 2. But, if you immediately released the key, it would instantly drop back to level 1.   It would not hold level 2 until 55C.  Same at 80C.   Fans would jump to level 3, but if you released the key, it would instantly drop back to level 2 (still above 75C)

I talked to Support and then sent me a new sensor and capacitor modification kit.   I installed them both.   I ran the dummy load test described above and the amp fans worked perfectly.

Now a couple weeks later, it is back to the old problem.   It will not lock into the fan speed as it hits 70C or 80C.   It's really irritating in CW when you are keying and the spaces between characters allow the amp to cool ever so slightly.  At temperature transition points, the fans just oscillate back and forth between the two fans speeds.  Level three is bad enough, but have it jump back and forth and it will  run you out of the room

I have advised Support of the return of the problem.   Any comments or similar experiences

Don
WB6BEE

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