[Elecraft] Interesting KPA100 partial power control loss

Ted Turk tedg7bqm at fastnet.co.uk
Sat Jan 23 16:55:19 EST 2010


Good evening all,

K2/100 #5822 started yesterday producing nearly double the requested  
power.  Below 10W fine.  50W requested gave about 100W and max power  
was approaching 160W on lower bands!!  I searched the reflector  
archive to confirm my memory of posts on the problem over the years.  
As suggested by Don W3FPR in the past, I replaced D16-17 with 1N4148  
diodes to see what happened before delving further, fearing damage to  
U5 or U6.  On reassembly the problem was fixed!

I checked the removed 1N5711's with a Peak DCA55 and both appeared  
normal (low voltage test only I suppose).

Have been playing with a kit built Linear Amp Ranger (572B) recently  
(circuit similar to Ameritron AL-811HD) and I am suspicious that on  
powering down the heavy toroidal mains transformer may be producing a  
small 'EMP' since occasionally it freezes up a nearly G4 iMac.  The  
antenna is either disconnected or switched to a dummy load when off- 
air.  No recent thunderstorm activity here - midwinter.

Does anyone have any ideas on how to reduce this turn-off spike, in  
case this is the cause?  All gear is fed from UK 240V mains via surge  
protected power outlets.  Meanwhile I will be even more vigilant with  
antenna disconnection and grounding.

73,

Ted G7BQM




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