[Collins] KWM-380 Final Transistors!

[Kenneth Grimm]

I recently asked for help in selecting a set of final transistors for my 
KWM-380.  Although I already had a matched pair of SRF 3662 transistors 
and the general consensus was that they would work well, it was also 
suggested that the MRF 421 in matched pairs would also 
suffice...although perhaps not quite as well.  It was also reported that 
RF Parts are selling 2SC2879 in matched pairs as a KWM-380 replacement. 

I had the SRF 3662s on hand and proceeded with their installation.  They 
seem to be a very good transistor and it was necessary to reset the bias 
and also reset the ALC to reduce the output down to 100 watts.  With the 
original ALC setting the 380 wanted to produce about 140 watts.  As 
tempting as it might be to use all that newfound power, it should be 
remembered that the other components on the final amplifier board and in 
the power supply are engineered for a 100 watt output.  Not really 
wanting to let the smoke out of any other parts, I set the ALC for 100 
watts.

In the course of removing the old finals, I discovered that one of the 
10 ohm transistors in the base circuit of one of the finals was burned 
open.  However I decided to replace both R41 and R42.  Probably not 
necessary, but I thought if one had burned open, the other might not be 
very healthy.  Cheap insurance.

I want to thank all those who offered information, advice and 
encouragement, especially  Ian McLean - VK3JQ, Colin Lamb - K7FM,Yves 
Pautard - WN4I, David Knepper - W3ST,  J. Fred Riley - W8OY and Jack 
Giehl - WB8BFS.

-- 

Ken K4XL
k4xl at arrl.net

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