{Collins} KWM-380 and ALC adjustment for Amplifier



My KWM-380 (HF-380) has all the SB and SIL's up to Dec 1984.  After many
reads of the service manual and self-study course I did a detailed bench
tuneup (huge thanks Jim WA9Z) and it is working really quite well now.  I
would like to pair it with Alpha 87a with my vertical antenna.    I have two
requirements that I was hoping to find one solution: 1) reduced tune power
setting around 10-15W for my remote antenna tuner (cannot use the 30W output
by grounding pin 25 on the control adapter interface) and 2) reduced power
output around 35-40W for the amp.  

 

I tried the suggestion by Ken, K4XL using a 4v battery negative supply
through a 1Meg pot but that appears to be way off as I found a very narrow
range of control (0 W - 100W) between 14.7k - 21.1k.  I did this with the
radio set for CW, carrier knob set for 100W key down.  Measuring the voltage
drop across the pot at these resistance settings I found a negative voltage
to the ALC jack in the range of:

 

-1.7vdc  = 0W output

-1.6vdc = 35W

-1.5vdc = 80W

-1.4vdc = 100W

 

Checking the schematic the external ALC input to Q106 does go through an 11k
voltage divider to ground confirmed by me checking the resistance acorss the
external ALC input RCA jack.  I also checked and reset all of the internal
ALC circuits per the service manual.

 

This seems like an extremely narrow ALC range.  Is this correct or what am I
missing?

 

Yes, I know my other choice is to use a 6dB power attenuator in front of the
amp but I like to understand the radio better.

 

73

Jerry NY2KW

 




This archive was generated by a fusion of Pipermail (Mailman edition) and MHonArc.