{Collins} KWM-2 idle current changes with tuning



This is my first post to this knowledgeable group.  I have an interesting problem and perhaps someone in this group has had this issue and would be able to share a solution or point me in the right direction to investigate the cause.
 
I can set the idle current (bias) to 40ma, no problem.  But when I tune the PTO, change the exciter, or change the plate tuning, the bias current changes!  It can swing to 70ma or more or down to 20ma depending on the tuning!  Changing the PTO, Exciter, or PA tuning will all change the indicated idle current. 

The radio is working fine otherwise and I continue to make contacts with a modest dipole antenna 10 feet off the ground with good signal and audio reports.

What I have been doing is to tune up first and then double check and reset the bias if necessary to 40ma.
This behavior started recently as I recall this was not a problem.  I have had a few mishaps that let the smoke out on several occasions the causes of which I located and damage repaired.  But perhaps I missed something?  It was suggested that the KWM-2 will do this if the PA tubes are not neutralized properly.  But I have changed the finals and checked the PA neutralization many times now and I believe the issue is elsewhere. 

In the meantime I will continue to investigate but perhaps someone has seen this issue before and can save me a great deal of time.  

I have checked the reflector archives.  There is one mention of a similar situation but no solutions or explanation posted.  

I do have an electronics background and a fair amount of test equipment so a technical discussion is welcome.

Thank you for the support.  

Regards,

Mike

KD9CNV


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