[Collins] KWS-1 PA current

[Dr. Gerald N. Johnson]

On Wed, 2006-11-29 at 15:26 -0500, Roger Shultz wrote:
> 
> > Hi Keith,
> >
> > Welcome to the KWS-1 misery group! Actually, when everything is working 
> > right the KWS-1 is a great sounding transmitter.
> >
> > You wrote a couple of key clues. "When in the tune position the PA current 
> > rises slower but will still peg."
> >
> > First, have you tried disconnecting the HV line from the RF unit to 
> > isolate and sectionalize the fault? When you go to CW from Standby you 
> > energize the HV so you can do this while taking safety precautions with 
> > the HV line. If the fuse doesn't blow then read further.

But be sure to pull the PA tubes too, because pulling the HV line will
leave screen voltage and without plate voltage you will damage the tube
screens from them glowing very hot.
> >
> > This then suggests to me two more things to check, one common, the other 
> > not so common but happened on one of the ones I have worked on. C-416, 
> > which is a .01 mfd HV bypass frequently fails and when it does it will 
> > cause the fuse to blow usually quickly but not always. This should be 
> > changed "just because". The because is that it was only rated at 2500 VDC 
> > when new 50 years ago and with today's higher line voltage you have very 
> > little margin in the HV supply.
> >
> > There are several ways to do this but one inexpensive way I use is to 
> > fabricate a ceramic feedthrough insulator on a smaller filler panel to 
> > bolt to the rear of the PA cage and then use a 3 or 4 KV .01 mfd disc cap 
> > as the bypass of the HV line. These are available through Mouser, Newark, 
> > and others for only a few dollars. The Mouser P/N is 75-440LS10.
> >
> > The second thing to check and one that bit me was that the connectors 
> > going into the RF unit had carbonized. Collins used several different HV 
> > connectors ranging from BNC to a couple Amphenol varieties and even the 
> > latest version on mine had carbonized. Change both connectors.
> >
> > Another thing for clarification is that your screen voltage should be 
> > close to +175 volts in the Tune position then rise to +350 or so in the 
> > Operate position. You mentioned you have checked PA bias voltage but not 
> > where. Make sure you have bias on the grid line going to the PA which can 
> > be reached under the chassis at the rear in the driver plate compartment. 
> > To make sure it is getting to the RF unit you can easily check with a VOM 
> > to ground with the other lead on either of the RF unit multi-meter 
> > terminals when in grid position. The same place and technique can be used 
> > to verify screen voltage.
> >
> > Hopefully this will eliminate some common problem areas. IF I can help 
> > further please let me know. There are plenty of guys with KWS-1 experience 
> > so you should get lots more tips!
> >
> > 73, Roger, NJ2R
> >
> >
-- 
73, Jerry, K0CQ, Technical Advisor to the CRA
All content copyright Dr. Gerald N. Johnson, electrical engineer