[Collins] KWS-1 PA current

[Roger Shultz]

> 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.
>
> 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
>
>
> ----- Original Message ----- 
> From: "Keith Sorensen" <radiorm1 at comcast.net>
> To: <Collins at mailman.qth.net>
> Sent: Wednesday, November 29, 2006 1:17 AM
> Subject: [Collins] KWS-1 PA current
>
>
> Hello to all,
> I am currently bring up a KWS-1 serial 0071.  I have a situation with the 
> initial set up, when going from STBY to CW the PA current will peg and HV 
> fuse will blow with in seconds.  I have done all the initial checks for 
> low voltage supply bias -39 to -96vdc, screen at 350vdc and low VDC 
> running at 300VDC, 25 vdc high.  I have installed new caps as decribed in 
> the "deadly caps" tip.  rebuild the final and dropped in new 7580W, and 
> added the service bulletins. The 2KV supply holds up fine until the 
> "operate/tune switch is in the operate position and moved to CW. I get no 
> screen, ALC, or Bias reading on the panels multimeter  When in the tune 
> position the PA current rises slower but will still peg.  Any tips would 
> be a great help.  I am a new member and this is my first request for help 
> from the pro's at CCA. I really want to put this old beauty on the air.
>
> Keith KE7GPO
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