[Collins] KWS-1 PA current

[Dr. Gerald N. Johnson]

On Tue, 2006-11-28 at 22:17 -0800, Keith Sorensen wrote:
> 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 
> described 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
> ______________________________________________________________
There are several possibilities. It could be the grid bias circuit is
going open between the easily read bias voltage and the PA grid. An
adjustment pot would be the most likely culprit. You might want to check
voltage at the PA tube grids, and then don't necessarily trust the tube
sockets.

You need to isolate things from the tubes (are you sure one of them
isn't gassy?) by pulling the tubes. Pulling the plate connector alone
will damage the tube from excess screen current. It would be a really
good idea for vintage tubes to run them with heater power alone for a
day or two, to let the cathode heat warm the getter deposit and to let
that deposit gather gas. 

Its possible some thing in the high voltage is breaking down, more
likely that tubes are gassy or are oscillating.

Just some things to start with.

Is there any RF output showing on the dummy load when switching to CW
with the key open while the plate current is rising? That might indicate
PA oscillation.
-- 
73, Jerry, K0CQ, Technical Advisor to the CRA
All content copyright Dr. Gerald N. Johnson, electrical engineer