[BC] FM10K

[Tom Taggart]

It's the final game of the regular season.  If the team wins this game, they will be 10 & 0.  Last time that happened was 1984--before that 1963, and 1948.  Into pregame and.....hisssssss

So I called the ops mgr, who was running the game, talked him into getting the Nautel V1 on air.  Off about 2 minutes.

The main is a venerable Harris 10K, from 1988.  Have had little trouble until recently--and that has been minor stuff.  Lost the plate current meter last winter, easy to find replacement meter for about $90. Lost the plate voltage meter multiplier and the plate voltage meter about  a month ago--had both from another 10K I've parted out.

What I have now is IPA trouble. No drive to final, the transmitter will come up, but with a boatload of PA current, and no output (have Bird in line).  Only small amount of IPA cathode current.  Outboard exciter drives 4cx300A IPA which drives the final.  Thought the IPA might have gone--been in the transmitter since July 2012.  However, same symptoms with new IPA tube, Eimac, bought 2 years ago when they were being discontinued.

IPA B+ derived from main B+ though a 50 watt, 200 ohm resistor. Resistor checked OK, no signs of heating. There are 500 pf/5 KV ceramics bypassing either end of the resistor--replaced both since we had them. Underside of IPA box--nothing obviously burned.  Filament voltage measures normal.  There is another resistor for the cathode DC ground on the front panel door (that's where all the control relays are located in a 10K)--it checks OK as well.

My main exciter did go out, too.  A Continental 802B, apparently blew the Darlington--went to full power as they do when the Darlington fails in the power control circuit.  Since it only takes about 15 watts to drive the IPA, all that would have done is saturate the grid in the old tube, unlikely to have been a cause of the transmitter problem.  Now have an FX-50 driving the transmitter, reflected on 15 watts is a 1/2 watt.

Had a similar problem 15 years ago which was a breakdown under voltage in the tube socket.  Have to check Monday to see if I can even get a tube socket.  Sure would be nice if this is something else.

Any ideas?