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McMartian BC TX problem...




 
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KL7OF
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« on: January 20, 2018, 04:23:05 PM »

Mcmartian 1 KW..4-400s x 4-400s....I converted this to 75 meters a couple years ago. Using an AF 67 for RF driver and a McMartian SS audio amp for audio driver...It was working very well when it was last operated a year ago...Now it still works well but it has a problem on key up...I have it set up to key the driver and the transmitter at the same time..When it is keyed, the driver comes up in resonance with output power but the transmitter pegs the plate current for 2 or 3 seconds and then comes into resonance showing the proper output....everything works as it should after the initial 2 second scare (with tubes getting red) ...I have checked everything I can think of but no joy...any ideas??   As I said The transmitter worked fine before and nothing has been changed..It has just been sitting in the shack unused for a year..   Steve KL7OF
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KL7OF
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« Reply #1 on: January 20, 2018, 04:58:34 PM »

It has been suggested that there may be a problem in the screen voltage not coming on with the HV...checking on that now...
update : screen voltage is correct and comes on with the plate voltage...antenna relay is working properly.. 
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W9BHI
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« Reply #2 on: January 21, 2018, 01:48:44 PM »

You need to apply drive first then HV to the finals.
If you apply HV with no drive, the the finals will draw lots of current especially if you are running grid leak biasing on the finals.
I had the same problem with my pair of 4-400's modulated by a pair of 833's.
I use a sequencer to switch the antenna, then apply drive and then apply HV to the finals.
Works FB for me.
Don W9BHI
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KL7OF
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« Reply #3 on: January 21, 2018, 04:36:02 PM »

Thanks Don....I daisy chained the relays so that the antenna comes in then the drive then the final...but that was not the problem...I changed out the final tubes and now it works as it should....The old tubes had full output once they came into resonance so I still don't understand what was going on...It is working now......Steve
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KD6VXI
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« Reply #4 on: January 21, 2018, 07:32:52 PM »

I'll venture a guess:  As the element(s) heated up under power, the input, output or interelectrode capacitance values came into play.  Once drive and everything else was removed, tube cooled, element(s) sagged, wash, rinse and repeat.

You might have prevented a short by swapping tubes.

--Shane
KD6VXI
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