Plate current but no output

[n4joy]

I was testing my Lettine 240 last night and experienced a few problems.  I was tuning the transmitter into a dummy load and, with tuning controls at default for 80 meters, read 100 ma plate current (where it should be per manual) but there was absolutely no RF output on the monitor scope.  I could increase the drive slightly when tuning the oscillator and could even dip the plate current slightly; but again, there was no output.

I tried different amplifier/oscillator coil and crystal combinations but the results were identical; however, during testing the 5U4 rectifier started arching, plate current dropped to zero, and the transformer loudly hummed when attempting to transmit.  I waited a minute, checked the tubes, and quickly re-transmitted.  Once again, there was 100 ma plate and no output; but the 5U4 arched and transformer loudly hummed after a few seconds and as before.

The tubes tested okay on my Mercury 990 tube tester, but I am leaning toward a bad 807 final.  The transmitter is in original condition (except the caps and a resistor or two I replaced).  I am going to double check all circuits and hope that I did not damage the transformer.

[KA2DZT]

Check the loading caps,  you may have a short on the output side of the Pi network.

[KC4VWU]

Can you hear the oscillator producing a signal into a nearby receiver?

[N9axl]

Along with this --- I'm working on this Viking 2 which has the original metal caps on the 807s. Is there any value in replacing these with some ceramic caps? I'm thinking that having some insulation on these which have 700 plus volts might be useful just from the "ooh..sorry I accidently bumped by hand there" standpoint.

[WD8KDG]

Along with this --- I'm working on this Viking 2 which has the original metal caps on the 807s. Is there any value in replacing these with some ceramic caps? I'm thinking that having some insulation on these which have 700 plus volts might be useful just from the "ooh..sorry I accidently bumped by hand there" standpoint.

Ceramic vs metal: I'd leave the metal ones in place. Push them down almost all the way, but not all the way. With that little air gap, insert a small screw driver and give it a twist to remove the plate cap. With the ceramic ones which I've seen, gotta pull them off and maybe take the plate cap with it.

Ain't no accident with HV..............could be the worst day of your life.

YMMV
Craig

[KB5MD]

Take it from someone who knows from experience, REPLACE THE 807 CAPS WITH CERAMIC INSULATED ONES.  That 807 plate voltage makes for a nasty burn on the fingers.

[KB2WIG]

Put wire nuts over the plate caps ifin u want to work on it hot.  I wonder if Ozona Bob would aprove??                   KLC

[n4joy]

Thank you for the suggestions.  Lo and behold, I indeed had a bent rotor!  I need to start checking for the easy stuff first!   

[KA2DZT]

Thank you for the suggestions.  Lo and behold, I indeed had a bent rotor!  I need to start checking for the easy stuff first!   

Bent rotor on WHAT???  Loading cap??

Fred

[KC4VWU]

I looked at the schematic. Parallel link coupled tank on that one Fred.

... Phil

[KA2DZT]

I looked at the schematic. Parallel link coupled tank on that one Fred.

... Phil

OK thanks Phil,  Maybe he'll tell us what rotor was bent.

Fred

[KC4VWU]

 

[n4joy]

Thank you for your help and suggestions.  The amplifier tuning capacitor had a bent stator--it was minor but enough to make contact during adjustment.

I replaced the 5U4 rectifier and applied power yesterday evening--there is no plate current now but fortunately no arcing (at least during the brief transmit attempt).  I noticed the 6V6 oscillator filaments were not lighted, so I checked all the tubes once again; the 6V6 tested bad on the tube tester.  I am reasonably certain that I did not overlook this tube when I checked prior, but a mistake is certainly possible. I do have B+ and checked the transformer--all good.  I did not verify whether the oscillator was producing a signal during to my first test, so perhaps the 6V6 was going south during my earlier attempt.

I have a new 6V6 and 807 coming in and will triple check the schematic today.    

Chris

[n4joy]

Just an update... The problem was resolved and it was an easy one.  My 6v6 oscillator was not wired correctly.  A .001 capacitor should have gone from the 6v6's plate (pin 3) to the oscillator coil.  Well, the .001 was going to ground!  All is working well now. 

[DMOD]

Just an update... The problem was resolved and it was an easy one.  My 6v6 oscillator was not wired correctly.  A .001 capacitor should have gone from the 6v6's plate (pin 3) to the oscillator coil.  Well, the .001 was going to ground!  All is working well now. 

Don't ya just hate it when RF is shunted to ground at the wrong place?

Phil - AC0OB