Sudden Valiant Problem

[wa2fxm]

Valiant was performing admirably on air up until a few weeks ago. I started hearing arcing sounds from the modulator section when speaking into the D-104. On the bench I find that tune up in CW is fine but in AM on the first modulation peak final plate current pegs the meter and power output drops down to about 30 watts. It feels like it's going into some kind of oscillation. I've never encountered symptoms like this before so if anyone can point me in the right direction to sorting this out I'd appreciate it.

Mark - WA2FXM

[DMOD]

Valiant was performing admirably on air up until a few weeks ago. I started hearing arcing sounds from the modulator section when speaking into the D-104. On the bench I find that tune up in CW is fine but in AM on the first modulation peak final plate current pegs the meter and power output drops down to about 30 watts. It feels like it's going into some kind of oscillation. I've never encountered symptoms like this before so if anyone can point me in the right direction to sorting this out I'd appreciate it.

Mark - WA2FXM

The first items I would check are the 6146's in a conductance tube checker or try to subsitube known good tubes. You may possibly have one of more interelement shorts.

Second, make sure no critters have setup housekeeping in C74, the neutralization cap.

Third, check C37 (DC Blocking Choke) and L11 (main RF choke) for discoloration. Later C37's may be 1000pF caps.

[wa2fxm]

The first items I would check are the 6146's in a conductance tube checker or try to subsitube known good tubes. You may possibly have one of more interelement shorts.

Second, make sure no critters have setup housekeeping in C74, the neutralization cap.

Third, check C37 (DC Blocking Choke) and L11 (main RF choke) for discoloration. Later C37's may be 1000pF caps.

Tubes, caps and choke all checked ok. Then I turned the lights off to see if anything would light up when I keyed the D104. The back section of the plate tuning cap had a nice blue arc across one of the gaps. None of the fins were exactly centered but one had a particularly narrow gap. I bent the rotor plate away and the problem is apparently fixed. Not sure how a rotor plate that's been sitting in one place for 60 years suddenly decides to move closer to the stator but there you have it. Thanks for the tips and getting me to use my eyes instead of staring at a schematic to fix the problem.

Mark - WA2FXM

[W3SLK]

One thing you might check is the rubber grommets going into your meter! My Ranger I had an issue like that and it took me to cycle the meter switch! I know you said it happens on modulation peaks.

[WBear2GCR]

there is a small tubular choke at the bottom of the larger PI wound RF choke.

On my Valiant, it fried...
Also the bypass caps at the bottom of that choke may go bad.

I had to repair the ceramic mounting hole at the bottom, but this pic shows
the not yet fried choke as the brown thin thing...

[DMOD]

Tubes, caps and choke all checked ok. Then I turned the lights off to see if anything would light up when I keyed the D104. The back section of the plate tuning cap had a nice blue arc across one of the gaps. None of the fins were exactly centered but one had a particularly narrow gap. I bent the rotor plate away and the problem is apparently fixed. Not sure how a rotor plate that's been sitting in one place for 60 years suddenly decides to move closer to the stator but there you have it. Thanks for the tips and getting me to use my eyes instead of staring at a schematic to fix the problem.

Mark - WA2FXM

BEAR has identified another problematic area of the Valiant.

I can't tell you how many of those 4.7uH L12 cylindrical chokes and the C47 and C48 .01 uF caps I have replaced in Valiants.

An indication the 4.7uH cylindrical chokes has been overstressed or has failed is by examing it with a light and looking for a dark band around it, usually around the middle or lower area.

I replace those L12 chokes with a TDK B78108E1472K.

C47 and C48 can be replaced with 3kV types such as the HCE103MBCDJ0KR.

Phil

[wa2fxm]

there is a small tubular choke at the bottom of the larger PI wound RF choke.

On my Valiant, it fried...
Also the bypass caps at the bottom of that choke may go bad.

I did look at that choke. It's an old original component but I didn't see any obvious evidence of overheating. The 2 bypass caps are the newer blue 3kv types installed by the previous owner so I think they should be ok.

One thing you might check is the rubber grommets going into your meter! My Ranger I had an issue like that and it took me to cycle the meter switch! I know you said it happens on modulation peaks.

Huh, very interesting. I've already buttoned it back up and hauled it up to the top of the rack where it lives so I'm rather averse to straining my back muscles again, but I'll keep that in mind if it acts up again.

Mark - WA2FXM