[Johnson] Valiant Modulator Help !!!

[Darrell, WA5VGO]

The problem is the terminal strip. It has broken down and is carbonized. 
This is a very common problem in Valiants. Replace the strip and everything 
will be okay. Actually, I usually place a couple of small ceramic 
stand-offs in place of the terminal strip.

Darrell, WA5VGO



At 06:33 PM 10/8/2006 -0700, w0ng at adelphia.net wrote:
>It appears (?) that the original modulation transformer in my 1957 Johnson 
>Valiant is toasted. At least when I see whisps of smoke coming out from 
>under it, I assume it is. Gee... it only lasted 49 years!
>
>It's been doing fine until this afternoon when the xfmr started 
>"singing-it's-head off" with modulation applied from my Heil Classic mic. 
>The modulated waveform now looks TERRIBLE!  Doesn't even look like a 
>modulated AM signal anymore.
>
>Very distorted waveform...hard to describe but flat-tops with just a 
>little modulation. Using an audio oscillator with 1KHz sine wave output 
>instead of a mic, the waveform has sharp multiple peaks instead of rounded 
>ones (??). With the audio oscillator at its lowest output, couldn't get to 
>100% modulation, positive or negative. Started sounding like the mod xfmr 
>was going to vibrate apart.
>
>Before this, it was fine. AM waveform now LOOKS TERRIBLE on the scope. 
>Carrier waveform is fine.
>
>It was out of the cabinet and on the workbench at the time I saw smoke 
>(when modulated) from under the xfmr. I also saw a brilliant "arcing" on 
>the terminal strip under the chassis where the plate leads (new HV wire) 
>from the modulator tubes are connected to the xfmr leads. It was arcing 
>between the two plate lead terminals when modulation was applied.
>
>Changed out the 6146's with a pair of known good tubes and the arcing 
>stopped (?).
>
>Idle mod current is good. I used to have a smooth rise in mod current as 
>the audio pot is advanced while talking, but now it's acting erratically. 
>I haven't yet unhooked the wires to the xfmr to do an ohmmeter reading. I 
>thought I'd ask the group for some advice before I heat up the soldering iron.
>
>Maybe I'm missing something else in the modulator circuit that's going 
>haywire??? Any advice would be greatly appreciated.
>
>If it is the mod xfmr, I'd appreciate some suggestions for a replacement. 
>I know Peter Dahl ($$$) sells them and Gary Brown, WZ1M can re-wind mine 
>but was curious if I'm overlooking other possible sources.
>
>Thanks in advance,
>Bill
>w0ng