[Johnson] Johnson 500 modulation?

[Roger Shultz]

I recently did a complete rebuild of the power supply/modulator for a local 
OM and thought that finally everything was working properly, however it 
ended with a problem that has me perplexed and I can't seem to resolve.

This 500 supply was pretty much a rusty basket case but I rebuilt it from 
the chassis up with the aid of the original assembly manual.

Everything seemed to work properly RF wise,  on 80 meter CW,  but the 
previous owner(s) had changed the low level modulator by removing the 
clipper and had used an external audio driver to the high level modulators. 
I was able to get the on board driver all working again, still with the 
clipper bypassed and had lots of drive out of the 6B4. Looking at the 6B4 
plates with a scope the audio looked reasonably decent with a 1 KHz sine 
wave input though the phone patch jack. The mod cath meter on the RF unit 
indicated plenty of modulation but here is the rub.

When I listen off air and when another station listens, while there is 
modulation, it is quite weak and perhaps only my imagination but it seems 
offset from the carrier!

I am getting plenty of power out on AM (325 watts) and from the modulator 
cathode indication the modulator tubes sure are working properly but what I 
don't get is "where is the modulation"!

It would seem that if modulation cathode reads like it is supposed to read 
then the 572B's, though of unknown history, would seem to be OK leaving only 
the modulation transformer as suspect. I then checked the transformer open 
circuit and find the primary to have about 120 ohms from each side to the 
center tap and the secondary 200 ohms and nothing is shorted.

Any thoughts, off the wall or not?

73, Roger, NJ2R