[Johnson] Johnson 500 modulation?

[Craig C. Heaton]

Roger,

You didn't mention if the original modulation transformer is still being
used? On page 53 of the manual, it can be downloaded from BAMA, the
resistance each side of center is about 85 ohms, the secondary is about 120
ohms.

Of course other mod trannys will vary. For instance, my "500" came with a
Peter Dahl mod tranny and might of be abused, lots of talk-back. Enough it
caused feed back because of the small shack its in. Resistance readings on
it were 100 ohms one side of center, 70 ohms the other side of center. The
secondary was 60 ohms.

Bought another from Peter and new the readings were 67 ohms each side of
center, the secondary was 80 ohms, it works just fine. No talk-back in the
shack, thus no feed back.

Don't think at this point the 572B's are of fault, but the rig was made for
811A's. My money is on the mod tranny.

On amfone.net, WA1HLR, has an audio mod for the "500". Bypassing the audio
clipper and such were a common item to get better audio from this
transmitter. Compare your RF deck with Tim's mod and see if anything obvious
stands out.

Craig,


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