The AM Forum

Looking for some advice. I've searched this forum, and elsewhere online, but would like to get the collective wisdom of this group. You guys have forgotten more about AM boat anchor technology than I will ever know.

I have owned a Johnson 500 for a while but have not had it on the air until recently. It was acquired at the auction of a SK radio station owner who collected E.F. Johnson gear. It was fully (and well) restored in 2011 by the radio station's engineer, also a ham, who did the work on all the owner's gear. It has the WA1HLR audio mods. More recently it was gone through by N6TLU just to make sure everything is up to snuff.

It operates well for a 60+ year old transmitter. Great audio. I am using an unamplified D-104, which has an Astatic cartridge acquired NIB at Hamcation a few years back. The second dot on the audio control gives plenty of modulation, but in the wrong direction.

I put my REA modulation monitor in the line to look at the waveform, positive and negative peaks (into a dummy load). I am getting negative peaks 20-25% higher than the positive peaks. Not good. I fear at 300 watts carrier and 100% positive modulation it will create splatter. I had the same problem with my K7DYY 80/40 Super Senior when I put it on the air a few years back. It took an Inovonics 223 to solve the problem. It's my voice.

In my research, an old thread on the AM Forum stated that swapping the plate caps on the 811s in the Johnson 500 modulation circuit will solve the problem. Hmmm. I am not sure what doing that brings to the party.

I was hoping not to have an audio chain for this rig, but so far in my research the best solution seems to be in a homebrew peak limiter I found online, schematic attached. Phase reversal of the mic did not help.

Any thoughts or suggestions will be appreciated.

Bud WO4K

Hi,

Bill Orr, in the July 1950 issue of CQ, swapped the plate leads of a Collins 32V
transmitter modulator, to correct a similar problem, where the negative peaks
exceeded the positive peaks on A.M..  It certainly can't hurt to try.

73

Ed  K4PF

Yes, in a stock 32V, that is true.  But if any of the mods done by the op added a negative feedback loop that included the modulation transformer, then we have an oscillator.  That issue should be determined prior to the plate cap swap test.

Swapping the plate leads on the 811s solved the negative peaks issue. Thanks!

Glad the issue has been resolved, but.....If there was a global negative feedback network from the modulated B+ back to a low level stage, then the plate cap swapping would cause positive feedback, and likely oscillation...could even blow the Mod transformer.

An alternative would be to go upstream to that MC-320 Mic element, and at the rear terminals, unsolder, and swap the two wires there. Then resolder.

Glad another V500 is back on the air.

I wish I had a Big Johnson!

Jim
Wd5JKO

Jim, swapping the mic wires was the first thing I tried. No joy: it was somewhat better, but not fixed. Swapping the plate leads did the trick.

Bud,

   If the modulator were linear, and without phase shift across the audio frequencies, then either method of swapping the phase should work equally.
Since there are two audio transformers inline, and R-C couplets (maybe?), you are seeing a difference.

Besides phase, sometimes swapping the modulator tubes (811's) can make a difference if one tube has more emission than the other. Worth a try if the positive peaks are struggling to hit > 80% upward modulation.

Jim
Wd5JKO

Back in 2010 a Viking II, a Ranger, and a Valiant followed me home from the Huntsville hamfest.   I suppose you could say I have Three Little Johnsons.  I have always wished for a big one too!
The Ranger is so little you can't even swap the modulator plate caps.

 ;D