[Hammarlund] HX-50 mods

[Robert Nickels]

I reckon there may not be many who have put these updates in their 
transmitters, but for the benefit of anyone who may scan the archives in 
the future, I'll share what I found.

There are 6 or more separate mods and updates for the HX-50 and to be 
blunt, most are pretty badly needed on those transmitters they haven't 
been done on.  The original audio and VOX circuits were just strange, 
and the modifications fix problems like the interaction between Mic gain 
and VOX gain controls.  However it requires significant rewiring and the 
addition of a matching transformer to drive the balanced modulator.   
After some experimentation I determined the original transformer must 
have had a very high primary impedance, as even a new 25K interstage 
transformers  put too much loading on the audio stages.   Knowing that 
engineers tend to be creatures of habit, I guessed that they may have 
just used the same transformer that was already installed for the 
Anti-VOX circuit, and this turned out to be correct.   Since this 
transformer is riveted-in place I didn't measure it's impedance but the 
DC resistance of the primary is 2.95K, which tells you there's a lot of 
wire in it.

It's a simple change to extend two wires and connect the existing 
(riveted) transformer to the balanced modulator, providing the correct 
amount of audio drive as the designers intended.   I then mounted my 
"next best" transformer (Mouser 42TM002) in the space provided and wired 
it to the less-critical Anti-VOX input.   A Mouser 42TM117 (50K to 1K)  
would be my best suggestion, but I didn't have one on hand to try.

As described in the update, it's necessary to add a small trimmer 
capacitor to achieve AC balance of the balanced modulator (the exiting 
pot provides DC balancing).   I had to attach the trimmer to the 
opposite diode in order to achieve best carrier null (this was commonly 
done but is not noted in the update info). Note that the exact carrier 
frequencies are marked on the side of the crystal filter - every 
transmitter is different so you'll need to adjust the carrier osc. 
trimmers accordingly.

The relay modification is highly recommended in order to obtain a PTT 
circuit that is switched to ground, and thus is usable with normal PTT 
mikes.

73, Bob W9RAN