[Heathkit] Update (was DX-100B on Phone - bad audio distortion /oscillation)

[Rick Poole]

OK, here's where I'm at.

I have reversed the SB-10 mod and put the RF section back to where 
it's was originally (there are some mods in the audio section I 
haven't backed out yet but I'm not using the audio section right now 
and have removed the modulator tubes).

I replaced all of the caps in the PA section, and loosened and 
retightened all the mounting screws that ground anything.

Objective is to get it absolutely stable and reliable in CW and then 
deal with any remaining AM problems.

Connect the dummy load and tune it up on 3900 KHz.  At 5 ma grid 
current, 200 ma plate current, and 800 volts plate voltage (160 watts 
input) I get about 80 watts out on the wattmeter... 50% efficiency, 
not too good, but it is what it is (so far).  I'm not sure of the 
accuracy of the wattmeter, the VOM, or especially the DX-100's plate 
current meter, so any of those readings and calculations could be off.

Turn off the plate... and then, as the plate voltage is falling 
(takes about two seconds to fall to about 200 volts), the transmitter 
goes into some kind of self-oscillation at very close to the 
operating frequency (makes some kind of sense I guess, since the tank 
circuit is tuned to that frequency).

If I put a scope probe on the input to the PA section, the problem goes away.

So... neutralization, right?  OK, if I have to, I'll make up another 
neutralization wand and put it on (had to cut the other one to get it 
off).  But people keep saying that DX-100's don't need no stinkin' 
neutralization, especially with 6146A's, so I don't have a lot of 
confidence that the problem isn't somewhere else.  But, we'll see.

Someone here said that tuning the plate shouldn't affect the grid 
current... well, it does, big time... if the grid is 5 ma at 
resonance, tuning the plate off resonance makes the grid vary from 
just under 4 to about 6.

I'll try the neutralization and see what happens.

Rick WA1RKT