[Heathkit] DX-100B on Phone - bad audio distortion / oscillation

[Rick Poole]

I got my replacement 1625's from RF Parts and put two of them in the 
DX-100B, and fired it up on AM into a dummy load.  All looked good, I 
was putting about 90 watts of carrier into the dummy load (finals 
might be a little soft), got what looked on the scope like 100 
percent modulation and occasionally a little more, and about 350 watts PEP out.

So far so good.

Connected up my off-center fed dipole for 80 and 40, which I have 
used for a couple of years on a few rice boxes and a Collins KWM-2A 
and it always worked acceptably if not spectacularly.  Tuned it up on 
3885 and listened for a while, putting in an occasional call.  Seemed 
to work OK for a while but then the output on the scope just took off 
into some kind of oscillation.  I backed off the audio gain and the 
drive to zero and no joy, had to switch from Phone to CW to make the 
oscillation go away.

Since then, it has been kind of random... sometimes it works fine and 
gets good audio reports on 3885, other times it takes off into 
self-oscillation.

The transmitter is rock-stable in CW, this only happens when I switch 
to Phone, and then not all the time.

The transmitter is grounded about as well as it can be in my 
second-floor shack.  The line cord was replaced with a 3-wire cord 
and so the chassis is grounded to the ground connection in the AC 
socket, and also I have grounded the chassis to a heating 
pipe.  Grounding and RF-in-the-shack has never been a problem with 
either the rice boxes or the KWM-2A (which is to say, we do get a 
little RF in the shack when using some antennas but not enough to 
hurt anything).  Still, I thought it was RF getting into the audio 
input so I put there large ferrite chokes on the mic cord, and when 
that didn't work I disconnected the mic altogether, which helped but 
didn't make the problem go away.

I've monitored the MOD current while this is happening and if 
anything, the mod current dips a tiny amount when it goes into this 
oscillation.

Just wondering if any of you DX-100 owners have run across this and 
have a fix for it, before I wrestle all 100 pounds of iron over to 
the workbench and start digging into it.

Rick WA1RKT