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

[lee]

Rick,

Yes, if the rig is neutralized, the plate tuning should not affect the grid 
current.  (Assuming the power supply is not changing things when you do this 
or from some quirky problem and all other things being equal).  Since 
nothing is perfect the grid current should change very little, worst case. 
If it changes BIG TIME, the rig needs neutralization.  I think when guys say 
a DX-100 does not need neutralization, they really mean most don't need very 
much and you can live with the rig as is without neutralizing it.  That does 
not mean it is neutralized.  Couple this to the fact many hams don't use the 
rig on 10 or 15 meters so they easily could use their rigs without 
neutralization.  I know many guys way back when did neutralize they DX-100 
and also the Apache using that wire wand trick.  I did it way back when and 
it worked great on both my DX-100 and my Apache.  I was able to have the 
grid current hardly move at all to the eye looking at the meter when 
neutralized on 10 meters.  This was a Heath approved mod, so it must have 
been warranted by many.  What I don't understand is why your grid current 
change is so dramatic.  I never experienced this with my un neutralized 
DX-100.  Neutralization helped, but the rig was never that far off to begin 
with.  My grid drive change before neutralization but it was never that 
extreme as you indicate.  Of course, different tubes could produce different 
results, but I would guess the changes would just be slight from one pair to 
another.  I have even used 6146B tubes in a DX-100 with no problem.  Again, 
I don't understand why you see this big a drive change.  I'm guessing, but I 
would think there is more to this then just needing neutralization.

I know you changed caps out in the PA.  You said the  oscillation  goes away 
when you put a probe on the PA input.  That said, as a suggestion,  how 
about doing this same thing with the 5763 driver as you did with the PA plus 
check out the resistors there too plus the caps on the input and output 
sides of the 5763 driver pi-net which go to the 6146 grids?  You have a 
bunch of .005 caps there and a .001 blocking cap.  Do you have a bad one or 
again, maybe a bad solder joint on one of them?  Are the voltages good on 
the 5763?

I don't know if any of this helps, but you need to try something.  I think 
this would be my next step.  Just my thoughts.

Lee, w0vt


-----Original Message----- 
From: Rick Poole
Sent: Friday, December 16, 2011 12:35 AM
To: heathkit at mailman.qth.net
Subject: [Heathkit] Update (was DX-100B on Phone - bad audio distortion 
/oscillation)


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

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