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

[ne1s at securespeed.us]

>
> The SWR goes off the scale, which is a dead giveaway that the
> transmitter is putting out RF on pretty much any frequency but the
> one it's supposed to.
>

Perhaps a VHF parasitic, then? If it was a neutralization problem, the
final would be oscillating fairly close to, but not exactly on, the
frequency it's tuned to. Can you locate the frequency in a monitor
receiver? Check out the parasitic suppressors - have they been modified or
otherwise screwed up? Another thing that might help if it's a parasitic is
"stopper resistors" on the 6146 grids - say, a 33 ohm or 47 ohm resistor
in series with each control grid, right at each 6146 socket.

> It's happening in CW now, too.  It happens reliably when I tune a
> little off-resonance on the high side, i.e. the Plate pointer points
> to a higher number, and the plate tuning capacitor is more
> meshed.  Maybe that's why I didn't notice it on CW before.
>

OK, that makes sense now. The modulator is out of the picture.

> The PA has a neutralization modification whereby a wire is routed
> from the driver capacitor to somewhere near one of the PA tubes, then
> you move that wire back and forth to find the sweet spot for
> neutralization.  I have tried every position of that wire and nothing
> works.
>
> I've ordered some 6146A's to replace the 6146B's installed now, since
> some report problems with the 6146B's, and I know that some Collins
> KWM-2's, including mine, don't play well with -B's.  We'll see what
> happens when those arrive.
>

I think you are on the right track.

> I'm just trying to figure out whether I should remove the
> neutralization mod before installing the 6146A's.
>

I wouldn't - I'd see if I could neutralize it after replacing the tubes,
and only modify or remove the neutralization stuff if that wasn't
possible.

73/GL,
-Larry/NE1S