[AMRadio] 6146A's Arrived (was DX-100B on Phone - bad audio distortion / oscillation)

[Jim Wilhite]

You should take a very close look at the audio section Rick.  I recall 
you saying that you could turn the mic gain down and nothing happened 
but the mic gain is between the first and second stage of the audio amp. 
If you have a scope then put it on the grids of the 1625s and turn it 
on.

If you see an oscillation there move to the grid of the 12BY7.  If its 
there, you need to look in the vicinity of the 2nd half of the 12AX7. 
This sounds like a poor ground somewhere in the audio chain to me. 
These oscillations can, sometimes, be a bit difficult to locate but 
when you narrow it down to a stage then check all components in that 
stage, resistors, caps, grounds and solder connections.  Be sure the 
tube socket has a good ground by loosening the screws holding the 
sockets and retightening.

For years I avoided Heath equipment because one never knows who built it 
and with what.  Recall the statement in the manuals telling everyone not 
to use acid core solder..  Construction practices varied widely on these 
old transmitters so, it appears, you will have to do a lot of searching.

Also don't forget to change the 12AX7 and 12BY7 just for grins.

Jim/W5JO

----- Original Message ----- 



> Well, the 6146A's arrived today, to replace the 6146B's in my DX-100
> which are suspected of causing my spur / self-oscillation problem.
>
> I bought four of them (I could use a couple of spares for my KWM-2A
> anyway) and took the two that matched most closely on my TV-7D and
> put those in.
>
> No joy... self-oscillation is as bad as ever.  Tried several
> positionings of the neutralization wand and then removed the
> neutralization wand altogether... still no joy.
>
> So, I got the XYL to help me wrestle the thing over to the workbench
> (feed me the line cord through the back of the cabinet and then move
> the cabinet out of the way), whereupon my Simpson 260 fell on her
> foot, so she is not happy... :-(
>
> But it's now out of the cabinet and ready for me to start going over
> it from front to back and try to figure out what's happening.  Stay 
> tuned...
>
> Rick WA1RKT
>
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