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

[Garey Barrell]

Rick -

Just a quick check.  I don't recall whether it's possible or not, but can you swap the plate caps on 
the two 1625s?  If so, you may have reversed them from original.  If impossible, disregard!  :-)

73, Garey - K4OAH
Glen Allen, VA

Drake 2-B, 2-C/2-NT, 4-A, 4-B, C-Line
and TR-4/C Service Supplement CDs
<www.k4oah.com>


Rick Poole wrote:
> 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
>