hw-101 carrier null

[Stewart Lyon]

Hi Jim (and reflector gang),

I have used 10-turn "trim-pots" that I found in a surplus store**  to
replace the carrier null pot. They are about 3/8" square by 3/16" thick
with an adjusting screw sticking out the top and 3 wire leads out the
bottom.   I mounted it on the component side of the mixer board in the
same holes as was the null pot.  I couldn't find a 200 ohm unit so I used
a 100 ohm wire-wound resistor unit.  A purest would probably increase the
end resistors to compensate but I didn't bother. As on indicator, I used
an hi-freq oscilloscope connected at the antenna jack  (careful, it's hot
in there).  It was neat to watch the carrier go through null as the
trimmer jumped over its wire turns.  I used this technique on both a
SB-401 and a HW-101 and was able to null the carrier absolutely out.  **I
think the store was All Electronics in Van Nuys, Calif. They advertise in
QST and also have a web page.

Another problem I found on these rigs was that both the LSB and USB
crystal were low in frequency -- by about 3-500 Hz. This places the
carrier in the wrong place on the slope of the bandpass filter. The
symptoms are higher pitched audio on LSB and lower on USB. If you have a
SB-401 manaul, there's a good description on page 131.  The fix varies
with the rig but generally involves slightly *decreasing* the capacitance
in the grid of the carrier generator oscillator. A counter is a nice
thing to have here.   In the case of the SB-401, the problem is
complicated because the carrier oscillator can be in a -301 (or maybe a
-303)!!

Hope this helps.
73, Stu - W6CUX
Winnetka, CA
ARRL Life Member

On Tue, 22 Dec 1998 14:57:13 -0700 n7kbj <n7kbj at ONEIMAGE.COM> writes:
>Season's greetings to all you heath guru's!  Once again I need your
>advice on an HW-101 problem.  My rig is showing almost full output
>when the mike is keyed and and the mike/cw gain is fully off.
>Attempts to null the carrier produce only erratic changes and no nulls,
unless
>I pull or push a bit on the shaft of the null control.  I have read from

>the list that the 6EA8 at V1 should maybe be replaced with a 6CQ8 to
>prevent V1 from going into occiliations.  However, I have also read in
>the service bulletins that there was apparently quite a problem with
>failures of the carrier null control.  My question is this - which
>problem do you think I have here (or both)?  and if it is the control,
anybody
>have a good replacement source for these little things - manual say it's
a
>200 ohm?  Thanks in advance!!
>Jim - N7KBJ
>

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