SB401 Carrier Null

[Stewart Lyon]

Hi John,
Sounds to me like you have a bad (open) null pot. According to the
service bulletins, Heath had a lot of problems with this pot. I bought a
basket-case -401 and had the same problem. I replaced the pot with a
10-turn 100-ohm trim pot. It is abt 3/8" x 3/8" x 3/16" thick with the
adjust screw out the top and the 3 wire leads out the bottom. I mounted
it on the top side of the board - the wire leads are the same spacing as
the original pot. Even though it was wire-wound and 100 ohms instead of
200, it worked fine.  I used a high frequency scope connected to the
antenna jack and set at its most sensitive range as an indicator.  It was
neat to watch the output stair-step through null as the pot jumped over
its wires. I was able to get a calculated -60db differance between full
CW and nulled out SSB.

As to the note from the -301 --- are you using the transceive or separate
mode?? If you are using the separate mode, it could be that the LSB
crystal in the -401 and in the -301 are not quite the same frequency and
some of the -401 LSB is getting to the product detector of the -301.  Try
disconnecting the cables between the BFO and HET I/O or going to the
transceive mode.

Another problem I found was that the LSB crystal in the -301 was slightly
low in frequency, putting the carrier way down on the slope of the both
the -401 & -301 filters. You can tell if you have this problem by
switching back & forth between LSB/USB on a noisy band.  The noise "hiss"
frequency should be the same. I was able to raise the frequency of the
LSB crystal by juggling the capacitors in the BFO oscillator.

Hope this helps es 73
Stu -- W6CUX



On Sun, 24 Jan 1999 16:29:10 EST John Garko <Jgarko at AOL.COM> writes:
>Hi all Greenies... I need a little help please. I am unable to null
>out the
>carrier using the carrier null control. It seems very sensitive to the
>touch.
>Also when listening on head phones from the SB301 in lower side band,
>there
>seems to be a low tone of sorts when i key the mike. It is not dead
>silence.
>Should it be? A little insight on this would be great.
>Thanks in advance,
>John
>WA4KWK

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