Help: SB-401

[Gerald C. Lemay]

Using a 10 turn trimmer pot for carrier balance is an excellent suggestion
and one. I've done this in a number of old rigs with great success. Also, be
sure to replace the carrier balance resistors with modern carbon film or
better resistors. Those old carbon composition resistors are bad news. They
absorb moisture over time and change value and are also unstable with
temperature.

73/Gerry/W1ID

-----Original Message-----
From: Heathkit Owners and Collectors List
[mailto:HEATH at LISTSERV.TEMPE.GOV]On Behalf Of Stewart Lyon
Sent: Tuesday, January 11, 2000 12:01 AM
To: HEATH at LISTSERV.TEMPE.GOV
Subject: Re: Help: SB-401


Chuck,
I've recently rebuilt three SB-401s that had problems similar to yours.
In all 3, I replaced the carrier balance pot with a small 10-turn trimmer
of the same value. The trimmer pots are Bourns 68WR200 and were available
at a local radio store. Newark and Digikey both should have them
(unconfirmed). You have to bend the wire leads out slightly to fit the
hole pattern of the old pot but otherwise the fit right in. And having
10-turns makes it easy to obtain a zero balance.

Using the technique in the SB-401 manual and a hi-frequency scope at the
antenna jack as an indicator, I was able to adjust for 50db ratio between
the suppressed carrier and a full cw output -- which is not bad.

73, Stu - W6CUX
Winnetka, CA
ARRL Life Member

On Mon, 10 Jan 2000 19:58:47 EST Chuck W4MIL <CEMILTON at AOL.COM> writes:
>Fellow Heathens--
>
>I have been restoring a very mistreated 401 and finally reached the
>point
>where alignment is the only thing remaining.  New caps, New tubes, ALL
>crystals installed for stand alone operation, wafer switches cleaned,
>et. al........
>
>The problem I'm having is in the Carrier Null adjustment.  The R and C
>components just won't allow me to balance the thing.  I get good
>voltage
>readings in the USB position but when switched to LSB, the voltage
>cannot be
>brought to the level achieved in the USB position.  I checked the
>usual
>components-----no problems.  Front-to-back resistance measurements of
>the 4
>diodes in the bridge indicate they are o.k. in that they measure about
>the
>same each.  Tried another LSB crystal and no help.
>
>Anyone got a clue?  Could there be anything in the instructions that
>may be
>printed incorrectly?  I'm open for suggestions/help.
>
>Thanks in advance es 73 de W4MIL
>
>Chuck

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