SB-104A Problems

[Kechkaylo, David]

Bill:

There is a CW keying modification which is part of the SB-104 upgrade which
involves a couple of wiring changes on the rear section of the CW/mic level
pot, plus the addition of a 5uF electrolytic cap. Your SB-104 level didn't
come that way in the first four production runs. I'd check the wiring
changes there. I have an SB-104 update mod kit manual if this would help.

I had the very same problem you described recently on my SB-104 manual #1
unit and this pot was pretty old (early 1974 vintage). Direct replacement of
this two-section pot from another SB-104 cured the problem.

The use of either the new (85-1931-1) or the old (85-1459-2, 85-1666-1 or
85-1708-2) receiver front end board installed in compartment "G" of your
SB-104(A) will not cause the RF gain control to work incorrectly; rather,
I'd check for a wiring error there. The "A" version upgrade requires a
wiring change there as well, and effectively removes one section of this
dual pot which was used on the older SB-104 version as part of the RF gain
control (adjusted bias on a PNP transistor which shunted RF in the 8.65MHz
1st IF filter to ground). Altho no longer used in the "A" version, the RF
gain control in the SB-104A really should be termed "IF Gain", as it now
only controls the gain in the MC1350P (IC501) IF amp on board "F".

Hope this helps.

Dave Kechkaylo, W8DLK

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