SB 100 Frequency Shift/Jump Problem

[Mark V. Johnson VE3DJU]

Hello all;

I just solved a problem on my SB-100 that I thought I would pass along.
Feedback always appreciated.

I picked up this SB-100 a few weeks ago, no mods, nice shape just very
dirty. Fixed the usual dirty contacts etc. but had an intermittent jump or
change in frequency when it warmed up. Usually this is a dirty variable cap
in the LMO. I pulled the LMO and cleaned out ancient grease and crap, but
that did not fix it. I verified this by putting my general coverage
frequency on 5300kc's and listening to the LMO output and the SB-100 at the
same time. The SB-100 receiver jumped or shifted frequency but the LMO
output did not.

Long story short....The carrier balance pot on the modulator board was bad.
I know this usually shows up when you adjust carrier balance, but when I
aligned the rig and set the balance, it adjusted up OK.

Once I replaced the carrier balance pot, no more frequency shift or jump. I
can only assume the carrier balance circuit loads/affects the 12AU7 carrier
balance/BFO circuit which will affect the receive signal/frequency.

No more intermittent problem and life is good again. Thanks for the
bandwidth. 73's.

Mark V. Johnson

Sponsored by the City of Tempe 

Listserver Submissions:  heath at listserv.tempe.gov
Listserver Subscription: listserv at listserv.tempe.gov - "subscribe heath 'name' 'call'"
Listserver Unsubscribe: listserv at listserv.tempe.gov - -"signoff heath"