SB-102 mods

[Kenneth G. Gordon]

On 17 Aug 2010 at 9:20, Paul Kraemer wrote:

> Have encountered several modifications to an SB-102
> Not being intimately familiar with the 101 / 102 I am asking group for
> anyone recognizing the mods and their possible validity #1 An extra
> trim pot in series with the original S meter zero.

Paul: Heathkit "experimented" with several very different "S" meter circuits in 
the HW/SB transceivers. There are at least two that I know of for the SB-102 
alone. In your case, probably the resistors associated with the "S" meter 
changed value enough that the original owner could not zero the "S" meter 
and added that pot to compensate. 

However, I also suspect that your "low volume" problem may have 
contributed to this problem. 

Your calibrator should show at least +20 db over S-9 on 80 meters with 
everything peaked. If it doesn't, something has low gain: probably that mixer 
I mentioned in a private e-mail. I know that in my own case, fixing the "low 
volume" problem also fixed "S" meter problems.

> There are several
> other resistor changes as well. #2 An added trim pot added to the
> board near V19. Turning the pot changes the frequency.

If I am not mistaken, V-19 is a 12AT7, both sections of which are used in the 
heterodyne oscillator, one section is the oscillator and the other section is a 
cathode-follower. It would help if you could tell us where the pot was added 
in that circuit.

Changing the heterodyne oscillator frequency WOULD change the frequency 
you are tuned to. 

One thing to keep in mind is that the heterodyne oscillator frequency 
relationships determine the center-tuning of the VFO: when the het crystals 
change frequency with age, center-tuning can be vastly different on each 
band, which can be very annoying in that the calibrator points will be all over 
the dial rather than centered, so that you have to re-center the tuning every 
time you switch to a different band.

Perhaps the original owner added this pot to compensate for this problem. 
However, if so, that is poorly done.

What SHOULD be done instead is to adjust each crystal for the same 
relative frequency so that the dial is centered on each band.

Ken W7EKB

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