SB644 Ext VFO & SB644A Ext VFO

[Kechkaylo, David]

Gene Holcombe:

I don't have an SB644(A) remote VFO. However, the major differences between
the two are:

A).  The VFO has been updated to those of the electrical equivalent of the
VFO in the SB-104A; namely, the zener-regulated supply has been stabilized
and the the temperature compensating capacitors modified to reduce thermal
drift. The first condition was especially apparent when the +11-volt supply
was too low, and the original VFO utilized a 10-volt zener diode reference.
The result was FM-ing of the VFO output signal, especially noticed when the
SB-104 was used in mobile applications. Subsequent versions changed this
zener to a 7.5-volt unit. Interestingly, the very original VFOs of the
SB-104 used a reverse-biased #417-118 E-B junction as this junction had a
reverse breakdown of about 8 to 8.5 volts, but was removed in favor of the
zener diode after the proofbuild stage.

B).  The original SB-644 had problems of VFO feedthru between the two VFOs.
This was caused by lack of isolation when the remote VFO was in the off
position. In the original SB-644, one could hear interaction on the SB-104
while tuning the VFO on the remote SB-644 even in the off, or bypassed,
mode. The SB-644A attempted to correct this problem.

Regards,

Dave Kechkaylo

> ----------
> From:         Gene Holcombe[SMTP:n5zdf at NETDOOR.COM]
> Reply To:     Gene Holcombe
> Sent:         Saturday, July 24, 1999 3:52 PM
> To:   HEATH at LISTSERV.TEMPE.GOV
> Subject:      SB644 Ext VFO  &  SB644A  Ext VFO
>
> Having made the plunge into obtaining a SB104A, I have to make a decision
> for an ext VFO, one is a SB644 and the other is a SB644A.  Question..What
> is the difference in the units and will either work on the SB104A.  Anyone
> have one available at a reasonable price?
>
> Gene  N5ZDF
>

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