SB-640

[John Dolan]

I have a SB640 with a complete station... It works well together.  The
VFO box is the same as the one that came with the SB101, factory sealed
from TRW.  The thing I just discovered last month is, in the manual for
the SB640, they for the first time provided a schematic of the VFO
circuit.  Both VFOs are parallel-tuned Colpitts circuits with temperature
compensating capacitors.

Also interestingly, there are no values on any of the components in the
schematic.  I thought it was unusual for Heath to sell the SB101 without
a schematic of the VFO, and was annoyed that I had to guess at the
circuit when I wanted to add a Motorola C7 Frequency Steering Circuit
Chip. (by the way has anyone done that or anything like that?)  The
Heath/TRW  VFO was spec'd to drift less than 100 Hz per hour but this C7
is supposed to improve it to 10 hz.
--
Regards,
John Dolan  K3FIG
74 Glacier  near Washington, DC

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