Hi, Tony

It could be that someone added an emitter follower
to the receiver BFO circuit, in order to bring the BFO
signal to the transmitter via coax. A drawback to the S/Line
is that the transmitter BFO crystal can be different
in frequency than the receiver BFO crystal due to manufacturing
tolerances, making exact zero-beating difficult in transceive mode.

73,
Ed Knobloch

Antonio Vernucci wrote:
Noone answered this question in the Collins newsgroup, so I try it here.

I recently inspected a Collins 75S-3B receiver, and I found that someone had modified the BFO circuit with the addition of a transistor (I have not determined whether the transistor actually replaces the BFO oscillator tube, or it instead just amplifies the BFO signal).

The receiver works fine, but I cannot figure out the reason for the modification.

I would tend to believe that:

- either it is a well known modification (but for which purpose?)
- or it was just a way to circumvent the problem of a weak BFO crystal that does not start oscillating with the BFO tube alone. As BFO crystals are not easy to find, that could be a possible justification.

Thanks in advance for any suggestion.

73

Tony I0JX