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

Back in the 70s Howard Sartori had an article published in QST concerning
Solid State tube replacements. He developed a line of "tubsters" for Drake
gear and later did some for Collins gear. You may just have one of his in
the rig or one someone built from a construction article.

Dick K8WHA
__________________________
"Antonio Vernucci" wrote in message
...
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

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