Re: {Collins} S line zero-beat issue in Transceive Mode?



When you connect the 2 RF cables, these only carry ver the crystal local oscillator and the PTO. The BFO signals do not get shared as they were assumed by the designers to be exactly the same.

Unfortunately, they were not.

Replacing the crystals will not guarantee that the BFO’s in the receiver and transmitter will be right on. Other aged components can affect that as well.

There is a “BFO Test” RCA jack but this does not act as a means to share the BFO’s between the two units.

Still, the best way to ensure exact frequency tracking is to add the trimmers to the BFO on either the TX or the RX.

Gary


> On Nov 24, 2014, at 10:51 PM, Mike Waters <mikewate@xxxxxxxxx> wrote:
> 
> I didn't look at the schematics --and it's been too many years-- but in the
> transceive mode with the 2 cables connected, would the BFO crystals in the
> transmitter matter? It seems like there was a reason that I had to trim the
> BFO crystal in the 75S-3  rather than the 32S-3.
> 
> 73, Mike
> www.w0btu.com
> 
> On Sun, Nov 23, 2014 at 6:27 PM, Gary Follett <dukeshifi@xxxxxxxxxxx> wrote:
> 
>> Would it not be simple enough to add trimmer caps across the BFO crystals
>> in the transmitter? The mechanical,filters,are very correct in terms of
>> passband position so the error in crystal frequency can only cause one or
>> the other sideband to be incorrectly positioned anyway. I picked the
>> transmitter, for this alteration since the effects old be less noticeable
>> for,the user. By adding these trimmers, you would zero beat the TX and RX
>> just the way you do when matching USB and LSB on the PTO.
>> 
>> Alternatively, one could ditch the notion of using the PTO offset for
>> matching USB and LSB and use the TX keying line to pull the PTO's when
>> going fro TX to RX. This would be quite easy to do.
>> 
>> Getting two new XTALs from INRAD is no guarantee these will operate on the
>> same frequency anyway. Either of the simple alterations I just described
>> will accomplish the feat perfectly.
>> 
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