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



Before doing anything, you should measure all the BFO frequencies with a frequency counter.  You might save yourself some expense and headaches.  It's always best to confirm the source of the problem before paying for the assumed solution.

-----Original Message-----
From: Collins [mailto:collins-bounces+gahart=ix.netcom.com@xxxxxxxxxxxxx] On Behalf Of Gary J FollettDukes HiFi
Sent: Tuesday, November 25, 2014 12:18 AM
To: Mike Waters
Cc: collins@xxxxxxxxxxxxx
Subject: 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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