{Collins} Re: Xtal Calculator


Actually there isn't much to the modification - just install the coax
between the grid of V3B (pin 2) and the spare RCA jack on the 75S-3B rear
panel, and disconnect C78. I just disconnected the hot end of C78 so it
could easily be reconnected if the modification were removed. I used some
old teflon dielectric coax that I had that is about 3/16" O.D., but any
small 50 Ohm RF coax should work fine. I dressed the coax along the
bandswitch shields. One thing to consider is the length of the cable from
the signal generator to the receiver. Since this mod results in an
open-circuit transmission line connected to the generator, the cable should
not be longer than about 1/8 wavelength at the highest crystal/generator
frequency. If it were 1/4 wavelength long, for example, you would present a
short to the generator, and you don't want that. So keep this in mind and
use as short a cable as possible.

Check to see if your signal generator output is AC coupled or has a DC path
to ground. Although the DC voltage at the grid of V3B drops to about zero
when the oscillator is disabled, I would recommend AC coupling to your
generator if it is not already AC coupled. A .01uF capacitor in series with
the generator should work fine. You can add this to the internal mod in the
receiver if you like.

The handy thing about the mod is that the crystal oscillator is
automatically disabled when you plug the signal generator into the 75S-3B
rear panel connector. I did some checking and found that loading the V3B
grid with a resistance of about 500 Ohms or lower would kill the crystal
oscillations, which must be done if the substituted signal generator is to
function properly. Since a connected 50 Ohm signal generator results in a 50
Ohm load to the V3B grid, this works fine to disable the crystal oscillator.
Keep this in mind if your generator has, for some reason, a high source

Here are some thoughts about the injection levels. Before installing the
mod, I measured the existing oscillator levels at the grid of V3B and found
them to vary from perhaps 8vpk-pk to a bit over 20vpk-pk, depending on the
band. I was concerned that with my signal generator level, too much gain
would be lost if I didn't have enough injection level. My generator will put
out +17dBm into 50 Ohms, which winds up being about 9v pk-pk into a high
impedance. When I later compared receiver gain after installing the mod, I
found the gain with the modification to be about 3dB lower on the upper
bands (20m-10m), and perhaps 1dB higher on the two lower bands. Sensitivity
was not impacted. Also, since I operate my 75S-3B and 32S-3 in transceive
mode, I wanted to see if the 32S-3 power was impacted. The difference was
insignificant. On 15m, for example, I only had to increase the Mic.Gain by
about 1/2 an increment to achieve the same operating condition I have when
using the crystal oscillator.

That's about all I can think of. As mods go, this one is pretty darn simple.


----- Original Message ----- 
From: "Jerry Kessler" <N4JL@xxxxxxxxxxxxxxxx>
To: <don.jackson@xxxxxxxxxxx>
Sent: Saturday, July 10, 2004 10:35 PM
Subject: Re: {Collins} Xtal Calculator

> Sounds like a nice piece of work, Don.  I would certainly like a copy of
> the spreadsheet and the synthesizer mod details if you already have that
> prepared.
> Thanks for sharing.
> Jerry Kessler
> N4JL
> At 07:50 PM 7/10/2004, you wrote:
> >Hi All,
> >I recently did a minor mod to my 75S-3B to allow injection of my Fluke
> >6062A sythesized generator into the crystal oscillator (a poor man's AOR
> >DDS-2) to allow general coverage operation. This works great, but I
> >a convenient way to quickly calculate the proper crystal frequency (or
> >signal generator injection frequency), preselector band, and preselector
> >tuning for a given desired receive frequency. Although I suspect there
> >many computer programs out there already, I haven't actually seen one. So
> >I created an Excel spreadsheet that does this. In addition, you can input
> >a known crystal frequency and it will calculate the receive frequency.
> >program calculates only the ideal frequencies and stttings per the
> >manual, not other possible frequencies due to image, spurious, etc.
> >
> >If anyone would like this spreadsheet, just drop me an email and I'll
> >it to you. Also, if you want details on the modification, I'd be happy to
> >pass it on. It is very simple, requiring only installation of a piece of
> >miniature coax from the grid of V3B to the spare RCA jack on the back
> >panel, and disconnection of C78, which has about the same capacitance as
> >the unterminated piece of coax. (C78 is omitted in the 75S-3C for the
> >basic reason)  The beauty of this scheme is that when you disconnect the
> >signal generator from the coax connector on the rear panel, the 75S-3B
> >returns to normal operation, using its installed crystal bank. Of course,
> >you really need to have a synthesized signal generator lying around to
> >pull this off for good SSB operation.
> >
> >Don
> >W5QN
> >Sponsored by the Collins Collectors Association
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