Plectrons

[N9JIG]

Well, my BC396 has really got me into the Fire Tone bit big time! I have acquired a few Plectron 720's and would like to set them up for some local stuff. The frequency is controlled by a drop in crystal, similar to the old crystal scanners, and I can order the crystals from a couple sources. I even figured out the formula, it is pretty simple (For Highband, just subtract 10.7 from the actual freq and then divide the remainder by 9).

These R720's use tuned coils for the tones however. For that I have a few questions I will be researching:

Is there a fairly simple way of tuning them to different tones than they are on now?

If they are so tunable, is there a limit as to how far I can retune one? For example, on of the tones is now set to 1600.0 Hz according to the ,lable and prior user, can it be adjusted to say 881.0?

If I am able to make these work for me I plan on replacing the old RCA jack antenna connector with a BNC as I did with my existing units.

 

[wa8pyr]

These R720's use tuned coils for the tones however. For that I have a few questions I will be researching:

Is there a fairly simple way of tuning them to different tones than they are on now?

If they are so tunable, is there a limit as to how far I can retune one? For example, on of the tones is now set to 1600.0 Hz according to the ,lable and prior user, can it be adjusted to say 881.0?

Simple answer: no.

The way these things are set up makes them almost impossible to retune the tone boards without major surgery on them. The L/C relationship is what determines the frequency; if you have a service manual and can figure out which capacitor is the primary determining element in conjunction with the coil you *may* be able to shift the frequency a few tens of hertz one way or the other, but I suspect if you went too far you would adversely affect the stability of the tone board.

An easier solution would be to look at the aftermarket pager decoder boards; some of these had dipswitch-selectable tones. The only one I can think of offhand is the Comm-Spec SD-1000, which has unfortunately been discontinued. If you can find one that would be the way to go.

http://www.com-spec.com/sd1k.htm

Might be time to put that ham training to use and build a decoder ;-) Here's a chip for the purpose:

http://www.eetasia.com/ARTP_8800303291_499481,499493.HTM
http://www.cmlmicro.com/products/innovate/docs/Inn823_1.pdf#search='twotone paging decoder'

Tom WA8PYR

 

[Al42]

I even figured out the formula, it is pretty simple (For Highband, just subtract 10.7 from the actual freq and then divide the remainder by 9).

Be aware that the loading capacitance can pull the crystal quite a way off frequency. Always order from a company that has the specs for the receiver you're ordering for, and tell them what model and rev number receiver you want the crystal for. It won't cost any more, but it will ensure that a) the crystal will be on frequency and b) if it isn't, they'll take it back and recut (or replace) it.

 

[Hoseman292]

Sorry to bring an old thread back to life.

The decoder can be set by reading the RMS at the decoder for tone A at a test point on the decoder then documenting it.

This requires test equipment.

Generate Tone A and watch for a voltage change at TP1. Tune the corresponding capacitor to the documented voltage.

Repeat this step for Tone B.

Generate two tone sequence to check decoder operation.

This is from memory. Sorry I could not be more specific.

TMB

 

[BamaScan]

I used a Plectron to monitor my local police for about 6 years until they went digital. So I set it up to listen to my local Fire Department. I bought the crystal from Bomar and it worked for about 3 years. The crystal drifted away form the frequency. I was looking for two reed tones and could not find them anywhere so I gave up trying to put in the reed tones. I still have the Plectron and thought about getting a crystal for a ham repeater or my local fire department again. The crystals were real expensive and drifted away so I'm scared to buy another crystal.

 

[parnass]

I still monitor a few local VHF high band frequencies using Plectron R720 receivers. Had to replace a bad pass transistor in one of them recently which died after several years of faithful service.