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scratchytcarrier

Tapping your PRO-97 or 2055 made easier

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Based on a post from a certain popular scanner mods site, without the author's or site's permission. This is all public knowledge anyways. Have phun.

FM discriminator tap access
Radio type: Rat Shack (GRE) PRO-97 or PRO-2055

0. This procedure is shown using the GRE Pro-97 handheld from Rat $hack. If you have the Pro-2055 tabletop unit the procedure is mostly the same since they are both the same radio, just packaged differently. The test point #4 used to access the baseband output is in a different spot on the 2055's motherboard but it is wired the same way. Depending on the type of jack you decide to use, you might be able to skip connecting a ground wire to a point on the board and use the metal chassis as your ground point instead, but I don't have a 2055 anymore and haven't tried it.

1. Prepare a 1/8" or 3/32" stereo or preferrably mono headphone (TRS) jack or a panel-mount RCA jack by soldering a piece of wire (signal) to a 10000 ohm 1/4 watt (brown-black-orange-gold) resistor. Solder the resistor to the "tip" terminal of the jack (as shown in the picture below). The article's original author used a Radio Shack part number 274-249 1/8" jack here and used blue and black wires for the signal and ground lines respectively. I (scratchytcarrier) used a RCA jack which provides a little more leeway with placement inside the radio housing. You will only have audio on the left channel if using a stereo patch cord and TRS socket. Solder another piece of wire (ground) to the "sleeve" terminal of the jack as shown above. After soldering, wrap all the terminals with electrical tape, shrink tube or pot them in epoxy or silicone sealer.
3849_pro-97_discriminator-2.jpg

2. Remove the battery cover and battery holder.

3. Remove the four screws and lift the back cover off of the scanner.

4. Locate "TP4" (test point #4), which is a small wire pin. This is connected to pin 9 of the 3361 chip (this is the discriminator output, providing baseband FM audio). It's shown in the picture below with the blue wire attached. CAREFULLY solder a wire to TP4 or crimp it on if you can get a small enough pressure fitting like I did.
3849_pro-97_discriminator-1.jpg

5. Now you'll need a place to connect the ground wire. I tinned the end of my ground wire and slipped it under a prong on one of the metal shields. Easy peasy.

6. Next, run the two wires through a hole in the chassis. The original author ran his through the lower right screw hole. For a more permanent connection, drill a hole through the plastic case to mount the jack. (See rebuttal in step #9.)

7. Replace the battery pack and the battery cover.

8. Secure the chassis with 3 screws (or all 4 if you drilled a new hole).

9. The original writeup specifies using the belt clip to hold the stereo jack as shown in the photo below:
3849_pro-97_discriminator-3.jpg
Rebuttal: this is not only ugly and unprofessional but also negates usage of the belt clip, so forget true portable operation if you do it this way. I drilled a hole in the housing just a little bit southeast of the computer jack when the radio is stood upright (shown near the bottom left in this photograph) and mounted mine there. This positions it just above the 24-pin header shown in the middle picture.

10. You're done. Plug one end of a patch cable, with an appropriate adapter if necessary, into the jack and the other end to the line input of that old dedicated XP box in the corner's sound blaster. Use a Y-splitter if you need to. Now go find a nice hot packet/DMR/P25/paging frequency, fire up your decoder software and go nutz. If you've only used the headphone jack for monitoring before then you will immediately notice the increase in performance, since you're getting nice undistorted, wideband audio straight off the demodulator IC (in other words, without the audio preamp, filtering, AGC and all that crap getting in the way. Yuck-oh, folks). You will also be able to use a data slicer if you have one (cough cough L0PHT cough cough). The baseband output, with the resistor, is effectively a standard 600mw 10K-ohm line-out port, so you could even connect it to an input on the stereo system or a tape recorder you wanted to. Note that the squelch (carrier and CTCSS) is not usable since that stage comes far after the discriminator, thus there will always be audio (including static) present on it. You will, however, obtain clear CTCSS or DCS tones from the discriminator output which can be useful e.g. for activating an automatic recording device, or keying the output side of an improvised repeater.

EOF

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