IC2SRA Summary: Discriminator output tap Modification instructions for the Icom IC 2SRA Handheld kingpin@l0pht.com L0pht Heavy Industries May 22, 1997 Abstract The ICOM 2SRA is a 144MHz FM transciever and wideband (50-905MHz) receiver HT, ideal for mobile communications and ham radio. With the increasing popularity in digital data monitoring, using protocols such as POCSAG, FLEX, and MDT4800 for Mobile Data Terminals, audio signals from the speaker output are not the most reliable. Modification of the radio to get a tap from the discriminator output will let us receive the best quality digital signal, with no audio filtering and amplification (which will "massage" the signal for the best quality within the voice range). Schematics and system block diagrams are supplied with the Icom 2SRA. By using these, I could find which IC was the FM IF and tap the output signal before it went to the filter circuitry. I added a tiny external SPDT switch to switch the "Speaker Out" jack between standard speaker output and discriminator output. This leads to hardly any external mess and keeps the radio looking nice, with minimal destruction to the internals of the radio. The modification I performed consisted of disassembling the entire radio, desoldering a board, drilling holes in the PC board, and resoldering connections. If done correctly the first time, you will be able to skip a lot of the heartache I had to deal with (which is part of experimentation, of course). The modification can be simplified by not adding an external switch, and just bringing out two wires (discriminator output and ground) to connect to your POCSAG decoder, etc. Upon examination of the block diagram, I found that the best place to tap the discriminator output was after the Motorola MC3372 Low Power Narrowband FM IF. This chip can "perform single conversion FM reception and consists of an oscillator, mixer, limiting IF amplifier, quadrature discriminator, active filter, squelch switch, and meter drive circuitry." Pin 9 of this IC is the "recovered audio". This is what you want. Modification Disassembly of the radio is a time consuming and delicate process. Be prepared to spend quite a few hours (if not days) working on this project, as you don't want to screw up a perfectly good (not to mention expensive) radio. The modification is well worth it, though, and will make your data reception quality magnitudes better. 1. Remove all screws from external casing and pull apart 2. Take out the small PC board at the bottom of the circuit. This UT-63 Tone Squelch Unit comes standard in the U.S. versions of the radio, but is optional for other countries. 3. You will see two metal boxes in the center, these are the transmit (TX) and receive (RX) modules. They are removable to keep the rest of the radio circuit modular and so a technician could easily replace a faulty unit or convert a 2-meter (144MHz) rig into a 440MHz rig. Disconnect both boxes from the connectors (they are screwed together). Take off the screws so you have two separate modules. 4. The MC3372 IC is inside the receiver module (the top one). It is a 16-pin SOIC package. Peel off the gold tape and pop off the cover. 5. If you are going to mount an external switch to switch between standard speaker output and the discriminator output, be prepared. The easiest thing to do would be to solder a piece of wirewrap wire to pin 9 of the MC3372, drill a hole in the case, and pull the wire through. This is simple but messy, unstable, and not recommended. 6. The next steps involve finding the traces on the PC board that go to the "Speaker Out" jack. Using a multimeter, find which pad on the PC board nearest the jacks (Jack PCB) is connected to the tip of the "Speaker Out" jack. The tip is the speaker output, the middle is for microphone input, and the sleeve is ground. Once you've found the pad (it is a fairly large one), mark it with a permanent marker so you don't forget where it is. 7. Look towards the bottom of the PCB, you will see a single row of solder pads. Find which one of these is connected to the pad you just found in step 7. If you look on the schematics, you see that the "Speaker Out" jack is connected to another board via an interconnect, which is connected to the audio filtering circuitry. What we are going to do is cut the trace before it gets to the speaker jack, so we can connect our switch. I believe the pad you want should be the 5th one from the left, but double-check on your own. 8. Once you are SURE that you have the right pad, you need to cut the trace that connects it to the pad in step 7. Unfortunately, this trace is on the UNDERSIDE of the PC board and requires desoldering and even more disassembly of your radio. If you do this right the first time, you won't need to go underneath and can leave the rest of the radio intact. Using a very small drill bit (I used a small hobbyists handheld drill), drill above and below the wire soldered to the pad. This way, you free the wire from connecting to the "Speaker Out" jack. Use an X-Acto knife to make sure no traces are touching each other, and use the multimeter to double-check that the connection has been severed. 9. Now you need to solder the connections to your small SPDT switch. Using wirewrap wire, you need to solder to three points: o The wire you just drilled around in step 9 connects to the middle terminal of the switch. o Pin 9 of the MC3372 connects to the left (or right) terminal of the switch. o The pad from step 7 connects to the right (or left) terminal of the switch. 10. Plug the TX and RX modules back into the radio, connect the AC adapter, and turn the radio on. If everything has gone right, the audio output (both through the internal speaker and the "Speaker Out" jack) will work only with the switch in one position. When you switch to the other position, you should only hear a faint hiss. Discriminator output is unfiltered and unamplified, and the signal is usually 800mV - 1V P-P. Volume adjustments and squelch do not work with the discriminator output. 11. If something isn't working right, go back and check all the connections and cuts you made. My modification did not work at first, and as mentioned above, I had to disassemble the rest of the radio to get to the underside of the Jack PCB. I am not going to go into specifics, since its practically impossible to describe step by step. My problem turned out to be a shorted connection between pins 4 and 5 where I drilled through the pad. Be very careful if you have to take the whole radio apart and make sure you don't lose any parts or stress the flexible interconnect cables, and don't forget how to put it all back together. 12. If your radio behaves as expected, you are at the final stage. I used a small amount of Epoxy to mount the switch to the left side of the radio (the side WITHOUT the "transmit" and "function" buttons). 13. Mark and cut (using a Dremel tool) the other half of the case where necessary - in order for the switch to fit snugly. 14. Double-check that it all works correctly and screw everything back together. Depending on your switch type, you might need to protect the metal contacts with a dab of Epoxy and/or electrical tape to keep them from shorting against the inside of the metal case. 15. Close up the radio and hope for the best! If you are unsure of anything, feel free to contact kingpin@l0pht.com. I take no responsibility if you destroy your radio. I am assuming you can read schematics and follow block diagrams. The step by step above is to be used as a guideline, and not taken as gospel. This modification does work, though, and if you do it right you will be very happy. Adios. -kp Appendix Motorola MC3372 data sheet: http://sps.motorola.com/books/dl128/pdf/mc3371rev1g.pdf Other modifications of this radio include a 138-174Mhz extended receive and transmit mod. This can be found on any radio modification site. You could also try to contact TPDugan@dockmaster.ncsc.mil. For more information on discriminator modifications, see "Discriminator & Baseband Audio Outputs for Scanners" by Bill Cheek (bcheek@cts.com), posted often on rec.radio.scanner.