Converting Maxon HTs for 9600 Baud Operation

NOTE: This page is still under construction, and is only a skeleton of what I hope will be here soon. I just wanted to get the conversion details written down before I forgot what I did (I'm getting old).

NOTE NOTE (12 June 2000): I'm not sure this is the correct conversion procedure. I'm having problems with the transmitted signal, and am going back to the drawing board. I may need to use a large isolating resistor and a smaller coupling cap to feed the audio in; I think the low impedance of the currently coupling circuit is loading down the varactor and its bias, causing Bad Things to happen.

NOTE NOTE NOTE (7 January 2001): The problem is solved and the radios now transmit properly. I sent myself on a wild goose chase with the impedance idea mentioned above. The problem was a couple of components in the existing audio circuit that created a whopper of a low pass filter that destroyed the modulation data.

Our radio club recently came across a lifetime supply of Maxon model CP-530 data radios. These are actually slightly modified versions of the 80's vintage Maxon crystal controlled handy talkies that used to be very common. Except for details of the data connector, the conversion information here should apply to the regular version of the radio as well as the data version.

These radios are 2 watt UHF units that were originally used in the 465MHz range. They tune down to 440-450 quite nicely. They use a 10.8 volt battery pack, and one challenge is coming up with a simple way to use them from a 13.8 volt supply.

The CP-530 data radio seems to differ from the standard version of the radio only the addition of a 6 pin data connector which takes the place of the channel selector switch on the top of the radio -- the CP-530 is a single-channel, crystal controlled radio.

Conversion Steps

First, please note that these mods were designed to be the easiest, most reproducible way to the get radio converted -- This is the template for what I hope will be a group project in our club, and I wanted to avoid complex and dangerous tasks like removing the shields soldered to the bottom of the board. Elegance gave way to simplicity. I apologize to all purists...

One goal, in particular, was to avoid having to unsolder the shield on the bottom of the circuit board that covers the part of the board we're working on. As a result, although it might be neater to desolder components and use the PCB holes for new connections, we're not doing that. All the wiring in the shield area is done on top of the board. It's ugly, but it saves a lot of time.

0. A way to provide (almost) the proper voltage from a 13.8 volt source is to use 3 rectifier diodes (M2.5 type or similar) in series with the positive lead. The voltage drop across the diodes will reduce the voltage to something around 11.5 volts on receive, and 10.8 volts on transmit. That seems to work OK.

1. Make sure the radio is operating properly on its original frequency. It should put out about 2 watts, and have a receive sensitivity of 0.5uV or better.

2. Desolder and remove the original crystals.

3. Before installing the new crystals, locate D5, which is mounted on-end just to the right (looking down on the radio, with the top away from you) of the receive crystal. Clip the exposed lead as close as possible to the top of the diode, and solder a short piece of wire to the now-free lead that used to be the cathode lead of the diode. We'll hook up the other end of this wire later.

4. Now, you can install the new crystals. If the TX crystal has a ground wire soldered on, make sure you resolder it to the new crystal.

5. There is an unused ground hole right next to where the ground wire for the transmit crystal case may be. Mount a 6.2 volt, 1/2 watt, zener diode with the anode end in that hole.

6. Locate the red wire connected to pin 5 of the data connector. Clip it 1/2 inch or so from the connector. This is a source of voltage from the battery; if you don't have the data version of the radio, you'll need to find another Vcc source that is active on both RX and TX. Solder the end of the red wire to a 4.7k ohm resistor, and solder the other end of the resistor to the cathode end of the zener diode.

7. Solder the anode end of a 1N914 or similar diode to the junction of the resistor and zener diode, and solder the wire you attached in step 3 to the cathode end of the diode.

NOTE: The previous steps solve a problem that otherwise makes the radio very slow to key up when used with a 9600 baud modem. The bias voltage on the varactor diode that serves as a modulator is keyed only on transmit. The large coupling cap that's normally used to bring 9600 baud data into the varactor forms an RC circuit with the output level pot in the modem, and on every keyup there is a large frequency swing because the bias voltage is sucked down by the capacitor as it charges. This change supplies an unkeyed bias source to the varactor; now, there's no charge cycle and the radio keys up very cleanly.

8. Now it's time to inject that data. In the data version of the radio, pin 1 of the data connector is data in. However, this is designed for 1200 baud and won't work for 9600. However, we can still use the connector by removing R87, which is a small 22k resistor located just about smack dab between the RX and the TX crystals. Remove it and put one lead of a 47k resistor in the former R87 hole that's closest to the top of the radio. Solder the negative lead of a 5uF or so tantalum capacitor to the other end of the resistor. Attach a wire to the positive lead of the capacitor.

9. In the upper right corner of the radio, just inside and above the PTT switch, locate C58, which is a 1uF teardrop cap -- it's the only large cap visible next to the PTT switch. Dyke that sucker out. (This isolates the varactor diode from the existing audio circuits, and gets rid of the low-pass characteristic that otherwise renders the data unreadable.

10. In the same area, locate R55, R56, and/or R58. These three resistors all have one lead common with one another. Solder the other end of the wire from the capacitor to whichever lead is easiest to get at.

11. That's it for modifications. The data output signal available at pin 2 of the data connector comes from the discriminator through a buffer; it's essentially flat through about 6kHz, so works pretty well for 9600 baud with no changes.

11. Tuneup is pretty straightforward. The only touchy point is that the transmitter is prone to many spurs if you simply tune for maximum smoke. You need to use a spectrum analyzer to get a clean signal. I'm not kidding.