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23CM PRM80 Coming.....

The Phillips/Simoco prm80 radio has proven itself to be a great performer on the 2M and 70cm amateur bands, particularly when dealing with pager interference, and there is quite a few being used as “High Power” UHF CB's. They have up to 160 channels, software programmable, and too many options to list. There is an E Band version of this radio that has finally been converted to 6 metres and there is an AM Aircraft Band modification for them as well.

Here it is 23cm in progress:

I am not a writer by any means, nor am I an RF engineer, so bear with the grammar and if you have any suggestions or changes please contact the author. In this article I will show how the conversion process is progressing to get a PRM80 up on 23cm. While inside the radio it is probably not a bad idea to replace all the surface mount electro caps as well. These can be replaced with SMD tantalum or Jaycar has electro's in the correct size for a replacement.

Parts Required so far

You need to obtain the following parts:

  • 23cm version of the Firmware
  • 330ohm resistor x 2

Firmware Mods

The idea behind the 23cm version is to use a 420 - 440 Mhz radio and triple it up to 23cm. This is ok but that leaves us with a pretty ordinary frequency step. I did think about changing the reference crystal to either 6.667 or 8.33Mhz and modifying the firmware accordingly as that would give us a 8.33khz step when programmed with either a 10khz or 12.5hz step. I would also have to modify the firmware to suit an IF offset 1/3 of the 21.4Mhz (so when the LO is tripled we get a 21.4 offset as normal).

Looking at the price of a special cut crystal, I had another read of the PLL IC's datasheet. I found that it keeps an R register, which is the division ratio for the reference crystal. After reading the formula to get R, I went looking for that hex code as well.

We now have a 23cm version of the firmware that gives us a 8.33khz step and pre-loaded words that allow us to start the channels at 424.333 Mhz (1273Mhz after tripling). It also has a 7.133Mhz IF offset on the low side for the RX VCO (gives 21.4 at 23cm)

Here is the firmware and a “Cheat Sheet” for programming a T Band radio for use as 23cm. The radio type has to be set as W4 in FPP or this will not work. 500Mhz in FPP equals 424.333Mhz programmed into the PLL. See the Cheat sheet. I will be writing a VB program that will allow us to type in the required 23cm freq and it will spit out a W4 freq to type into FPP.

Hardware Mods

So far:

The RX VCO refused to oscillate at some frequencies due to the shifted IF. One Varicap diode may need to be removed from it to lift the circuit resonance.

The VCO reference fed back into the pre-scaler (and into the PLL) has to be moved from the end of the buffer to in the middle of it. This is achieved by moving the 330 ohm smd resistors so they attach to the base of the BFR92 and then feed into the PLL. On the receive VCO there is a 100 ohm resistor going to ground that will have to be removed so the reference signal is high enough for the PLL to read it.

On the TX VCO bufer there is a 68 ohm resistor biasing the BFR92. I have found by replacing this with a 27ohm resistor this provides enough biasing to get the transistor to act as a trippler. The output is not very high, but with a 1/4 wave stub (at 433.33333mhz) attached here I was able to read 1300Mhz on my frequency counter.

At this point I think I need phil to come back from ireland with my bits to go furthur….

I'm open to suggestions from here on in…

23cm_prm80.txt · Last modified: 2018/09/14 22:49 by vk3smb