70 MHz 4m 28MHz IF 'Dogs Breakfast' Transverter Project

Spring 2020

 PW Meon Board
Meon Tranverter Board

This project started when I purchase a Practical Wireless Meon transverter board on Ebay. It was advertised as not working but on trying the unit I discovered that the transmitter mixer IC was U/S and I could not find a replacement device. The local oscillator and dual gate RF amplifer and diode ring receiver mixer worked fine. The drive to the transmit mixer was measured at +17dbm. I had a 1 GHz high level Mini Circuit SRA mixer to hand so added this along with a 70MHz band pass filter. The 70Mhz Tx signal is amplified by an old Pye Westiminster P.A strip that I had to hand and modified to give about 3-4 Watts output. I have used this on a few occasions to enter the UKAC contests however 4 Watts is just not enough power from my home locations to work many of the more distant stations.

Westminster PA
Modified Westminster PA.

Modified Pye Westminster PA Strip

I had previuosly used this Westimster PA strip in my 2m multi-mode home brew rig that I built in 1976!. It have bias resisters/diodes added to each stage to make it a linear amplifier for SSB operation. It was simple enough just to change the inductors to get it to work on 70MHz. A simple modification to the second stage increased to saturated power output to 7 Watts.

 Pye A200 PA
Modified Pye A200 P.A.

Modified Pye A200 Low band 35W P.A.

The A200 amplifier was purchased on Ebay and had been previouly modified to add a mode switch FM/SSB It used the original RF switching circuit with a simple R/C network to increase the hang time on SSB. This provide to be totally useless. There is an article on the 6m forum about converting the A200 for 6m use. As supplied by Pye the A200 is not linear and the PA is biased some where between class C and class B, Ok for AM but not for SSB.The bias circuit is derived from a 47 ohm resiter from the 12Vdc supply and is stabilised by two diodes in series to ground giving a crude 1.2V supply. A single dropper resiter feeds the 1.2V to the base of both transisters. To provide RF stablity the base of each transister is ground by 3x12 ohm resisters! You cannot just reduce the value of the resister from the 1.2v supply.The 47 ohm dropper resistor is already 5w and gets very hot.I replace this resitor with two 100 ohms 5W resitors the second soldered to the underside of the PCB. The solution to increaseing the bias voltage is to lift the grounded end of the 12 ohm resitors and decouple each with a 10n capacitor to earth.The two resitors close to the transitor collectors looked impossible to unsolder easilly so I left these and only added four capacitors, this is enough to allow the standing bias to be set to 150mA total collector quiescent current. I used SOT fixed resisters as I did not have a small variable with a low enought value. Check that you still have 1.2V across the stablising diodes. I bypassed the RF sensing circuit and made the PTT line operate the internal relay which just switches the bias on and off. The 12dc supply to the transistor collecters is perminantly on and the RF switching relay on the input and output has ben removed and the RF leads wired direct to the RF in and out on the amplifier section.

A200 PA
A200 Amplifier RF Section

A200 Amplifier RF Section

The finished transverter will give 45W saturated power output with a 13.5 Vdc supply. I usually run with about 30 Watts max output. Whilst the A200 amplifier is a quick solution for an FM transceiver,I personally think there are better and easier units available for SSB.