Microwave Modules MM144/28 SSB Transverter

This is a classic 2m to 10m TX/RX transverter made from at least 1979 to the late 1980's. Considering how many are traded on e-bay there is very little information around for them so this will become another resource page!

My version is not dated but is probably mid 1980s as it does not quite match the 1979 circuit I have and isn't quite up with the latest circuit I have from 1988.

Lots of pictures, circuit diagrams and board layouts to at the bottom of this page....

The reason for the purchase is that I want SSB/CW capability on 2m and the Shimizu SS105D has a transverter output and separate receive input so the transverter. It only cost 5 GBP from the North Wales radio Rally so a bargain was had but the lid had been removed so danger signs were there!

The PA unit looked in good condition with no obvious signs of fiddling but I was still dubious.

Before powering the unit on I decided to take it apart and do a very detailed inspection. To remove the main board the PA needs to be removed but this is a simple job. I recommend taking photos so you can reconnect properly and not how you think it should be.

The main board looked poorly. Three of the trimmer capacitors had corroded, two were the drive ouput trimmers and one was the LO output trimmer. I replaced these before switching on. I also spotted the two bias resistors for the RX mixer were burnt out (10 ohm) I feared the worse for the mixer but replacing the resistors and powering up showed the TX mixer and driver were working fine.

I connected up the PA and got no output - not even a minor deflection. I decided to tune up all the trimmers just in case and then got a solid 10 Watts out when all aligned properly. The alignment is ctritical and very sensitive - lack of drive to the PA is much easier to achieve than full drive!

I had believed the RX path was working fine as I could receive my handheld ok but connected up to an antenna there was nothing. The local repeater was active and on my hand held but nothing on the transverter output. As I could resolve signals close by then I think the mixer is fine but the RF amplifier is dead.

Looking at the photos of the underside of the transverter showed some pretty horrendous modifications, it seems the RX/TX switching transistor and both the RX amp and RX mixer Mosfets have been replaced probably due to the transistor failing and TX RF blowing both devices. I am planning to replace the existing mixer with a BF981 as I have a few spare - I need to make sure the new circuit fits in with the old and I have little confidence that this is like any circuit Diagram I have so it will be a trace it and draw it exercise.