RAT5 Topband AM Receiver

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After completing rebuilding the Codar AT5, I decided that I would like to have a matching Topband AM Receiver to go alongside. After doing some searching on the internet, I found a nice circuit which used a single TDA1072A chip. This chip was used a lot back in the 80’s and 90’s for AM car radios. Building the circuit would mean starting from scratch and that would involve making a printed circuit board and finding all the components. As I soon found, some of the components needed were now obsolete, especially the TOKO coils which are like hen’s teeth nowadays!
After hours of searching I came across a kit which was made years ago called the RAT5. This used the almost identical circuit of the TDA1072A, but with additional features including a squelch circuit. The RAT5 was initially designed to compliment the FAT5  A 50W+ Class E solid-state AM transmitter, which I also sold as a kit. Mine will of course compliment my trusty valve Codar AT5.

The kit was supplied by Eric GW4GTE  http://www.shortwaveradio.co.uk/rat5.htm

I sent an e-mail to Eric to ask if he still had one of the kits in stock, he does….so my problem with making a PC Board and sourcing components was solved!

I was really looking forward to starting this project, all the components ready including the TOKO coils.

The completed board ready for testing. The kit comes with a very detailed set of building instructions along with testing and alignment. The receiver worked first time when I switched it on (after doing some initial measurements to ensure that I had not made any big mistakes) My next step is to find a suitable case for the receiver, I wanted one that will match the Codar AT5. This was not easy because at one time U Shaped boxes and chassis were 10 a penny, but now nobody makes them. I eventually found a UK source of nice aluminium cases, but the one I wanted was out of stock, so I had to settle for one smaller.

This is a neat little case, very well built. Once I had tested the circuit board I removed the potentiometers from the board and extended them to fit into the case. I also wanted a slow-motion drive for the VFO. My first topband transmitter had an Eagle Vernier Dial, I was going to try to find one for my receiver, but I realised that it would be too large, instead I found a smaller, but similar drive on eBay.

Now I know that this is Topband AM Receiver, but it would be nice to be able to listen to SSB and CW stations as well. To do this I made a simple BFO using a 455khz IF coil (left) It took a bit of fiddling about, but I got it working fine. The BFO just couples loosely to the board to insert the 455khz signal.

Looking good! Just the final alignment now, although the Vernier Drive was a bit troublesome, it kept sticking and needed to make sure that everything was mounted in line.

For the lettering, I sent for a sheet of white decal paper and experimented with letter size on my computer.

This has been a very satisfying project, the RAT5 is very stable and sensitive receiver, the S Meter and Squelch work well! I now use the receiver along with the Codar AT5 transmitter for the Sunday morning 1963khz AM net.

My thanks to Eric GW4GTE for his advice and additional material. A nice project!

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