Ray, GM4CXM, a good friend of mine was extremely fortunate to be given the makings of a 13cm transverter earlier in the year. The boxes in question were the SSB Electronics SLO13 (local oscillator), SRM13 (RX converter) and STM13 (TX converter). Back in the early 1980s these were ‘state-of-the-art’. Ray was of the impression that these had once formed part of a ‘mast-head’ transverter which wasn’t terribly well sealed to the elements. I joked that they looked like they had been stored at the bottom of the North Sea. Knowing that I like a challenge, Ray asked me if I would like to undertake the task of producing a 13cm transverter based on these boxes.
The 3 SSB Electronics boxes before renovation
The SRM13 in particular was so badly corroded that I had to use a hammer and chisel to get the lids off! Once ‘inside’ all three boxes, it was clear that I had some work ahead of me. The logical place to start was with the SLO13 Local Oscillator, which Ray had been told, had a tendency to self oscillate. However my main problem to begin with was that the U310 FET was not oscillating. When I did get it to ‘start’, I found that the adjustment range was very limited and low in frequency. But at least it was something that I could work with. The next problem that I encountered was a clear indication that T3 (BFR90A) was faulty. This was replaced and the rest of the multiplier stages were tuned up with the aid of a simple volt-meter and my Systron-Donner spectrum analyser. The output was however very low and I soon found that C6 & C7 on the output of the 1st multiplier were incorrectly set. Obviously tuned to secondary response. Once correctly set, both outputs of the SLO13 were well in spec. There was however an instability in the signal which was eventually traced to a faulty ceramic capacitor (C4, 2p2) between the oscillator and the 1st multiplier. This was replaced and a simple Murata crystal heater placed over the crystal for added thermal stability. Replacing the faulty capacitor cured the tuning range issue, but it was possible to ‘push’ the frequency by varying the supply voltage despite there being a 9V regulator in line. This turned out to be faulty and was replaced by an LM317LM and a couple of carefully
chosen resistors . . . Problem solved! Satisfied that we had a viable project, I carefully ‘cut’ the SLO13 from its corroded case and re-fitted it into a brand new identical tin-plate box, with one noticeable change . . . I replaced the nickel-plated BNC connectors with gold-plated SMA ones. In fact I intended to replace the BNC on all three boxes with SMAs for two reasons. Firstly, BNCs are fine for bench equipment but are somewhat cumbersome when used for internal connections. Secondly their performance at 2GHz is questionable. So they had to go.
The SLO13 in its new shiny box. The LM317LM is top right. The pink wire is the 12V line to the Murata heater. Some extra decoupling has been added to the oscillator circuit.
Not a bit of rust in sight