GM4CXM gets 13cm!

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 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 it was low in frequency and that the adjustment range was very limited. 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 and tuned to a 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 with 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 BNCs 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.

Left: 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!

The SRM13 was next in line and as with the SLO13, it took a few carefully aimed blows from a hammer and chisel just to get the top cover off! The tin-plate was completely gone and the lid was literally fused to the box by aggressive ferric oxide (plain old rust!). The bottom lid was only slightly corroded and came of easily and it was a simple job to cut the board out.

I didn’t have a suitable size tin-plate box but instead cut a larger one down to size. I replaced an over-size resistor with one the correct size and quickly found that both GaAsFETs were faulty. These were originally both MGF1200s which probably cost cost ‘an arm and a leg’ back in 1982. I replaced them both with MGF1302s (old stock, but new) and once I was satisfied that the bias currents were correct, I was pleased to find that the converter worked quite well. I don’t have a signal generator that goes beyond 1.12GHz, and at the time there wasn’t a ‘local’ beacon but I do have a personal beacon which I put into low power mode and tuned the SRM13 up on that. Curious to see what difference a HEMT would make in the front end, I replaced the ‘1302’ with an FHX14LG and despite the absence of an independent negative bias rail, the device actually outperformed the MGF1302 at 2.32GHz.

The STM13 (TX converter) was in relatively good condition, Ray having already cleaned most of the rust off. Since I didn’t have a similar size box available, I opted to work with it as it stood ... Once I replaced the BNCs with SMAs first though. Initial tests were promising with all stages tuning up more or less correctly, although the output was admittedly somewhat lower than expected ... About 20dB lower to be precise! What did strike me as odd was that P2, the mixer balance control was set to one end of its travel, indicating that there was a substantial difference between T2 and T3 (both BFR91A). Both transistors checked out more or less OK but after replacing both devices with new ones, P2 maximised mixer performance at more or less mid travel. This accomplished, the output of the STM13 was measured at just over 600mW.