GM4CXM gets 13cm! (/cont.)
The SRM13 was next in line and as mentioned previously, 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 (see previous page) and came of easily and it was a simple job to cut the board out.
To be honest, I was quite taken aback by the state of the soldering on the SRM13. The soldering on the SLO13 by comparison was good. Whereas this was down-right shocking. Since both GaAsFETS were needing replaced it was a simple case of attacking the board with solder-wick and re-flowing every single joint. The photograph on the right speaks for itself.
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.
There are several disadvantages with modular designs like this. OK, this kit is almost 20 years old and it was indeed pioneering stuff in its day. But it does make for bulky projects by today’s standards. Secondly, there are no less than 8 coaxial connectors involved. Changing the BNCs for SMAs was quite expensive. Thirdly the user has to provide their own I/F switching and PTT control. In this case, I put together a small PCB incorporating PTT control along with DC and I/F routing. This is the little board situated to the right of the STM13. The antenna coax relay is just to the right of the SRM13.