I have always been impressed by the design of military radios, they are nicely designed to handle the worst treatment and survive. The German SEM52A is no exception. I saw one come up on Ebay for $100 so jumped at it.
The SEM52A covers the frequency range of 47-57MHz, so the plan was to convert it for use on 6 metres FM. One of the impressive things about this radio is its minute DC power consumption. It uses only 5mA on power save receive and 30mA on normal receive (muted). This is because the radio is not a synthesised design, it uses a single crystal per channel. There is an offset oscillator that creates the transmit frequency from the single crystal.
The 6 metre net is 52.525, so the challenge was to find the cheapest way to obtain a crystal (hard to get these days) that would select this frequency. Quotes for a crystal from local suppliers was $40 plus. The radio has several “channel modules” on a rotating turret behind the selector. Each module has pair of resistors that electronically tunes the radios RX and TX circuits for the channel frequency.
The crystal frequency is 20MHz above the channel frequency as the IF and offset oscillator are 20MHz. This requires a crystal frequency of 72.525MHz.
The 72MHz band is used in the USA for remote controlled (RC) model airplanes. A bit of a search on the web revealed that a US channel 14 crystal for high side receiver injection has a frequency of 72.525MHz. Another search found the crystals for sale on a Japanese site (Aircraft World) for USD2.99. This sounded great, there was a crystal package available that was the same as the micro one used in the SEM52A. Just in case there was a problem with capacity loading, I also bought the next crystal channel (to hell with the expense!).
The crystals arrived about a week later and fitted nicely into the tight module space. I put a pot where the tuning resistor pair went in the channel module and used the pot to tune the radio for max sensitivity on 52.525. The pot was then taken out and replaced with two resistors (68k and 120k).
The receive sensitivity is excellent and the transmitter power about 600mW.
The next challenge was replacing the bone conduction mic and earpiece with something a little more friendly. I used a speaker microphone from a commercial radio and found the radio had ample audio power to drive the speaker. The electret mic levels had to be reduced and some HF roll off introduced into the mic circuit.
The radio works nicely, had a contact with a mobile operator a few km away using the tape measure style whip antenna on the unit.
The radio came with two AA size battery packs, which I put some varta rechargeables into. It will be interesting to take the radio up to some high ground (maybe SOTA) and see what can be worked.