Dittel FSG71M fixes
This is an excellent radio, especially for a sailplane. It performs well, even with the often lowish DC voltages often found in glider panels. The Dittel is very reliable, especially compared with some later radios that have appeared. However after several decades even the best can fail....
Push to talk (PTT) relay fault
This is a known problem that occurs as the radio ages. The symptoms are either lack of TX audio, or loss of RX audio (and possibly both!) The onset of the fault may be of a gradual and highly intermittent nature. While the TX fault might be noticeable (by others) but the RX fault might be missed (unless you open the squelch). Blipping the PTT button may restore function for a period. The fault may be more common at altitude (or perhaps in the cold). Sometimes there is very faint (thin) audio present.
What happens is that the contacts on the relay appear to simply wear out, resulting in poor contact (high resistance). This relay switches the output of the TDA1020 audio power amplifier, routing it to either the loudspeaker (for RX), or to the modulator stage (for TX). The relay, K301, is a little blue job. I struggled to find an exact replacement: the small size is hard to find. I used a slightly bigger relay from Radio Spares (part number 683-9255). It fits into the printed circuit board (PCB) if inserted at an angle, lengthening two pins as shown in this pic. You'll also need to remove the adjacent electrolytic capacitor (C354). This capacitor is not too important as it simply feeds the loudspeaker. Replace it with a modern, smaller unit. Even 100uF (16VDC or greater voltage) will work, but see if you can get a 470uF to maintain speaker audio quality. To remove the old capacitor you'll also need to cut the glue that holds it firm to the bigger companion next door. Of course you'll need to be somewhat skilled with a soldering iron to perform this repair. Be very careful not to damage the tracks on the PCB: use a solder-sucker and solderwick. There are tracks on both sides of the board. Make sure there is nothing poking out below the PCB that could short to the aluminium below.
Memory backup battery
This retains the programmed channel frequencies when the power is off. The battery should easily last 20 years, perhaps even 30, however it is easily replaced so should be done if ever the radio is out for major maintenance. Again Radio Spares can supply this part. I used part number 668-4544. Before replacing take note of the current channels because you'll lose the programmed frequencies.
This has been known to fail. Avail. from eBay perhaps. The TDA1010 can also be used. Remove the heatsink from the dead IC first.
Front Panel labels
Over time the white lettering deteriorates. Remove the knobs using 3/64"" (0.05") Allen key. The grub screws do tend to seize over time. Add some thin oil and try again the next day. The front plastic cover will now pull off. Protect the LCD with tape. The remaining letters can be scratched off with a sharp knife. Sellotape will help pull off the leftovers. Use rub-on white Letraset transfer lettering or similar, lightly spray clear coat over the top to protect the nice new lettering.
Many thanks to "3U" for useful info regarding this fault. He has more detail at: https://sites.google.com/site/threeuniform/dittel-fsg71m