Marti STL 15-C. These were the microwave equivalent of a swiss army knife in the 1990s. Frequency agile and affordable. There are still quite a few putting in a day’s work. This one isn’t. Let’s find out why.
This is the Audio processing board. Roughly half the capacitors on it have failed, some are leaking electrolyte. It is passing audio, but barely and it sounds terrible.
This is a fairly common failure with aging equipment. Capacitors typically have a 7-10 year lifespan, some much longer and some much shorter. A lot depends on the temperature and transmitter buildings tend to be pretty warm.
Whitish crud leaking past the rubber seal- that’s electrolyte and it’s corrosive. A few areas on the board needed the PC traces repaired as they were eaten away.
2nd converter/IF amp. This got all new capacitors too. Wurth makes some good quality replacements over in Germany and we’ve used them here.
1st converter. Not a lot to do here other than align it once the unit is reassembled. These units are frequency synthesized, but still require a crystal in the converter. Marti shipped these with a styrofoam “oven” around the crystal to keep the temperature stable. Over the years those disintegrate, so we’ve made a new one.
Power supply and squelch board. Generally I remove these in Marti equipment and install a 13.5V switching supply. It’s $20 well spent as the units run much cooler. This one is not getting that so we’ll finish replacing the capacitors, apply fresh heat sink compound and put it back in service.
Reassembled and operating with a test Transmitter, the audio response is much better now. I sweep these from 20-20kHz then spot check frequencies from 20khz up to 53khz. This one again meets factory spec of +/- 0.2 dB. Noise is back in the -70 dB range. After 24 hours burn in it will be returned to the customer and go back in service.