Replacing Selenium Rectifiers in Zenith Trans-Oceanics
All the miniature tube Trans-Oceanics, from the G500 to the B600, were equipped with selenium rectifiers instead of the traditional tube rectifier. Selenium rectifiers are notorious for failing gradually over time. Their forward voltage drop increases to the point that the tubes stop working. As this happens, the increased resistance causes the rectifier to heat up, which eventually causes it to burn out. When this happens, it emits a highly pungent odor, and could also start a fire. Even if a selenium rectifier is working now, it is only a matter of time before it fails. Fortunately, a Selenium rectifier is easily replaced with a silicon diode; however, added resistance (typically a 40-50 ohm, 5 watt resistor along with the existing 130 ohm "sand" resistor) or a larger voltage dropping resistor in place of the original resistor (typically a 170-180 ohm, 5-watt resistor) is needed to compensate for the lower forward voltage drop of the silicon diode. Here's a step-by step guide on how to replace the selenium rectifier in your Trans-Oceanic.
1. Unsolder the wire from the bottom lug of the selenium rectifier.
2. Unsolder the 130 ohm sand resistor from the top lug of the selenium rectifier.
3. Solder the wire to the top lug of the selenium rectifier.
4. Solder the unbanded end of the diode to the top lug of the selenium rectifier. The rectifier is now out of the circuit, but is usually kept in place for appearance purposes.
5. Unsolder the other lead of the sand resistor from the 50A1 socket (600-series models) or the filter capacitor can (500-series models).
6. Solder one lead of the new resistor (value depends on which method you're using) to the point where the sand resistor was originally connected.
7. Solder the banded lead of the diode to the other lead of the new resistor. Or, if you want to add some resistance onto the existing sand resistor, solder the sand resistor in between the banded lead of the diode and the other lead of the new resistor.
An example: The old selenium rectifier in my B-600 had been working fine, but one night, after around 40 minutes of listening to shortwave, reception totally faded out. I switched to AM, where I got reception, then I switched back to SW, where I got reception for around a minute (maybe less) before reception faded out yet again. I knew that the selenium rectifier was on it's way out. I couldn't find a 40 or 50 ohm resistor that wasn't as long as the existing sand resistor, so I replaced it with a 750 ohm resistor and a 225 ohm resistor tied together in parallel, which calculates to ~173 ohms. However, I wouldn't recommend this method, as using series resistance or a larger single resistor would be more accurate.
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