Here's a circa '48-'49 Arvin-built Sears Silvertone AM/FM tube radio that I picked up recently. Upon first look, I discovered that the AC power cord had been replaced and one of the original non-switched volume control had been replaced with a switched control.
I get nervous when I see equipment that's obviously been worked on by others because they usually perform hack repair jobs. This radio was no exception, as whoever replaced the AC cord attached it to the original remains of the old frayed power cord (big "NO-NO").
Upon first power up, AM reception was weak and FM was totally dead. The first thing to do when troubleshooting a piece of equipment is to make sure it's power supply is working properly because missing/low power supply voltages can cause the equipment to malfunction.
This radio uses a selenium rectifier; which, is an earlier form of solid state rectifier that was used from the '40's until the early '60's. These types of rectifiers usually fail in two ways. The first way is that they short; which, can result in arcing, burning, and a fould odor that one will never forget. The second way is that they develop a very high internal resistance; which, causes them to have a low output voltage.
When I tested the output of the selenium rectifier in this radio, I found it to be around 63-64 volts and I was expecting to see around double that voltage.OK, the selenium rectifier is bad; so, now what?
Selenium rectifiers are no longer widely made and haven't been in decades; but, they can be replaced by modern silicon rectifier diodes. When replacing a selenium rectifier with a silicon diode, there are a few things to consider. First, the voltage drop across a selenium rectifier is around 10-15 volts. The voltage drop across a silicon diode is around .7 volts. If we were to install the silicon diode directly across the connections where the old selenium rectifier went, our B+ voltage could be too high; which, can cause overheating of components and possible damage. To get around this problem, we can install a power resistor in series with the new silicon diode to drop the excess voltage. It may take some experimentation; but, a value between 22 ohms and 68 ohms at 5W-10W is usually sufficient. This largely depends on the equipment that's being serviced. The easiest thing to do is try several value resistors until you obtain a B+ voltage measurement that's close to the voltage specified on the schematic.
When selecting a silicon diode, you want to make sure that it's forward current rating and it's peak reverse voltage (PRV) are sufficient. In most radios and phonographs, a 1N4007 (1A, 1000V PRV) is sufficient. In TV's or other larger equipment, it may be better to use a heavier diode.
With the new diode, our B+ voltage is back up; but, the performance is not what it should be.
Негізгі бет Repair of a circa '49 Silvertone AM/FM tube radio (selenium rectifier replacement) - pt. 1
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