Jeffhs
10-26-2003, 02:10 AM
I have a Zenith K-731 AM/FM table radio of 1959-60 vintage which works very well; however, something about the audio output circuit puzzles me. There is a plastic box mounted near the speaker, with two leads emerging from it. These leads connect to a terminal strip on the chassis by means of slip-on connectors. What is the purpose of this mysterious box? My best guess is that it may be some sort of audio enhancement device, perhaps a high-frequency tweeter. Given that this radio was one of Zenith's high-end table models in the late 1950s or early '60s, this seems to me very likely, as the set has several enhancements not found in garden-variety AA5s such as tone compensation circuitry with a wide-range tone control on the front panel, a large speaker, and a large, heavy walnut cabinet (all of which give this set incredibly good sound quality, especially in the bass range). This set sounds even better than my 1951 Zenith H511, which is in a black bakelite, oval-shaped "racetrack" cabinet (and as I write this has a bad speaker--the cone is torn in several places; haven't gotten around to replacing it yet).
I have another question regarding the K731. This radio uses a selenium rectifier in the B+ supply. Why did Zenith not design the supply to use a 35W4 rectifier tube, or better yet, silicon rectifier diodes right off the bat, rather than using a selenium in this position in the supply? I mention this because selenium rectifiers can and often do short, giving off a gas which smells of rotten eggs, not to mention the fact that seleniums can pose a shock or fire hazard when they fail. Add to that the fact that selenium rectifiers are obsolete and difficult, if not impossible to find today, and we now have the makings of a very good case for replacing them with silicons. I would suggest this to anyone wanting to use one of these sets (or any radio or TV set with one or more seleniums in the power supply) on a regular basis, as silicons are much more reliable. If you want to use an old radio or TV with its old seleniums, you can, of course (as long as they work as they should), but I would suggest that you unplug the set when you are not using it. The reason is that 40-50 year old seleniums have outlasted their useful life and can fail with little or no warning at any time. If your treasured antique radio or TV suddenly begins to smoke (and that smoke smells of rotten eggs), unplug the set at once, as the selenium rectifier(s), if your set has them, have likely shorted. (Your house fuses may blow at this time as well, as these rectifiers are connected directly across the AC line, and many transformerless radios and TVs have the chassis connected directly to one side of the line.)
Please be aware as well that the smoke given off by shorted seleniums can be toxic, so do not leave a set with smoking rectifiers plugged in for any length of time.
I have another question regarding the K731. This radio uses a selenium rectifier in the B+ supply. Why did Zenith not design the supply to use a 35W4 rectifier tube, or better yet, silicon rectifier diodes right off the bat, rather than using a selenium in this position in the supply? I mention this because selenium rectifiers can and often do short, giving off a gas which smells of rotten eggs, not to mention the fact that seleniums can pose a shock or fire hazard when they fail. Add to that the fact that selenium rectifiers are obsolete and difficult, if not impossible to find today, and we now have the makings of a very good case for replacing them with silicons. I would suggest this to anyone wanting to use one of these sets (or any radio or TV set with one or more seleniums in the power supply) on a regular basis, as silicons are much more reliable. If you want to use an old radio or TV with its old seleniums, you can, of course (as long as they work as they should), but I would suggest that you unplug the set when you are not using it. The reason is that 40-50 year old seleniums have outlasted their useful life and can fail with little or no warning at any time. If your treasured antique radio or TV suddenly begins to smoke (and that smoke smells of rotten eggs), unplug the set at once, as the selenium rectifier(s), if your set has them, have likely shorted. (Your house fuses may blow at this time as well, as these rectifiers are connected directly across the AC line, and many transformerless radios and TVs have the chassis connected directly to one side of the line.)
Please be aware as well that the smoke given off by shorted seleniums can be toxic, so do not leave a set with smoking rectifiers plugged in for any length of time.