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  #1  
Old 10-26-2003, 02:10 AM
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Jeffhs Jeffhs is offline
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Zenith K731 puzzler, and a question on rectifiers

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.
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Last edited by Jeffhs; 10-26-2003 at 02:21 AM.
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Old 10-26-2003, 07:13 AM
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Chad Hauris Chad Hauris is offline
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The little flat speaker is an electrostatic tweeter, it runs right off the high voltage at the plate of the output tube.

I think at the time this radio was made, seleniums were probably cheaper than the equivalent-rated silicon rectifier, so that's why they were used. Probably the filament voltages add up to enough in this set without another tube, so a tube rectifier could not be used.

Later Zeniths of this type (mid-later 60's) do use a silicon diode.
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Old 10-26-2003, 12:26 PM
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Re: K731 puzzler, and a question on rectifiers

Chad,

Thanks for your reply. I thought that little speaker might have been a tweeter, but I wasn't sure.

You were right as to why Zenith did not use a 35W4 tube as a rectifier in this set. I did some figuring and found out that the entire filament string (seven tubes) totaled 109 volts as it was. Adding another 35-volt tube to the string would have brought the total to 144 volts, well above the standard line voltage.

Thanks again.
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  #4  
Old 11-21-2003, 08:58 PM
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Exclamation Replace that rectifier!

When I bought my first Zenith (1950 AM/FM Model H724), the radio emitted a spark minutes after I turned it on, then smoked (and smelled like sulfer). I immediately shut it off and unplugged it, and sent it up to my dad for repairs. He obviously replaced the fried rectifier with a new silicon diode, and after replacing all caps and the power cord, plus a much needed cleanup and tube checkup, it now operates flawlessly (and sounds good, too).
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Old 12-12-2003, 02:31 PM
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jst1cav,

I just looked at the pictures of your current stereo system. Very interesting--that McIntosh amplifier with a modern digital receiver and Klipsch speakers. I hadn't seen many setups like that until I became a member of AK and started browsing the forums.

The cabinet styling of your 1976 McIntosh 2100 brings back memories. My dad had a hi-fi system with a McMartin SCA (storecasting) adaptor which he added in the early '70s, which was housed in the same type of cabinet as your amplifier. No McIntosh equipment in his setup, though.
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  #6  
Old 01-27-2004, 12:10 PM
joe_tbird
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Solenium rectifier question.

I have what appears to be a mid 1960s Zenith AM/FM tube alarm clock/radio. I don't have the model number handy, though I can scan in a picture of it if necessary.

Sounds like by the mid 1960s Zeniths didn't use Solenium rectifiers so I may be ok. Is there a way I can check which it has?

Just in case, though, is there a danger leaving it plugged in? Since the clock works I'd like to keep it plugged in. If I follow this thread correctly, it sounds like Solenium rectifiers could short out even if the radio is turned off but plugged in. Is that same danger present with an alarm clock/radio unit?



Thanks,
Joe
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  #7  
Old 01-28-2004, 06:52 AM
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Chad Hauris Chad Hauris is offline
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The selenium rectifier is mounted to the rear of the chassis on the outside...it has about 6 square, stacked plates about an inch square. If it uses a silicon diode, you will see a small silicon diode (small, black cylinder with band on cathode end) mounted on a terminal strip on the outside of the chassis.

I have never had a selenium short out when the set was off. Haven't had one short out by itself at any time, but have noticed excessive voltage drop (resistance).
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Old 01-28-2004, 09:19 PM
joe_tbird
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Thanks. I'll sleep better knowing my radio won't spontaneously ignite while turned off. Maybe I'll take a peek inside before using the radio to fall asleep to. This Zenith is a very nice set. As you can guess, it has a 60 minute variable timer to shut off the radio after the selected time interval.

Here's a picture of the set, with a modern alarm clock from Sharper Image sitting on top for comparison:
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