#1
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Scored A Zenith!
All I can tell about it is, it says Zenith Long Distance Radio on the back, has AM/FM/AFC has a pretty decent compliment of tubes, mostly smaller ones but it works great, sounds great, and looks darn near new.
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#2
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The back shot, hopefully some of you can tell me a little more about these?
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#3
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Here's a nudie, looks like all the original Zenith tubes are still in there and working well.
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#4
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Another nudie shot :
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#5
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I didn't think to look underneath it :
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Audiokarma |
#6
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Tube compliment :
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#7
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DanTana,
Congratulations! You have one of Zenith's best AM/FM table radios from the '60s, model K-731 (chassis 7M07). I also have a Zenith like yours (almost exactly, except for the grille cloth), an ebay score about a year and a half ago, and like it a lot, though I don't use it much. I cannot say enough good things about it. This model is second only to Zenith's model C845 in sound quality, and may be better than the '845 in that regard because of the K-731's larger cabinet and two-way speaker system. The '731 has a 5x7 oval speaker for lows/midrange and a small electrostatic tweeter for highs, the latter being driven directly from the plate of the 35C5 audio output tube. The only modification I'd make to this excellent radio would be to replace the selenium low-voltage rectifier with a modern silicon diode (don't forget to add a series resistor to get the same voltage from this as you did from the original selenium). Seleniums were good in their day, but have long since been superseded by silicons, so I'd replace the old rectifier as soon as possible. Selenium rectifiers can short, giving off a gas that smells every bit like rotten eggs (the smoke can be toxic as well)--another case for getting rid of that old, outdated selenium rectifier. You may and probably will also find that you have more voltage output from your power supply when you replace the selenium (the reason for the series resistor). I'd go through the tubes and replace any weak ones as well, not to mention replacing old wax or paper caps under the chassis. Any radio this old is bound to have a few doubtful or outright defective capacitors (I'd check the main 3-section electrolytic in the power supply as well), perhaps some out-of-tolerance resistors, and, as I mentioned, weak tubes. If your set works, however, I wouldn't do anything with it beyond replacing weak tubes and/or the power supply filter. In a radio of that vintage (Zenith introduced this set in the late '50s and offered it in two cabinet styles), I'd replace the filter cap as a matter of routine, even if you have no hum in the sound.
__________________
Jeff, WB8NHV Collecting, restoring and enjoying vintage Zenith radios since 2002 Zenith. Gone, but not forgotten. Last edited by Jeffhs; 12-09-2004 at 12:55 AM. Reason: Minor addition to post |
#8
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Wow, that's Niiiice! I love seeing something that old looking so new, kind of like having a time machine.
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#9
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I was wondering what that square plate was next to the speaker, I didn't think it was a speaker because it was so thin. Kind of amazing they put an electrostat speaker inside these back then. I'm not sure if the electrostat works or not, I'll try to check it later. As far as replacing stuff, that's probably not a bad idea, but I was amazed at how well this radio works, it picks up stations really well and there is no hum at all. Last time I replaced a selenium diode I just put in a 350v silicone one and didn't put any resistor in series with it. Let me know what value resistor I should be using with that.
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#10
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K-731 electrostatic tweeter
I don't recall offhand what the formula is to determine the value of the series resistor ordinarily used with a silicon rectifier diode and, unfortunately, don't have the information available right now.
You are right about the electrostatic tweeter. It is a rather unusual thing to find in a table radio, but the K-731 was a high-end table model designed for high fidelity. (Zenith's model C-845 also has two speakers, but they are standard cone-type units.) This receiver also has a wide-range tone control and incredible bass response (likely due to the large 5x7 speaker and the equally large wood cabinet)--I am amazed at the sound quality. I have often wondered why this model did not have a phonograph input, as does (I think, anyway) the C845. The K-731 probably cost quite a bit when new; I don't think the addition of a phono input would have added that much more to the price. Besides, I think with an audio stage as good as the '731 has, it's a shame the set doesn't have such an input jack (or an external speaker output, for that matter, not to mention a tapoff at the discriminator output, ahead of the audio, so that the receiver could be used as an FM tuner; I honestly believe the FM section of the '731 is that good). As to how one can check to see if the electrostatic tweeter in a K-731 (or any other radio using such a tweeter) is working, I don't know offhand. My set is the first radio I have seen which even has such a device, so I'm not all that familiar with it or the crossover circuit which may be used with it (if there is one). Perhaps someone here who is more familiar with this set than I can fill you in on the details of the electrostatic tweeter system.
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Jeff, WB8NHV Collecting, restoring and enjoying vintage Zenith radios since 2002 Zenith. Gone, but not forgotten. |
Audiokarma |
#11
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I'm sure there would be a way to tap into the FM signal and route it to a set of RCA's for a preamp. I know with just the standard antenna I was picking up at least as many stations as a few of my modern FM tuners do. Only thing I wish is that it was stereo, then you could even use it as an amp. I guess you have to compromise some places to be able to fit into a tabletop design. I was rather surprised with the bass response, I had to set the tone to about 3/4 toward the treble. I'm curious to see the underside of it to see what components are in it, I could probably swap out quite a few of the parts in an afternoon.
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