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Jeffhs - which schematic is the one for the K731 on techschematic.org? Is it the 7M07, as I couldn't find one specifically for the K731?
And thanks so much for the info! I glanced at the tubes in mine yesterday real quick and noticed that the 35C5 has been replaced with a 50C5. Gonna have to change that. I didn't pull the ones with the shielded sockets yet. Shane Last edited by Blooze; 03-29-2006 at 02:02 PM. |
#17
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It should play fine with the 50C5, and the tubes will last longer. I'd leave it.
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#18
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Quote:
You are more than welcome. I had the same problem when I was trying to find a schematic for my K731. Try looking under K725, which is practically identical to the K731 as far as the chassis goes; the only difference I can see on my schematic for this set is the chassis number (7K06 for the K725, 7K07 for chassis K731, L, M, R, W, Y) and a different style of cabinet used with the K725 (plastic as opposed to the '731's walnut cabinet). My 731 has chassis 7M07. It could be the later versions of the K731 (the ones that came out in the '60s) used chassis 7K06 and 7K07. As to that 50C5 in your set rather than a 35C5 I don't know what to tell you, except by all means replace the 50C5 with its 35-volt equivalent. I did some figuring as to total voltage drop across the filament string with a 35C5 and a 50C5 in the output stage; there is a difference of over ten volts between the two (114 with a 35C5, 129 with a 50C5). Look at the tube layout diagram on the bottom of your '731. Is there a 35C5 or a 50C5 shown? If it is a 35C5, and you have a 50C5 in the output tube socket, the set's former owner may have switched tubes, either inadvertently or not knowing any better. It is always best to use the exact tubes specified in the tube layout chart, especially with a series-string filament circuit; using a 35-volt tube in place of a 50-volt one, or vice-versa, could (likely will) overload other tubes in the string, leading to their premature failure. Series-filament tubes with heater-cathode shorts can also upset voltage drops along the string, causing one or more tubes to fail as well. The total voltage drop across a series filament string must very closely approximate the line voltage. If there is a difference, it is compensated for by a filament dropping resistor.
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Jeff, WB8NHV Collecting, restoring and enjoying vintage Zenith radios since 2002 Zenith. Gone, but not forgotten. Last edited by Jeffhs; 03-29-2006 at 01:43 PM. |
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Fisherdude, OldWolf,
Thanks much for the kind words. That's what we AKers are here for, to help each other out when we run into a sticky problem or need information on a piece of equipment (radio, TV, etc.) we are trying to restore and/or repair. I personally have been interested in electronics and radio since I was eight years old (I will be 50 in July), and an amateur radio operator since the age of 15; I must enjoy this stuff immensely if I've stayed with it this long. I'm always glad to help out wherever I can, although most of my experience has been with tube-powered radios, TVs and ham gear; I'll leave the solid-state stuff to the other folks here who know it better than I. Fisherdude, I also concur with your last sentence in your post. I love being a member of AK as well. Been here three years and have no intention of leaving any time soon. You guys are the greatest! Without AK, who knows what might have happened to all the antique/vintage radios and TVs we have brought to life, very often (more often than not) after they have been given up for dead on curbs, treelawns, etc.? Don't forget, we are also preserving radio and TV history by restoring these venerable old soldiers. Sets made by the original Magnavox, GE, RCA, Zenith, etc. are treasures, as the old ones (ca. 1920s-'60s for radios and '40s-'70s for electronic television) are becoming scarce. Every set we can restore to past glory is one more we have saved from the landfill.
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Jeff, WB8NHV Collecting, restoring and enjoying vintage Zenith radios since 2002 Zenith. Gone, but not forgotten. Last edited by Jeffhs; 03-29-2006 at 02:21 PM. |
#20
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50C5 in place of 35C5 is fine. I have tried this substituion when I didn't have a 35C5 with no trouble. It is only the other way around that there would be an overload to the tubes.
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Audiokarma |
#21
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This is what I was thinking. Just running the tube at 70% or so on the filament, correct? |
#22
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Blooze,
When I first looked at the chassis photo of your K731 and saw the enormous amount of splooge around the electrostatic tweeter, I laughed out loud and thought; "What idiot squeezed all that in there?" then it dawned on me. I've been having a devil of a time getting harmonic buzz out of my C730, so last night I pulled the tweeter out (it's held in with two fragile little clips that attach to the corners) and fashioned a neat little silicon 'bed' then re-attached the tweeter. Magic! The buzz is gone, and my favorite radio can once again be cranked and enjoyed as intended. See, it pays to look at the pictures! Last edited by Nolan Woodbury; 03-30-2006 at 09:47 AM. |
#23
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Thanks for that little bit of info, though it was in response to another post. I couldn't remember which substitution could overload the string; now I know. I'll file this with the rest of my Zenith radio info, of which I have quite a bit.
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Jeff, WB8NHV Collecting, restoring and enjoying vintage Zenith radios since 2002 Zenith. Gone, but not forgotten. |
#24
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I saw that picture of the "innards" of Shane's K-731 and also noticed all that gunk around the electrostat. I wondered myself what it was. I couldn't imagine Zenith would have used such a cheap method of securing the tweeter to the cabinet in a radio like this (or any of their other vintage receivers, for that matter); the original Zenith Radio Corp. had too much class for that, IMHO. Then I read the rest of your answer and saw your comments about the four clips that hold the tweeter in place in the cabinet; that made a lot more sense. I haven't had the back off my own K-731 in months, so don't know whether the tweeter has the same stuff around it that Shane's has. It probably does, though. Now you have me wondering. What on earth was the purpose of all that junk around the electrostat tweeter? If it was not to mount the thing in the cabinet, then I'm baffled. BTW, I will send you a PM addressing another matter after I write this.
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Jeff, WB8NHV Collecting, restoring and enjoying vintage Zenith radios since 2002 Zenith. Gone, but not forgotten. |
#25
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I'm sure yours is tidy, but my C730 has seen some wars before coming into my life. Still, I've been working on it for about 8-months now and not only does it have SOLID gain and excellent reception, it has developed a nice glow from all the attention and polishing. A VERY sweet radio and a Zenith classic! |
Audiokarma |
#26
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Nolan, I've got the blonde version of yr set...Really needs to go see Terry & get "tuned up". It was my granmother's & she kept it on a station in Black Mountain, NC, that played classical all the time. She had an antenna for it-I hooked it up to a portable TV/AM/FM once, & I swear, I was picking up stations from Atlanta...The entire FM band was just one big station.
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Benevolent Despot |
#27
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Aren't these models called "Long Range" receivers or some similar? I thought I read that somewhere.
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#28
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The earlier-late '40s ones were, I think...
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Benevolent Despot |
#29
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Jeff, WB8NHV Collecting, restoring and enjoying vintage Zenith radios since 2002 Zenith. Gone, but not forgotten. |
#30
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As powerline voltage is usually on the high side, close to 125V, the 50C5 in placce of the 35C5 would work fine.
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Audiokarma |
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