Zenith K725

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Zenith K725

Post by Zeek on Thu Jul 04, 2013 2:40 am

This is my first tube radio (child of the transistor age), so y'all forgive me if some of my questions are...well, stupid Laughing 

First things first, want to clarify some things first; in reading some information it stated that tube radios can catch on fire, but didn't bother going in depth under what conditions they can catch fire. Is there anything special to watch for beyond simple things that can cause any electrical equipment to catch fire (e.g. are old tubes more prone to catching fire)?

I plugged the radio in and tested it out...works fairly well (hooked up iPad to FM transmitter and tuned into an old time horror show), but did notice the case was a little warm (really, just warm, not hot). I assume this is normal? Is it safe to leave plugged in and is there a time limit that I should allow it to run before shutting it down?

Where do I even start on the restoration? I'd really like to make this radio like new, and I'm sure from the component aspect this should be easy to accomplish, but what about the plastic case and cloth speaker cover (unfortunately, I don't think replacing the speaker cover is feasible, not sure if you can even find one in this pattern).

Where can I find more information on this radio, for example, is the tuner dial / indicator supposed to be lit up? Schematics, and other information. Where is a good place to get replacement knobs and tuner dial plastic covers.

The case is in fairly good condition with only an extremely small crack on the underside that is completely unnoticeable sitting on a desk. Regardless, I think I'd like to repair that, but not sure.

Tons of questions, and just looking for a bit of guidance on where to even start. Thanks in advance!


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Re: Zenith K725

Post by simplex1040 on Thu Jul 04, 2013 11:49 am

HI,

Tube radios if they have not been recently serviced could short out resulting in fire.
the capacitors in these sets have a tendency to be old and dried out, they can then short and cause issues.

Changing the electrolytic capacitors is a must in an old set. When they fail they can take other important components with them.
Changing the paper capacitors will prevent other issues.
Tube radios do run warm.

Looks to me like your tune knob is only missing a a silver cover.

see this photo
http://www.radioatticarchives.com/radio.htm?radio=6090
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Re: Zenith K725

Post by Zeek on Thu Jul 04, 2013 1:11 pm

Excellent, thanks Simplex. I can understand shorted components causing a fire, just didn't know if there was anything additional to look for other than the standard electrical components (e.g. didn't know if dust laying on the tubes increased risk...hypothetical example of course).

You're right, the tune knob is only missing a silver cover; the silver coating on the on/off, volume knob and the other knob are flaking off leaving the raw plastic. Is there any way to recoat these?

I'm going to focus on bringing the operational components back up to spec however, I am not a body shop (before radios, worked mostly on cars) type of guy. Any recommendations on a shop that can restore the case? Overall, the case is in excellent condition, just that minor crack that I would like to restore.

Appreciate the response, thanks.

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Re: Zenith K725

Post by Wildcat445 on Thu Jul 04, 2013 10:58 pm

Simplex, with all due respect, I would be willing to wager that you have never actually seen an old radio short out and cause a fire. I would wager that you do not know anyone who has seen an old radio short out and cause a fire. I would also wager that you heard "somewhere" that it was possible and are simply parroting what you heard. What is there inside an old radio, housed in a metal chassis that could get hot enough to burn? The wires in radios are small enough that they will melt when they get sufficienctly hot, thereby breaking the circuit. Witness the failure of power transformers and audio transformers. They do not burn. They can put out lots of smoke, that's for certain. But the likelihood that they would actually catch fire and threaten their surroundings is unlikely. Telling anyone, especially someone just starting out, that failure to replace capacitors could cause a fire is unnecessary, IMHO. How that old wives' tale got started is anybody's guess. One could take a blowtorch to a radio and probably not get it to actually catch fire and burn (cabinet notwithstanding). If strict flammability was an issue, would not modern equipment with PCB's and lots of plastic in its construction be more of a fire threat than an old radio with a bakelite cabinet and metal chassis? It is more common for coupling caps to open or become leaky than to short. It is fairly rare even for an electrolytic to actually short. They more commonly become leaky and cause a hum. I have only run across a half dozen known shorted capacitors in my life. Capacitor failures can potentially cause harm to the radio and its components, not likely the radio's surroundings. Capacitors can become sufficiently leaky and raise the current draw on a power transformer and cause it to fail without actually being shorted. I was always taught that if the rectifier "sees" less than 25k ohms in the B+ line (from the rectifier cathode to B-) that the filter caps may be considered shorted. Zero ohms would technically be a dead short. And as I said earlier, I have seen very few capacitors dead shorted.

As far as environmental possibilities of radio fires, my grandpa ran an old Zenith radio for years in his milking parlor, actually a 5-S-56. When I rescued it from the old milkhouse during its demolition, there was so much hay and chaff and cobwebs covering the radio chassis that I couldn't see the tubes. It had probably run that way for 25 years. Did you ever see an old metal radio or tv chassis that had been through a house fire? There may be a surprising amount of it left. Try that with your new flat panel. You will have to produce a receipt of purchase to be able to prove that one actually existed. The dust normally found in radio chassis poses no fire threat. The bulb themperature of a glass rectifier or output tube is about 400 degrees or so, metal ones hotter still. So heat buildup in the cabinet is normal. Some radios, such as my three Hallicrafters sets, actually have a cardboard bottom under the chassis. One would think that such an arrangement would not have survived for 60+ years if radio chassis had the proclivity to catch fire.

Just rebuild this radio, replacing the caps for performance reasons, make sure everything is up to snuff, operate it like it was intended, use common sense and you will have many years of SAFE enjoyment from your radio. Can you injure yourself fooling with, and operating an old radio? Sure you can. You can also get killed going to the mailbox if you step out in front of a concrete truck that you didn't see.

Regards

WC

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Re: Zenith K725

Post by simplex1040 on Fri Jul 05, 2013 12:52 am

WC,
you can also set fire to the carpet in your house with the radio chassis setting on it when a cap let go. So can one cause a fire you bet. is it common , no. The question was asked I answered with my experience.

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Re: Zenith K725

Post by Wildcat445 on Fri Jul 05, 2013 1:06 am

Got it! I will be more careful next time I run a radio chassis while it is setting on carpet. Since my house has all porcelein tile, can I come over and play?

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WC

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Re: Zenith K725

Post by simplex1040 on Fri Jul 05, 2013 10:47 am

nah we have hardwood floor in this house, harder to set fire to than carpet ( yeah I know I took all the fun out) plus no radios allowed on the floor anymore.
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Re: Zenith K725

Post by Bill Cahill on Fri Jul 05, 2013 11:22 am

And, while this argument is going on, Ugghh, I have seen a radio catch fire.
I was working on a Philco 48-1201 radio chassis.
I had re capped it, and, put in new electrolytics.
I was getting an audio hum, no reception.
I started checking voltages , and, just at that time, there was a lud pop, and, one of the IF transformers caught fire.
Fire actually shot out the bottom of the can.

I shut it down, disconnected transformer.
Tried firing it up again, heard another pop, and, a sizzling.
Shut it down, and, found out audio output transformer had also failed, and, was burning.
Primary had shorted to chassis.

Now, what were you saying about no electrical fires??


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Re: Zenith K725

Post by Zeek on Fri Jul 05, 2013 3:34 pm

Here I was relieved when I read WC's post lol! So what I'm seeing is that I have a military grade anti-personnel device sitting on a wood end table next to my pillow on my bed.

Seriously, what I'm hearing is that it CAN happen, but not necessarily a common occurrence and just be cognizant of what I'm doing when working on it. If I've already run the radio, chances are there's no short and the fire hazard is pretty limited. What I should be worried about is potentially degrading other pieces of the radio that are working based on pieces that are not functioning at full capacity already.

Really appreciate the answers all!

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Re: Zenith K725

Post by Bill Cahill on Fri Jul 05, 2013 3:38 pm

NO, not quite correct. If you are using a set with the original caps, and, electrolytics in it, they have already degraded, and, it's merely a matter of time before complted failure.
Shorts can, and, will happen.

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Re: Zenith K725

Post by MinnesotaHam on Fri Jul 05, 2013 3:43 pm

Yes, fires in old radios are uncommon. But I would not so easily dismiss the ability.
The energy available on the secondary side of the power transformer is limited by the transformer itself.
The only limitation on the line cord or primary side is the house circuit fuse.
1800 Watts (120 volts * 15 amps) is several times that needed to ignite something on fire.
True, most caps don't dead short. I have two of these. But it does not have to be dead shorted to be a danger.
The line to ground cap used in many radios is connected directly to the line supply.
Many of us have pulled line caps out of radios that have shriveled or burst from repeated heating / cooling cycles. This is all the visual evidence I need to know of the awesome potential of the line supply used by old radios.

I may not feel an immediate need to replace caps in old radios. But I DO insist on knowing the status of the primary side of the power supply circuitry before plugging in a radio of unknown condition.

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Re: Zenith K725

Post by chrisc on Fri Jul 05, 2013 4:43 pm

Since this radio is one of the later tube type models most of the capacitors could be the 'disk' type which are generally OK. Paper capacitors are almost always bad to the point where it is hardly worth checking them. These are tubular in shape. The large Electrolytic capacitors , which are usually together in a big silver can, will very likely need replacing. This would probably not be hard and I'm sure we could help you do that. Zeniths still were using 'point to point' wiring with these sets into the 60's and this does make life easier when replacing parts. I have a similar set and they are fine performers.

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Re: Zenith K725

Post by Wildcat445 on Fri Jul 05, 2013 5:25 pm

We can talk "woulda, shoulda, coulda" all day and get nowhere.

My intention in making a comment was to reassure the OP that he was not likely to burn his house down messing with an old radio. Fortunately, he appears to have gotten my message in spite of the rather dramatic attempts at times to discredit what I said. I truly feel that causing undue concern by insisting that caps that fail can cause a short and burn his house down is simply untrue. That's all I have to say about that.....

Regards

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Re: Zenith K725

Post by Tony V on Fri Jul 05, 2013 7:04 pm

My advice is reguardless if the old radio, tv or record player has been restored or not, NEVER play them unattended. These are antiques after all and failures do occur even with new capacitors. Tube electronics were marginal at best for safety when they were new (hot chassis radio's is one example) so how are they going to be any better 40+ years later? Its common sense.
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Re: Zenith K725

Post by 75X11 on Fri Jul 05, 2013 8:42 pm

One other thing that hasn't been brought up is that if I remember correctly, that is a hot chassis radio and should be treated as though it carries line voltage at any time when plugged in. Bear that in mind when you have it out of the housing, or before you operate the controls without the plastic knobs in place to isolate you.
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Re: Zenith K725

Post by Resistance is Futile on Fri Jul 05, 2013 9:36 pm

Where do you people think the term curtain burner came from? I think that may have been a concern and just spread, resistance power cords evidently had a reputation of catching on fire, don't know personally though.
As with any electrical appliance if the power cord gets frayed and/or shorted you could have a fire especially with any type of flammable's. I plugged in a piece of test equipment and didn't realize the power cord was bare at the chassis and actual red and white hot sparks flew before the circuit breaker kicked off. So I suppose if you had that situation and had alcohol or flammable material next to the radio it could create some definite worries.

However for most intents most radios are pretty darn safe.
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Re: Zenith K725

Post by Guest on Fri Jul 05, 2013 10:31 pm

Well I hope we didn't scare the heck out of Zeek! Shocked 
 
All I'll say is; "Let common sense prevail and enjoy a get hobby!" Very Happy

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Re: Zenith K725

Post by 276merc on Mon Jul 08, 2013 9:32 pm

That is a fairly common example of the fine AM-FM table models Zenith built in the 50's, very little changed in the circuit and they used a different letter each year. Yours is a 1953 model and here is the schematic and one persons experience.

http://www.glcharvat.com/Dr._Gregory_L._Charvat_Projects/Zenith_K725_Photos.html#3

http://glcharvat.com/website%20pdfs/53_Zenith_7K01_192.pdf

Where do you people think the term curtain burner came from? I think that may have been a concern and just spread, resistance power cords evidently had a reputation of catching on fire, don't know personally though.

Curtain burners were replaced in the mid-late 30's when the various AC/DC series string tubes arrived and the Zenith in question is from 1953. It still has the hot chassis cautions as built and Id suggest a polarized plug type line cord at a minimum that mates to the radios connector.

Besides replacing paper caps and electrolytics Id also suggest replacing that selenium rectifier with a 1N5408 diode with a 33-47 Ohm 3W metal oxide resistor in series. Experiencing the room clearing stink of a failed one is not pleasant.

Carl

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Re: Zenith K725

Post by Art Hoch on Tue Jul 09, 2013 5:24 pm

I never go off somewhere while an old radio is playing.
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Re: Zenith K725

Post by Tony V on Tue Jul 09, 2013 8:36 pm

I agree with you Art...never leave vintage electronics alone while playing. Restored or not as you never know.
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Re: Zenith K725

Post by 75X11 on Tue Jul 09, 2013 9:13 pm

I wouldn't either. I like to get the maximum benefit of having the equipment on.
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Re: Zenith K725

Post by Doug Burskey on Wed Jul 10, 2013 12:56 pm

I have heard that remark about any vacuum tube electronics,"they run so hot you can heat your house with it!"True they run warm but make shure the vents on the case are not blocked. Worked on a few of those Zenith K725 like sets and always pleased with how they turned out. The thing to watch out for is that capacitor that is connected across the power cord input. If it is a ceramic disc you should be OK. If it's a black tubular with color code stripes it can short out and some times split open and go BANG! Replace it with a good quality disc capacitor.
I have a 725 that had a service order folded up inside the set dated in the early 1960's. The customer complaint was "made popping sounds and blows fuses" Then the remark from the service tech was "not reparable, offer customer a trade in on a new set" When I pulled the chassis that cap was all blown apart! Sounds like someone wanted to sell a new radio.

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Re: Zenith K725

Post by Wildcat445 on Wed Jul 10, 2013 1:19 pm

MEZLAW wrote:Well I hope we didn't scare the heck out of Zeek! Shocked 
 
All I'll say is; "Let common sense prevail and enjoy a get hobby!" Very Happy

I hope we did not as well. Zeek cannot ever say that he did not get a response!

Regards

WC

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Re: Zenith K725

Post by 276merc on Wed Jul 10, 2013 1:42 pm

The thing to watch out for is that capacitor that is connected across the power cord input. If it is a ceramic disc you should be OK. If it's a black tubular with color code stripes it can short out and some times split open and go BANG! Replace it with a good quality disc capacitor.

You mean like this from a H723? Those early disc ceramics should also be replaced, they were Zeniths attempt at making their own.....resistors:roll: 

Carl




And this is what happens when the cap in the IF can lets go. The puncture is the tiny dot at top center that resulted in thunderstorm static.


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Re: Zenith K725

Post by Zeek on Sat Jul 13, 2013 2:19 pm

Hey all! Didn't mean to have spawned such a hot debate, it really is due to some warnings I'd seen and having never worked on tube radios (yes, ashamed to admit, I'm a solid state era baby). I have a tendency to error on the side of safety and wanted to know just how much of a concern this actually was. Based on the responses, I definitely have a good idea and just with everything, it's a matter of a healthy respect for the potential regardless of the probability.

I have worked on electronics before, but it was over 20 years ago when I was in the military (so really rusty) and even then, it was tracing signals, determining bad components and plucking and chucking.

Y'all didn't scare me off, generally spare time on weekdays is relatively limited due to commute and other obligations, thus the reason I hadn't replied. I think it comes down to Mezlaw said about common sense and followed up by Art (never leaving the house with it playing).

I do appreciate the responses everyone and thanks much! I'll try and tender my questions to a less controversial subject in the future Laughing 

Thanks again...and to be honest, this forum has some of the best members I've seen on a number of forums...even in a debate.

I have the chassis out of the case and reviewing the schematics. I will probably do the maintenance on the electronics, and then bring it to a radio repair shop to validate the work that I've done.

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Re: Zenith K725

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