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 Post subject: Restoring a Zenith K731
PostPosted: Mar Tue 13, 2018 6:11 pm 
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Joined: Mar Tue 13, 2018 5:48 pm
Posts: 2
Hi everyone, I am brand new to the world of vintage radios and audio equipment. Right now I would love to restore my old Zenith K731, but I have no idea where to start.

It works, sort of. It has a lot of trouble getting a clear sound and the tuning indicator is actually one channel higher than the actual. It's a bit rough cosmetically, it is missing the tune knob and the screen is pulled up in a spot. The front panel is worn and doesn't sit quite straight anymore. The main issue is that, when you turn it on and give the tube time to warm up, you have to get a great tuning and turn the volume to max to try and get the speaker to kick in properly, otherwise it's extremely quiet. After that you can turn it back down and as long as you don't go too quiet, it'll keep working normally. Though sometimes it will start to lose the tuning and will go quiet again.

What I would like to do is make this a custom piece. It was my father's, so I'm attached to it and want it to last for a long time. The wood has some dings and scrapes, so I'm considering sanding down the finish and staining it a darker color. I want to keep a vintage look, so I'm thinking a moderately dark color and replacing the screen with a silver or brass-colored screen if I can find the material. I would also like to replace the speaker with a new, high quality one.

Please let me know what I can do to restore this awesome little radio, as I'd love to see it last for another generation or more. I have no experience with these old radios at all, but I am technologically inclined to a degree and I can work with a soldering iron.

Thanks!
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 Post subject: Re: Restoring a Zenith K731
PostPosted: Mar Tue 13, 2018 8:43 pm 
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Joined: Dec Wed 25, 2013 7:57 am
Posts: 2662
Location: USA
This is a good place to start.

https://antiqueradio.org/begin.htm

There are a few components that need to be replaced. Performance will not improve by itself. Please leave it unplugged while you figure out what to do next. Ask here on anything that's not clear.

Welcome :)

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 Post subject: Re: Restoring a Zenith K731
PostPosted: Mar Tue 13, 2018 8:54 pm 
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Joined: May Thu 14, 2015 4:15 pm
Posts: 949
Location: Dallas, TX
I'm going to guess there is a bad connection or short. The sound vibration clears the problem until the vibration gets too low. Could be happening inside a tube or other part. Always start with a re-cap. You may find the problem in the process.

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It's not the Destination, It's the Journey.


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 Post subject: Re: Restoring a Zenith K731
PostPosted: Mar Tue 13, 2018 8:55 pm 
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Joined: Jul Mon 06, 2015 11:25 am
Posts: 200
Take a look at this video to see what is involved:
https://www.youtube.com/watch?v=6R16kF_b6cw

Also, RestoreOldRadios has 3 videos on the K731 starting here:
https://www.youtube.com/watch?v=vfau9AeJtVE&list=PLjLu4V-71S1KMcp59HqwhBgSQBgdRMXJJ


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 Post subject: Re: Restoring a Zenith K731
PostPosted: Mar Tue 13, 2018 10:04 pm 
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Joined: Aug Sun 23, 2015 6:01 pm
Posts: 758
Location: South Jersey East of Philly
This model was one of the first radios that I fixed. The Zenith table tube radios of the early '60's had great tone and sensitivity.
I'm guessing from your description that you may have a scratchy volume pot that needs some DeOxit or CRC into it to allow it to work properly.
The radio finish doesn't look bad for a 50+ year old radio....I wouldn't worry about re-finishing it since this was your father's and it would seem to be a better memory of him in the original finish.


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 Post subject: Re: Restoring a Zenith K731
PostPosted: Mar Wed 14, 2018 2:12 am 
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Joined: Jan Mon 18, 2010 2:13 am
Posts: 17028
Location: Dayton Ohio
Hello and welcome to the Antique Radio Forums! :D

Well, this is your radio, and you may do whatever you want with it, but most of us here lean on the side of preserving what it is.
The cabinet may clean up with some good cleaning. Some waterless hand cleaner such as Gojo, Goop or NAPA (The smooth stuff WITHOUT pumice) will remove years of dirt, grime, wax and nicotine.
It may shine like a new dime. The cloth probably could be pulled back down and glued in place. Knobs can be found
The original electronics and speakers may be ok and it just needs restoration. New electrolytic capacitors is a must. Plus any wax/paper caps will need to be replaced. As I recall, these used at least a few "Bumblebee" types which have a black body and stripes like a resistor.

Once restored, it will probably surprise you in sound quality using the original radio and speakers (this radio has two speakers)

A story about one of these models. Long ago, I was trying to get a job at a local TV shop. I visited it often and became friends with the owner. One day he was having difficulty repairing a Zenith K731. It was intermittently cutting out.
He was at his wits end with it and I asked if I could take a whack at it. After a while of getting nowhere, I finally hit upon the area of the intermittent. It was around a terminal strip on top of the chassis where the speakers connect. There are a couple capacitors and resistors around the terminal strip and a splash of solder fell between a capacitor and the strip intermittently shorting one of the connections to the chassis. It was completely hidden down there. It may have been there since new, but anyway, I found the problem and fixed it.
He was duly impressed and gave me $7 for my help!
I feel this action helped me get hired not long afterwards. :D

-Steve

Oh, here is a photo of mine, which I recently acquired. No, this is not the one I worked on those many years ago.


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 Post subject: Re: Restoring a Zenith K731
PostPosted: Mar Wed 14, 2018 3:12 am 
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Joined: Jan Thu 01, 1970 1:00 am
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Location: Earth
You might want to read up on the importance of an isolation transformer. A good thing to acquire for working on these kinds of radios. Zenith used metal chassis and wired one side of the incoming power cord to the chassis. This can present a nasty shock hazard under the right circumstances.


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 Post subject: Re: Restoring a Zenith K731
PostPosted: Mar Wed 14, 2018 3:41 am 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 156
Location: parkersburg west virginia
I have this same model & i use it every night to listen to my podcast from my F.M transmitter. i've had this radio for 25 years & it has been in consistent use since. when i got it, it needed work done on it. the 35C5 had an open filament, & the 12DT8 was weak. this chassis is similar to many other models made throughout the sixties. there are a couple of things that you must do if you want to use this radio on a regular basis. one, you've got to replace the selenium rectifier with a silicone diode. two, you may need to replace the coupling capacitor for the 35C5 as some of the early models used bumblebee capacitors. third, you need to pull the tubes & clean the sockets with DeOxit.
usually, the first tube that fails on these is the 12DT8 F.M osc/conv tube.
these radios seem to last a very long time between servicing.


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 Post subject: Re: Restoring a Zenith K731
PostPosted: Mar Wed 14, 2018 1:40 pm 
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Joined: Mar Tue 13, 2018 5:48 pm
Posts: 2
Thanks for all of the in-depth replies, everyone. I'm honestly still unsure how to start with it, I wish I could find my multi-tester... I would like to replace the grill cloth, even if I don't do a custom job, because it's extremely tight where it's pulled up and it also has that hole you can see in the picture. There are some deep scratches and even a bad ding or two which can't be seen in the picture, which is why I thought a refinish might be a good idea - but I totally respect that most of you like to keep things original. I'd like to make it look as new as possible, so if there's a magic product out there that'll get rid of the light marks in the finish, let me know! :lol:

But, of course, function over fashion here. How can I tell for sure what needs replacing? I've only opened the unit once, and that was just to take some compressed air and give it a cleaning (WOW there was a ton of dust). I know it's a live chassis so I want to be educated on what it is I'm doing with it before I jump in.


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 Post subject: Re: Restoring a Zenith K731
PostPosted: Mar Wed 14, 2018 3:51 pm 
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Joined: Jun Tue 10, 2008 6:06 pm
Posts: 502
Location: San Antonio, TX
For just a general cabinet clean-up I would recommend using Howards "Restore-A-Finish" in neutral wiped on with a rag, allowed to set a while, then wipe off with a clean, soft rag (old T shirt). After this only a major, deep scratch will be noticeable. All this best done to an empty cabinet, stripped of dial face, grill cloth and knobs.

Welcome and good luck
Dale


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 Post subject: Re: Restoring a Zenith K731
PostPosted: Mar Wed 14, 2018 6:45 pm 
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Joined: May Thu 14, 2015 4:15 pm
Posts: 949
Location: Dallas, TX
Many times I've seen furniture restorer use Naphtha (aka VM&P Naphtha) to wipe down woodwork. It cleans the finish without otherwise effecting it. I use it for cleaning lots of things, even metalwork. It is very similar to "cigarette lighter fuel".
The scratches might be helped with a touch up pen. You will have to pick one with the same color or maybe a little lighter.
https://www.homedepot.com/b/Paint-Inter ... boZ1z0t40w

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It's not the Destination, It's the Journey.


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 Post subject: Re: Restoring a Zenith K731
PostPosted: Mar Thu 15, 2018 4:39 pm 
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Joined: Jun Wed 08, 2011 2:33 am
Posts: 7141
Location: Ohio 45177
What about wiping down with Gojo? That stuff will take soil off a finish pretty good. And then gives it a bit of shine for awhile, it seems.


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