Saturday, December 5, 2009

details of Zenith K725 restoration are now posted online

http://www.mit.edu/~gr20603/Dr.%20Gregory%20L.%20Charvat%20Projects/Restoration%20of%20a%20Zenith%20K725.html
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2 comments:

  1. Hello, I'm looking for some information. I just purchased a Zenith k725 at a yard sale. It's in great condition. When I turn it on it takes a while to warm up (tubes) but when it does come on it sounds great. My problem is that after several minutes of playing the sound drops out. Could this be a bad tube??? and if so how do I know which one to replace???
    Thanks
    Mark Booth markb112255@comcast.net

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  2. Hi Mark,

    No matter what is wrong with it you will have to follow the restoration procedure for antique radios which is to first replace all electrolytic and paper capacitors then try out the radio to see if it works. Whatever the problem, it usually has to do with those old capacitors. Check out this briefing i wrote on how to restore antique radios and give it a try:
    http://web.mit.edu/gr20603/www/website%20pdfs/MIT_Haystack_Open_Lunch_Oct_21_09.pdf

    I've found that the tubes are usually OK. Only once have i had to replace a tube during a restoration.

    If the radio is still acting goofy then you want to do signal tracing from to see at what tube the signal stops. Test that tube, if good then check the resistors around it. You will need an isolation transformer to perform any signal tracing because that radio is a hot chassis radio, meaning that the line cord is tied directly to the chassis.

    Good luck, hope you can get it working!

    Greg

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