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Repairing a 1937 GE cathedral super het

Discussion in 'Troubleshooting and Repair' started by dietermoreno, May 11, 2013.

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  1. dietermoreno

    dietermoreno

    238
    0
    Dec 30, 2012
    [​IMG]


    I have a knack for destruction of all things electronic.

    Unfortunately, when I visted my mom in Florida in the winter over winter break, they have an antique radio as a piece of furniture in their living room and in my fascination with the workings of radio I turned it on and destroyed it.

    When I turned it on, I waited for the tubes to warm up, then I turned the volume up to 3 and all I heard was mains hum.

    At night I tried tuning the dial to 780 cycles (the dial actually said cycles on it instead of hertz that's how old it is) to attempt to receive a Chicago station at night. No luck, only mains hum.

    I turned the dial from 550 cycles to 1600 cycles and all I heard was mains hum.

    I was very frustrated and in my frustration I cranked the volume to 11 to see if I could hear any stations over the mains hum, and all that accomplished was I heard a high pitched feedback squeal and then I smelled smoke and heard a pop and it went off and won't turn back on.

    On the back of the radio it says copyright 1937 by GE.

    When I looked in the back of the radio at the chassis underneath the grill, I saw that it has 4 power tubes and 4 pre amp tubes just like my guitar amp (except my guitar amp was mupped to have the 4 pre amp tubes replaced with transistors so the pre amp tube holder sockets in my guitar amp are empty).

    I saw its power supply transformer and its output transformer, just like my guitar amp.

    It plugs into mains, just like my guitar amp.

    It has no master volume control. It only has one gain/volume control. Compared to every guitar amp that I have has a master volume control.

    Now the similarities stop.

    I saw that the tuning dial was an old fashioned huge air variable capacitor.

    I saw that it had a ferrite loopstick antenna.

    I looked for the RF gain stage tubes and didn't see any, except I think I saw that there were 2 tubes that were not lit up but I ignored them because I thought those are just audio tubes so it will just make the volume quieter so I have to crank the volume to 11.



    So in actuallity, maybe I'm an idiot and those 2 tubes that were out were the RF gain stage tubes and that's why I'll that I heard was mains hum.


    I don't like listening to mains hum.



    Also probably I'm an idiot for cranking the volume to 11 when its not working. Maybe this is part of how I destroy so many things.



    Any guesses at the cause of the magic smoke and pop?



    I will be visting my mom's house in Florida on summer break in 2 weeks, so I was hoping to build up a knowledge base before I go back there and try to plug it back in.
     
    Last edited: May 11, 2013
  2. duke37

    duke37

    5,364
    771
    Jan 9, 2011
    You damaged an amp by overdriving it and now you have severely damaged this radio set.

    A loopstick antenna seems to be very unlikely for a 1937 set.
    How many sections are there in the tuning capacitor?
    Can you find a model number?
    There are 8 tubes but what are their numbers? Do not unplug them but try to see what they are in situ. Unplugging old tubes in the wrong way can break the bottom off.
    One tube will be a rectifier to give the HT (B+) voltage and the other seven all have to work since the signal passes from one to the other in series.
    You only have one volume control since you only have one speaker.

    The mains hum was probably due to faulty smoothing capacitors. If these are leaky, they can take out other valuable components. When you heard the hum why did you not turn it off immediately?

    In order to repair this, you will need a schematic and a good knowledge of tube radios. It will be necessary to disconnect some parts of the radio to find the fault and then work through from output to input.

    The set will use high lethal voltages and even the chassis may be live. It should not be touched without an isolating transformer.

    Damaging a modern amplifier is unfortunate, killing a valuable heritage item is unforgivable.
     
  3. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,496
    2,837
    Jan 21, 2010
    1) I hope you've owned up to it.

    2) I hope you don't even consider trying to fix it.

    3) Learn to stop playing with things you don't understand.
     
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