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Need to fix old valve radio

Discussion in 'Electronic Repair' started by [email protected], Mar 4, 2007.

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  1. Guest

    I have a very old valve radio from a company called Lafayette.
    Sorry, I accidently posted it to the wrong group.
    To add to the above, it is a MW/LW/SW type
    and the problem occurs on all of the bands.

    Thanks in advance.
     
  2. Guest

    ....and lastly, the radio has not been used since the last time I
    tested
    it about 10 years ago. Then it worked fine, no I tested it again and
    suddenly there is this problem. It is entirely possible that it was
    knocked (I hope not dropped) while in storage, but there is nothing
    visibly broken, and all the valves do glow.
    BTW, I tried removing the valves one by one and seeing if one valve
    did not "make a difference", but this approach did not work, any valve
    removed results in no sound at all.
     
  3. The power supply. Check the high voltage. Could be the rectifier valve.
     
  4. Ken Weitzel

    Ken Weitzel Guest

    Hi...

    It's been a long long long time, but I don't believe any of those quoted
    are a rectifier. Perhaps selenium, or silicon.

    Take care.

    Ken
     
  5. Hi...
    Could be selenium ... difficult to tell from what has been posted.
     
  6. DaveM

    DaveM Guest


    The 6X5 is the rectifier. After 10 years of shelf time, the most likely thing
    that you should look at is the electrolytic capacitors in the power supply.
    They are probably dry by now and need to be replaced. Since tube equipment runs
    hot, you should replace them with 105C temp range units.

    Cheers!!
    --
    Dave M
    MasonDG44 at comcast dot net (Just substitute the appropriate characters in the
    address)

    Some days you're the dog, some days the hydrant.
     
  7. 6X5 is a full wave rectifier.

    I'd check the main electro (multi part) and the resistors across it.
     
  8. Tubes/valves rarely go bad just sitting.

    I'd start by checking electrolytic capacitors.

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  9. hr(bob)

    hr(bob) Guest

    Got a model number? It sounds like a transformer powered all-band
    1940's superheterodyne set. Later sets used 12, 35 and 50 Volt
    versions of those tubes in series diretly across the AC power mains.
    Then they were replaced by miniature versions of the same type of
    tubes. The schematics for these sets are all almost identical, the
    6say is converter, 6sk7 is IF amp, 6sq7 is detector and 1st audio amp,
    and 6v6 is audio output, 6x5 is rectifier. Volume control is at input
    to 1st audio amp. Do you have a scope so you can look at the signal
    from that point to the speaker? If so, I can walk you thru the testing
    needed.

    H. R. (Bob) Hofmann
     
  10. PeterD

    PeterD Guest

    6X5 is the rectifier...

    Could be bad, but is there any hum? If yes, check the filters, too.
     
  11. Guest

    Bob, thank you. I don't have a scope.
    I think you are right about the "series" thing. They are protecting
    the valves these way, if someone takes one out everything stops
    working.

    Here is more information:

    The model number is very faint, but it ends with E629, maybe IE629

    There is also a 4 digits serial number, 2xxx
    Probably not too many of those about today :)
    Made in America
    Rated 110 to 125 Volts 50-60 cycles 40W
    Yet (strangely) was always operated from 220V.

    I need to change the power cable, it is probably the original and
    is crumbling in a very dangerous way, so I might get a glimps
    to the internal components. It looks like a big job simply to take
    the unit out from the wooden box without damaging the tuning
    mechanics. I wonder if it can simply slide out or if it needs to
    be lifted out (bad).

    Oh, and another interesting thing, the pickup coil doesn't use
    ferite core like nowadays, there is a huge drum and on it the
    coil is wrapped.
     
  12. Be aware that many radios of this time were 110 volts and used the mains
    cord as a dropper to obtain that from 240.
     
  13. hr(bob)

    hr(bob) Guest

     
  14. Guest

    I have just finished replacing the power cable. The unit came out as
    a whole, which was easy. I then was able to see all the
    components inside. It is very messy as in those days they were
    no printed circuit boards, but the general impression I got was
    that damaged was caused to the internal part/s due to storage
    in the attick, and last summer here in England the summer was
    the hottest on record, so you can imagine... Everything looks
    shining, a sort of wet-look, this must mean bad news.

    On the positive side, I did manage to fix one of the two front panel
    illumination bulbs - it seemed to be just a contact issue, and after
    whasing the bulb in some water, it was working again. Now remains
    only the audio problem.

    I have taken some pictures, I will post them soon.

    If someone wants this radio, make me an offer. I can ship
    worldwide, and I accept paypal.

    Thanks.
     
  15. hr(bob)

    hr(bob) Guest

    Dave is 1000% corret. The line cord that is crumbling is probably in
    that condition because it has gotten so hot so many times over the
    years. The cord has to dissipate at least 20 watts or maybe even
    twice that amount to drop the voltage down to 120 v when used on a
    230V circuit..

    The first thing to do is to measure the mains voltage - 230V
    approximately, and then measure the voltage at the set end of that
    crumbling line cord when the set is turned on and see if it measures
    about 115 Volts. If so, then the line cord must not be replaced with
    a regular line cord or everything in the radio will be fried beyond
    repair.

    The only way to salvage the radio will be to insert a fixed resistor
    mounted somewhere inside the radio that will drop the voltage the same
    as the original line cord is now doing. If it is possible, I would
    suggest measuring the resistance of the two conductors of the line
    cord, add the resistance of the two conductors together, and use a
    resistor of that ohms value as the replacement for the old line cord
    resistance. The wattage of the resistor will be 120V x 120V divided
    by the resistance, or 14,400 divided by the resistance of the line
    cord. It will be a sizeable resistor!

    Once we get the power input to the radio fixed, we can then start
    going through the audio stages to see what is wrong. A scope would be
    nice, but I think we can measure voltages at a number of places and
    get things fixed.

    H. R.(Bob) Hofmann
     
  16. Kurt

    Kurt Guest

    How's the speaker? Could be blown.

    Distortion could be caused by bias failure. Leaky capacitors (probably
    called condensors when the radio was built) are a big problem with tube
    (valve) musical instrument amplifiers.
     
  17. hr(bob)

    hr(bob) Guest

    When you plugged the radio in after replacing the pilot light for the
    front panel, were the lamps the same brightness as before the line
    cord was replaced. If so, then the line cord must not have been used
    to drop the line voltage from 230V to 115V. The set must be using a
    power transformer designed for an input of 230V and not the 115V shown
    on the back panel that you mentioned. I will see what info I can
    find. Do you have a high-empedance voltmeter available to check the
    votages on the tubes? Is the bottom of the radio accesible? Do you
    know how to count tube pins from the keyway?

    Bob Hofmann
     
  18. Guest

    Hello Bob,
    (and others)

    I am not sure about the brightness, since I received you message
    after I had done the work, I didn't pay much attention to this.
    I do have a digital DMM. Here are some pictures that I took
    while repairing the power cord:

    http://www.geocities.com//
     
  19. hr(bob)

    hr(bob) Guest

    I went to "nostalgiaair.com to see if the model number worked. I
    couldn't find anything close, perhaps you can go to the site and see
    if anything rings a bell.

    Ok, with a digital multimeter, we should be able to get going. Let's
    assume that the radio did not get fried due to a doubling of the input
    voltage. We will check the B+ voltage for some sanity, and then go
    from there. There should be a large chassis-mounted electrolytic
    capacitor rated 30 uf or larger, at 200V or higher. The can should be
    grounded to the chassis, and there should be two non-grounded lugs
    connected to various components.

    Check the DC voltage on each of the two lugs. I would expect each of
    the voltages to be at least 150V DC, maybe as high as 350V, one should
    be a few volts higher than the other. Is the speaker a permanenet
    magnet speaker, or is the place where the magnet normally is located
    actually a coil of wire with two wires leading to it (in addition to
    the 2 wires that go to the speaker cone)?

    If you don't see at least 150V on the electrolytic, check pin 8 of the
    6x5. That is the output of the rectifier circuit. It should be
    connected to one of the electrolytic terminals.

    Is the sound clear at low volumes? How does it distort as you turn
    the volume up, does it just get fuzzy, or does the volume go up and
    get fuxzzy at the same time.

    Bob Hofmann
     
  20. hr(bob)

    hr(bob) Guest

    Just looked at the photos, great. the large metal can is probably the
    electrolytic I wrote about as there is no chassis-mounted can visible
    in the above-chassis view. The big loop is the loop antenna, nowadays
    they use a ferrite stick, back then most coils were flat wound in a
    spiral form rather than a coil such as your radio has.

    Bob H
     
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