Replacing capacitors in an old Telefunken radio

Discussion in 'General Electronics Chat' started by RickW, Feb 3, 2015.

  1. RickW

    Thread Starter New Member

    Feb 3, 2015
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    I am trying to restore my Grandfather's old Telefunken Opus 6 (1955) to working order, and have a few questions. All the "experts" say to start by replacing all the electrolytic capacitors and the selenium rectifier. Since these are no longer available, they suggested using a silicon bridge rectifier with a dropping resistor rated at 100 ohms @10 watts. I assume (did I say that I am NOT an electronic engineer) that the resistor would be connected in series with the output of the bridge, but rather than do damage to the radio, I have chosen to appear stupid and ask someone that knows for sure. Also, there are a couple of 50MFD caps in it that I have been unable to locate so far. Would a 47MFD work as a replacement? Thank you in advance for your help with this!
     
  2. MrChips

    Moderator

    Oct 2, 2009
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    I would begin with a visual inspection with the power off. Check for bulging, leaking capacitors. Discoloured components such as resistors. Brittle or damaged insulation on wires and cables. Check fuses.

    If everything checks out ok, I would apply power and see what happens. Look, smell, listen. Be prepared to turn off the power promptly.
    Only after conducting a power on test would I decide to replace capacitors (and other components) after I have made a note of all symptoms.

    Yes, a 47μF electrolytic capacitor can be substituted for 50μF. Pay particular attention to the voltage rating on the old capacitor. It would not hurt to go to a higher voltage, go from 25V to 50V or 63V for example. If you are planning on replacing the dual filter capacitors, you'll be looking for a pair of 47μF electrolytics, greater than 250VDC. I would be looking for 300VDC or greater.

    Watch the physical size and case size of the old and replacement components. Make sure that the new component, axial or radial leads will mount suitably in the circuit. Most of the metal can capacitors will be difficult to replace exactly. Modern capacitors will be in smaller packages with plastic wrap over the can.
     
    Last edited: Feb 3, 2015
  3. MaxHeadRoom

    Expert

    Jul 18, 2013
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    On many of the older radios it also pays to replace the smaller value caps, especially if these appear to have a cardboard sleeve and are waxy in appearance.
    They can be replaced by their current equivalents Polyester or Polypropylene film or even mica.
    Max.
     
  4. GopherT

    AAC Fanatic!

    Nov 23, 2012
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    I would try it with just a capacitor replacement and check the selenium rectifier last. If you get that far, I have a selenium capacitor if you are sure you need it.

    image.jpg

    I believe these are Selenium too...

    image.jpg

    Let me know.
     
  5. Dodgydave

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    Jun 22, 2012
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    Last edited: Feb 3, 2015
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  6. alfacliff

    Well-Known Member

    Dec 13, 2013
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    use your nose, selenjium rectifiers usually have a rotten egg smell when they go bad. there is a problem with replacing selenium rectifiers, they have a higher voltage drop than silicon diodes, and may make the rectifiers fail. the added resistor will only drop the voltage after the tubes warm up the selenium diodes pictured above are not for power supply use, the long thin ones are for high voltage erctifiers in the horizontal stages of tv's and the one with 3 leads are low curent ones used for tv circuits also.
     
  7. AnalogKid

    Distinguished Member

    Aug 1, 2013
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    I agree about replacing the selenium rectifier with a silicon one, but it isn't as simple as it might seem. Each plate of a selenium rectifier stack is an individual diode with about 1 V of forward drop and good for about 20 V of reverse voltage. So you can get a 1st pass guess of the voltage rating for the replacement diode by counting the plates, multiplying by 20, and doubling that for margin (or just go with a 400 V part for 125 VAC, or 600 V part for 220 VAC). Those plates also act as heatsinks, so the replacement part almost certainly will need a little aluminum around it. Guessing the current rating is more difficult, but if you can post a photo I bet someone around here will have an experience-based guess.

    Another possible issue is that the total drop across the old rectifier could be 10 or 15 V, while the drop across a single 400V silicon diode will be less than 1 V. That extra voltage can cause issues in marginally designed circuits. This is one of the reasons for the external resistor. If the radio doesn't smoke when you power it up, consider inserting a small power resistor (10 ohms or fewer) between the original rectifier and the first filter capacitor. The voltage across the resistor will tell you the current rating you need for the replacement rectifier. Again, take the reading and at least double it for margin.

    An old radio is a very warm operating environment and nothing shortens semiconductor lifetime like heat, so I can't stress enough how important it is to overrate the replacement diode. Standard axial packages are good for 1 W, 3 W, and 5-6 W (P6 package), all of which are difficult to heatsink effectively. If using one of them, leave the leads as long as possible because they are the primary heatsink. Back when I was doing this semi-regularly I used a stud mount diode rated for 10A and 400V on a about 5 to 10 square inches of aluminum, whatever would fit. Never failed. That old stud package might be expensive nowdays, but a high voltage diode in a TO-220 package is an excellent option.

    ak
     
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  8. crutschow

    Expert

    Mar 14, 2008
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    I would think that the current in a tube radio (likely less than an amp) would be low enough that the power dissipated in a silicon rectifier (say a 2-3A unit) would not require any heat sink.
     
  9. RickW

    Thread Starter New Member

    Feb 3, 2015
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    Thank you for the advice. I have ordered a 35 amp, 600 PIV stud mount rectifier encased in aluminum and plan to mount it directly to the frame with a little heat sink compound. That should take care of any heat problems. Still sourcing some of the other caps and plan to replace every one that I possibly can. With the help of the fine people at this website, I may well end up with a great family heirloom. Thanks again!
     
  10. #12

    Expert

    Nov 30, 2010
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    100% wrong.
    1) One diode does not replace 4 diodes.
    2) This machine uses less than 1/2 amp through the rectifier, not 15 amps or 30 amps.
     
  11. RickW

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    Feb 3, 2015
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    Sorry if I misled you. It IS a bridge rectifier. I went with the 35 amp more for the way it mounted and the aluminum casing. I know it's overkill, but there were few options at 600 volts. Thank you for your comments.
     
  12. #12

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    OK. It's still more than I would do. Little plastic 4 diode rectifiers are available, effective, and cheap.
     
  13. RickW

    Thread Starter New Member

    Feb 3, 2015
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    I know, but if it's going to be a family heirloom, I want it to be "bullet proof".
     
  14. GopherT

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    Nov 23, 2012
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    You'll have to stock up on Digital signal converters or VCRs or DVD players if you have any chance of using it in the future. Unfortunately for your heirs, they will inherit a large, heavy, outdated technology that makes a better boat anchor than a source of entertainment or sculptured artistic beauty. Make sure you call it an heirloom several times in front of your children and have an unbiased party tell you the reaction of your heirs. I am not sure it will be positive.
     
  15. RickW

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    Feb 3, 2015
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    The creators of the same technology you are rebuffing almost conquered the world! I wonder if you would feel the same about a 1933 supercharged Bentley?
     
  16. GopherT

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    Some old technologies fascinate me, others don't. The old VW beetle was designed by the same crew and I wouldn't wish that vehicle on anyone as a daily driver.

    Good luck.
     
  17. RickW

    Thread Starter New Member

    Feb 3, 2015
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    Can't argue with you on that! My daughter had one of the "new" ones. Couldn't fix ANYTHING on it.
     
  18. snav

    Active Member

    Aug 1, 2011
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    Haven't seen your schematic but most amps use tube rectifier for B+ (Have you checked for a 6x4 or 12x4 tube first?) and the selenium is for the negative bias. Very low current for bias, may be perfectly fine.
     
  19. RickW

    Thread Starter New Member

    Feb 3, 2015
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    looking into it. Thanks for your reply.
     
  20. Dodgydave

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    Jun 22, 2012
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    schematic is on post #5
     
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