tube radio repair: HMV-1112

Discussion in 'General Electronics Chat' started by bug13, Mar 8, 2015.

  1. bug13

    Thread Starter Well-Known Member

    Feb 13, 2012
    1,208
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    Hi guys

    I got this radio from a second shop, it has been sitting in my garage from a while now. Now I have some ideas. My goal is using it as tube amplifier. (by passing the radio function)

    This radio is His Master's Voice - 1112, also called Marconiphone 919.
    Here is the schematic: http://www.radiomanuals.info/information/Broadcasters/sMarconi%20919.pdf

    So I found this faulty cap, what type of cap should I replace it with? It looks like C19 by matching the value and location, but I am not sure.

    Thanks guys!

    IMG_2528.JPG IMG_2529.JPG
     
  2. bug13

    Thread Starter Well-Known Member

    Feb 13, 2012
    1,208
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    Forgot to ask:
    Is there any harmful chemical/substance in these old caps that I need to worry about? As the one needs replacing is popped open.
     
  3. bertus

    Administrator

    Apr 5, 2008
    15,648
    2,348
    Hello,

    It looks like the mentioned C19 is part of the tone control in combination with VR2.
    The value mentioned is 0.05 μF / 400 Volts.
    Nowerdays the normalized value would be 47 nF.

    As it is a foil capacitor, no dangerous fluids have been used (as they would have in electrolitic capacitors).

    Bertus
     
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  4. flat5

    Active Member

    Nov 13, 2008
    403
    17
    Replace all the wax and paper caps that are in the audio chain (the circuits you are interested in).
    I would also remove the B+ from other stages not used. Don't trust these types of old caps.
     
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  5. BReeves

    Member

    Nov 24, 2012
    412
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    rca6v6pp.jpg I wouldn't..

    That radio is an early AC/DC with no transformer. This means the chassis is connected to one side of the AC line. You hope it is the neutral side and should be if the power cord is polarized. Just fix the radio and enjoy it for what it is, if you want a tube amp build one, it isn't that hard.

    Here ya go, build this and you will be happier with the results.
     
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  6. ian field

    Distinguished Member

    Oct 27, 2012
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    AFAIK: those are just waxed paper - some of ye olde ancient electrolytics had some dodgy chemicals in them, but I used to pull them apart when I was a kid and I'm more than half-way to antique so far.
     
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  7. bug13

    Thread Starter Well-Known Member

    Feb 13, 2012
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    So what is the green capacitor and the big cap behind the resistor? What type are they?

    IMG_4843.JPG
     
  8. Hypatia's Protege

    Distinguished Member

    Mar 1, 2015
    2,803
    1,237
    Mylar caps are good upgrade replacements for paper/foil types...

    In the interest of abatement of 'hot chassis' issues, it may be advisable to operate the device 'through' a 1:1 line-isolation transformer...

    Aroclors (polychlorinated biphenyl based potting compounds) were not deployed as dry (CIP wax) dielectric materials in that vintage/type of component...

    Despite the absence of polarity markings, the green-clad capacitor looks as if it may be electrolytic -- a better 'angle' is required for confident identification.

    Best Regards
    HP
     
    Last edited: Mar 9, 2015
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  9. Dodgydave

    Distinguished Member

    Jun 22, 2012
    4,999
    745
    They look like wax types, the green may be a polarized cap, need to know its location on the circuit,

    if your planning on using it as an amplifier, use the pick up terminals, but disconnect the radio signal feed first,by removing R15, looking at the circuit it has a transformer psu, if yours doesn't it will have a Live chassis.
     
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  10. Hypatia's Protege

    Distinguished Member

    Mar 1, 2015
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    FWIW I'd use a LIT in any event -- I've seen too many 'ground-faulted' power transformers in old sets (Can we say Crosley? ;))

    Best regards
    HP
     
    Last edited: Mar 9, 2015
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  11. snav

    Active Member

    Aug 1, 2011
    100
    10
    I think I saw the filiment/hv/rect windings in the diagram he provided. Why do you say it's transformerless?
     
  12. debe

    Well-Known Member

    Sep 21, 2010
    947
    184
    Because the original picture he has shown of the radio, it had no transformer but in its place were large wire wound resistors with adjustable taps. Typical of sets with no transformer. In those sets the valve filaments are all wired in series & a large resistor is used to control the filament voltage.
     
  13. Lundwall_Paul

    Member

    Oct 18, 2011
    220
    19
    By the way that cap is an electrolytic. The big black band tells me that.
     
  14. ian field

    Distinguished Member

    Oct 27, 2012
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    AFAICR: the big black band indicates which end is connected to the outer foil - if you peel the capacitor, you will eventually come to one of the 2 layers of foil - the one you get to first is connected to the lead at that end.

    In that era, the majority of electrolytics were chassis mounted, the polarity was marked with red and/or black paint dots on the solder lug rivets.
     
  15. BReeves

    Member

    Nov 24, 2012
    412
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    Because the schematic he linked to isn't for the radio he has.

    This has to be one of the first transformerless tube radios. Before they came out with the 35y4 and the other HV filament tubes to eliminate the need for the resistor. Thousands or maybe even millions of 5 tube AC/DC radios were sold based on that simple design.
     
  16. snav

    Active Member

    Aug 1, 2011
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    Ah, Now I understand. Different post same subject.
     
  17. bug13

    Thread Starter Well-Known Member

    Feb 13, 2012
    1,208
    38
    Hi guys

    Thank you all for your concerns, I do understand the reason of your concerns, and the risk involves in a transformer-less psu.

    Please understand that:
    • I am repairing my old radio at work with my spare time. (there is at least another person with me while I am working on it)
    • There is an isolation transformer that I can use, if I need to test it.
    • All the power sockets are RCD protected.
    • I will NOT measure/poke/replace component live.
    Thanks again guys!

    PS: My radio is transformer-less, but the schematic (transformer version) linked above is the best I can find so far.
     
  18. ian field

    Distinguished Member

    Oct 27, 2012
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    In the UK; TV sets were mostly AC/DC dropper type from quite early on. Toward the end of that era, they realised they could dramatically reduce dissipation in the heater dropper by inserting a half-wave rectifier in the feed. Some of the first sets with solid state tuners used this fact to derive the 12V for the tuner, simply by having a 12V MOV (VDR in those days) as the bottom most element in the heater chain.

    This can be used to advantage if you have to piggy-back a WW resistor to replace a burned out section of heater dropper - apart from anything else, you can use a smaller/lower resistance.

    The only thing to watch for, is whether anything derives a low voltage AC supply from the bottom of the chain - I can't think of anything on a radio that would.
     
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