Viewing 15 posts - 1 through 15 (of 25 total)
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  • #11087
    Bernie Pyra
    CVRS Member

    I am new to the restoration of old tube radios and am working on a 1960’s vintage Marconi stereo radio Chassis 638/645. I have the chassis but not the speakers. I would appreciate any help in determining what type of speaker set up I should use for this radio. I have the schematic for the radio, 2 pages of which are attached. It does not seem to indicate what impedance the speakers should be. It appears to me that there were 4 speakers for this unit, 2 for each channel. The schematic shows one speaker larger than the other and it appears to me that for each channel they are wired in parallel. I am unclear what the other items are that are pictured in the schematic for the speaker layout. Any advice as to how I should set up speakers for this chassis would be appreciated. Thank You.

    #11089
    Alistair Thomson
    CVRS Member

    According to the schematic you supplied of the 645 (the audio amps), there are only two speakers: Left and Right. The circuit uses one output transformer in each channel (T2 and T3), with no field coils, so any modern permanent magnet (as opposed to field coil) speaker with an impedance from 4 to 16 ohms will work. I suggest a diameter between 5″ and 12″ but of course the cabimet will have the correct size of cutout. There will have been an impedance specified in the original design, but in the absence of any more info you can try any of the above without causing any harm.

    #11090
    garykuster
    CVRS Member

    Bernie, I’m new to this as well and your schematic looks more like an illustration. The symbol for speaker(s) isn’t used, instead it’s sort of pictorial. Can you not tell by the cabinet if it had more than one speaker per side? My parents old 60’s era Grundig hi fi had tweeters as well as a set of 10 inch speakers.

    Good luck, Gary

    #11091
    Bernie Pyra
    CVRS Member

    Thank you to both Alistair and Gary. Gary, I do not have the cabinet for this Marconi radio, so I cannot view the speaker configuration. You are correct, I attached an illustration, but the schematic I have does not provide information about the speaker setup. I believe that you are correct in that it had tweeters as well as a larger bass speaker.
    Again, thank you very much for the information. Bernie.

    #11092
    Alistair Thomson
    CVRS Member

    It’s common to have a single drive unit, or two with a crossover, or even three with two crossovers, but the radio actually doesn’t care!

    If the receiver is AM, then there is little point in having a complex speaker system to listen to the radio, because the sound quality just isn’t there. But the gramophone would certainly benefit from a two-drive arrangement plus crossover on each channel, and if the receiver is FM then it’s definitely worth having a more hi-fi arrangement than a single speaker, even if the radio is mono.

    However, for testing and basic troubleshooting, any speakers will be fine!

    #12329
    betula
    Forum Participant

    Hey Bernie, I have the 646/636 amp and radio. It’s the same but different haha. Seriously it looks the same but four 6BQ5’s and two 6C4’s.
    I hope this works. Below is a link with a schematic for my unit. I have the speaker compliment for it. The output transformers are for 4 ohm speakers. I would love to get my hands on the schematic for the am/fm radio part of this unit.
    marconi646_zps9fec631b
    This second link is the speakers for it. I hope.
    https://www.flickr.com/photos/upload/

    #12330
    betula
    Forum Participant

    Further to my last reply re: Marconi is another link with (I hope) the speaker.

    https://www.flickr.com/photos/185468526@N02/?

    #12331
    Jean Marcotte
    Forum Participant

    For betula : Schematics for models 636/646 are in RCC Supplement 60, pages 41 to 48

    #12332
    betula
    Forum Participant

    Hey Jean,
    Thanks for your prompt reply. Sorry but I am not familiar with RCC. What does it stand for?
    Also, were you able to view the speakers?

    #12333
    betula
    Forum Participant

    I live in the GTA. Would it be at the Toronto Public Library?
    Thanks

    #12334
    Bernie Pyra
    CVRS Member

    For betula: Thank you for the information you have provided and for the various photos. I am able to view the speaker picture. When I was looking for the 638/645 schematic, I ended up purchasing it from justradios.com

    However, your chassis may be different and thus these schematics may be no use to you. If you want a copy, let me know.
    I, like you do not know what the RCC Supplement is so I will look forward to Jean’s response as well.

    #12335
    betula
    Forum Participant

    Hey Bernie,
    Thanks. Yes, Jean’s Radio/amp is single ended and mine is double ended. Hence the 4 6BQ4’s.

    #12336
    Bernie Pyra
    CVRS Member

    For betula: Found out that RCC is Radio College of Canada. I did some searching on Google and found that the Toronto Public Library does have 80 copies of an RCC Service Manual. They are at various branches around the GTA.
    Here is the link I found
    https://www.torontopubliclibrary.ca/detail.jsp?Entt=RDM224019&R=224019

    Not sure this is the book that Jean references but worth a look. Hope you can find the schematic for the chassis you have.

    Thanks
    Bernie

    #12353
    Jean Marcotte
    Forum Participant

    RCC was the tech’s bible at the time. Looks like that’s what you found at the library. Actually, models 638/645 are in Supplement 67 pages 42 to 47.

    Jean

    #12354
    betula
    Forum Participant

    Hey Jean,
    I’ve been to the Research Library and had no luck. They directed me to the Sams Publications and couldn’t find it. I will go again and focus on RCC(Radio College of Canada)
    and try my luck there.
    I will not be using the speakers for this unit. They are still attached to the wooden grills. If you’re interested I can let them go for a very reasonable cost.
    Thanks again for your interest.

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