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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!

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