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Vintage Delco 8 Track ...

Discussion in 'Electronic Repair' started by Arfa Daily, Sep 20, 2007.

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  1. Arfa Daily

    Arfa Daily Guest

    Any of you good ol' boys across the pond, familiar with a Delco 15CFMT3 (or
    might be a 25CFMT3 as it has two stickers on it, one red, one black, each
    purporting to be the "service model number" ).

    It's from a vintage Chevy, I believe, and a colleague asked me to have a
    quick look, and that's all it's gonna be, as I really don't have the time.
    Initially, it had instability at about 50Hz, dependant on vol control
    setting, It has a multi-section can-type cap in the back right corner, and
    this seems to be open circuit on one of its sections. I hung a 1000uF
    temporarily across the bad section, and that knocked the instability problem
    on the head. I then tried the radio. There is plenty of livliness on AM, as
    in it picks up a few stations, and there are lots of shortwave-like birdies,
    but that is without a proper antenna connected, and in an electrically
    pretty noisy environment (my shop!). On FM, it seems a bit insensitive, but
    again, that might be down to a poorly matched rod antenna that I am hanging
    on it. I don't know how well these old fellas performed in the first place.
    If they were anything like British ones from the same era, then it's not
    very well as I remember.

    So, on to the 8 track bit of it. Mechanically, it all works, including track
    selection, but audio-wise, there is just a horrendous mess on both channels.
    There may be audio at the back of it all, but nothing recognisable. It's
    hard to describe the noise. Sort of a very random, but continuous, distorted
    'crackle'. Both channels exactly the same, and controlled by the volume
    control. Given that the radio section / output stage works (sorta !) I am
    guessing that this other problem must be in the 8 track head amp somewhere.
    I really am not going to go looking for it, as I really don't have the time
    or the inclination. I am not looking for general repair advice on it, as I
    repair hifi equipment for a living as most of you know, so am well familiar
    with fault finding. All I am looking for is if anyone remembers the model
    and can say " Oh yes, That was a really common problem. It's C29 " or
    whatever.

    So, anyone got anything for me ? TIA

    Arfa
     
  2. Meat Plow

    Meat Plow Guest

    I would look at how the audio is switched from the radio to the 8 track.
    Is there a mechanical switch buried in there somewhere? IIRC I worked on a
    Delco am/fm/cb/8track long ago that had a mechanical switch in the audio
    path that could cause horrendous noise. I'd also look at the head itself
    although less common. I used to work on a lot of mobile audio but that was
    decades and a few lost brain cells ago. :)
     
  3. Jerry

    Jerry Guest

    Have you taken a close look at the 8-track cart you're using for an
    audio source? The pressure pad that is used to push the tape against
    the head is usually a cheap metal springy strip with an even cheaper
    foam piece glued in the middle. Those old 8-track carts are notorious
    for the foam disintegrating with age. If the tape doesn't make good
    contact with the head, it's not gonna sound too good.

    I discovered this a couple of years ago when I was trying to transfer
    some 30 year old vinyl LPs and 8-tracks to CD - had to refurbish the
    pressure pad on nearly every 8-track.

    Here's a good online resource:
    http://www.8trackheaven.com/

    Jerry
     
  4. Arfa Daily

    Arfa Daily Guest

    Thanks both. I'll look into them tomorrow, but I don't think that it's going
    to be either of them. The noise on the audio has got a distinctly
    'electronic feel' about it. It's very hard to describe, but if it was on one
    channel only, rather than both, you'd be looking for a rustling cap or a
    noisy transistor - you know the sort of noise ?

    Arfa
     
  5. Guest

    I know you're not looking for generic advice but I had a Delco Am/FM/
    cassete radio in my 1983 Buick years ago. It had small micro switches
    that engaged when you loaded the tape. They used to have to be sprayed
    regularly. And that would do the trick for a while. It was a bitch of
    a job getting at them without pulling the radio too. Finally I broke
    down and disassembled them and resurfaced the contacts inside and that
    was the end of the problem. That statically crackling you are
    describing sounds very reminiscent of my old car. Lenny Stein, Barlen
    Electronics
     
  6. Arfa Daily

    Arfa Daily Guest

    Thanks Lenny. It could well be something like that, but I didn't see any
    microswitches that you could get at, and it looks like a major dismantling
    job to either get below the deck, or above the radio PCB, and if that is the
    case, then I am not going to go further for lack of time / interest. I only
    got it (reluctantly) in the first place, because I do a lot of hifi repairs
    for the shop in question, and blundered in there last Friday and got cajoled
    into taking it "just for a quick look" by the lad who does all of the in-car
    stuff, and looks about old enough to have just come out of junior school !
    He had never even seen an 8 track, but figured that I would be ancient
    enough to know all about it. Boy, I feel old sometimes, now ...

    Arfa
     
  7. Guest

    Hey don't knock ancient. We have more experience in our little fingers
    than that all too well meaning kid probably has in his whole arm. In
    spite of all that though, I don't think I ever worked on a Delco 8
    track but I do remember the first time I looked at my Buick radio. It
    truly seemed impossible to work on the transport but a further really
    close inspection finally revealed 4 small screws that held the entire
    transport in. After unplugging and removing it, working on it was a
    simple job. Perhaps your 8 track is similar. Give it a really good
    eyballing. Some of the older Delco stuff was really miserable to work
    on but you might be lucky. Lenny Stein, Barlen Electronics.
     
  8. 15CFMT3 = Sams AR-114
    25CFMT3 = not listed.


    I have a copy of Sams AR-114, if you need any information from it.
    I'll try to dig it out for you. The biggest failure in most of the
    various models was noisy or dead transistors, or motor noise from the
    motor's speed control circuit, caused by poor grounds or bad
    capacitors. A failing motor would arc, and radiate noise into the
    preamp, as well. A scope made short work of that problem. I worked on
    hundreds of those Delco 8 track tape decks in the '70s.


    AR-114 was published in 1972, probably in February.

    http://home.earthlink.net/~mike.terrell/Sams-Index.html Is a chart by
    year of publication. This information isn't in the Sams electronic
    index. If anyone has any newer data, I would appreciate it.

    http://home.earthlink.net/~mike.terrell/HWSAr.html is a list of the AR
    manuals I have on hand at this time.


    --
    Service to my country? Been there, Done that, and I've got my DD214 to
    prove it.
    Member of DAV #85.

    Michael A. Terrell
    Central Florida
     
  9. Arfa Daily

    Arfa Daily Guest

    Thanks for the offer Michael, but the moment has gone now. The guy that gave
    it to me has had it back. There was an O/C section on a multi section
    can-type electro at the back right corner. Bridging that got the radio going
    reasonably ( there was a loud "hum" previously which I guess was LF
    instability rather than actual hum, it being DC car equipment ). When an 8
    track cart was pushed in though, it seemed not to produce any audio, other
    than a loud random crackle, which did indeed sound as though it might well
    be a noisy transistor, or even resistor. Still, it was only a 'favour' job,
    and I was never going to spend any real time on it, so I called it a day
    with the radio working. Thanks again anyway. I knew there would be someone
    on here that knew a bit about them ;-)

    Arfa
     
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