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help: repair to VHF marine radio

Discussion in 'Electronic Repair' started by Larry Gagnon, Dec 12, 2004.

  1. Larry Gagnon

    Larry Gagnon Guest

    I am attempting to repair a Midland Model 78-100 VHF Marine Radio for
    a friend. I have basic radio electronics knowledge, some
    troubleshooting skills, a 275MHz scope, VHF sig generator, freq
    meter, RF power meter, etc, but am unable to locate the problem.

    Symptoms are: completely dead on receive and transmit on all channels,
    but unit powers up good, all power supply circuits and voltages are
    good, display and frequency display circuitry are good, no detectable
    cold solder joints nor saltwater corrosion anywhere, power to all
    chips is good.

    I first suspected the PLL circuitry (due to its being common to RX and
    TX), but signals coming out of the PLL, prescaler and VCO chips appear
    ok.

    Any suggestions as to where I might start looking would be greatly
    appreciated.

    Larry Gagnon
     
  2. NSM

    NSM Guest

    | I am attempting to repair a Midland Model 78-100 VHF Marine Radio for
    | a friend. I have basic radio electronics knowledge, some
    | troubleshooting skills, a 275MHz scope, VHF sig generator, freq
    | meter, RF power meter, etc, but am unable to locate the problem.

    Don't you need an FCC license to do that?

    N
     
  3. Larry Gagnon

    Larry Gagnon Guest


    Oh please, do get a life you sad little man! And since when did you assume
    that I live in the USA anyways and that I am subject to FCC regs?

    Larry
     
  4. G

    G Guest

    Howdy Larry.......You didn't mention having a schematic so I'm assuming
    you're troubleshooting blind. Do you have any receive audio noise and/or
    does the squelch seem to work? If no noise is heard I might start with
    injection AF at the volume control to see if you have an audio stage (and
    make notes continuously when troubleshoot unless it's a very familiar radio
    to you) Does the radio seem to switch from receive to transmit? I'd
    probably start by looking at the basic transmit chain for activity starting
    with PTT and looking for some low level on a known frequency....say 156.300
    (CH6).....of course make sure you're dumping into a suitable dummy load
    (good for 25watts at least). Use that fancy scope of yours to look for the
    TX chain activity.......It wouldn't surprise me if you had something "like"
    a problem with the basic T/R circuitry if you don't have any activity at
    all - a typical setup might use a dedicated IC for this purpose. If you
    have a schematic.........and could attach it via e-mail I'd be glad to walk
    you through some troubleshooting.

    Note: Having a second receiver capable of receiving these frequency's
    (cheap scanner for example) can make quicker work by finding out whether
    you have any low level activity when placed in the vicinity (same room
    etc).

    73 - Gord VE1AJF
     
  5. NSM

    NSM Guest

    | On Sun, 12 Dec 2004 19:52:26 +0000, NSM wrote:
    |
    | >
    | > | > | I am attempting to repair a Midland Model 78-100 VHF Marine Radio for
    a
    | > | friend. I have basic radio electronics knowledge, some troubleshooting
    | > | skills, a 275MHz scope, VHF sig generator, freq meter, RF power
    meter,
    | > | etc, but am unable to locate the problem.
    | >
    | > Don't you need an FCC license to do that?
    | >
    | > N
    |
    |
    | Oh please, do get a life you sad little man! And since when did you assume
    | that I live in the USA anyways and that I am subject to FCC regs?
    |
    | Larry

    Plonk!
     
  6. Jamie

    Jamie Guest

    i assume that you have acknowledge that the unit is actually attempting
    to RX/TX via the mic button?
    like for exampl a TX light coming on etc..
    in any case, you may have a mixer/IF stage in the frequency section that
    is common to both RX and TX.
     
  7. WbSearch

    WbSearch Guest

    Not sure if this is applicable to your situation, but I just repaired a
    different brand radio with similar issues that had controls in the mike. There
    was a small 8 pin connector on the board in the radio from the mike cable that
    had broken loose from the PCB. I wrapped a small wire around the end of my
    soldering iron, put a couple pairs of reading glasses on, (bad eyes),
    resoldered the connector, and have a happy customer.
     
  8. bg

    bg Guest

    Not sure if this is applicable to your model, but a common problem with
    marine rigs is that the antenna or coax opens up. When TX is pressed, the
    transmitter being unloaded, tends to blow out the pin diodes that switch the
    front end between Tx/Rx. The Rx front end winds up with 25 watts directly
    into the input. The transmitter power module might cook as well.
    bg
    Larry Gagnon wrote in message
     
  9. Larry Gagnon

    Larry Gagnon Guest

    On Sun, 12 Dec 2004 09:42:40 -0800, Larry Gagnon wrote:

    thanks to all who replied to my query. I found the problem in the VCO. 3
    cold solder joints. It meant removing the VCO compartment shields and all
    the beeswax it was sealed in. Fun that was, but it worked!

    Larry
     
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