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ICOM 751a meter lamp

Discussion in 'Radio Circuits, Repair & Performance' started by KB3UWC, Jan 22, 2012.

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  1. KB3UWC

    KB3UWC Subscriber

    the meter light burned out in my 751a. can anyone tell me if this is the small 12v wheat grain light bulb? all the service manual gives is a part number. If its still available its probably at least $5 plus shipping. I have access to the wheat grain bulbs locally. I don't want to take it apart untill i have a bulb in my hand. So i am asking here.

    Thanks and 73 Steve
     
  2. W9GB

    W9GB Ham Member

    Steve -

    Good question!
    I will have to do some research, to see if this was answered before on QRZ, or if someone has a photo of that disassembly.
    The Icom IC-740 used the small 12 V bulb - T1 bulb size commonly found in Kenwood and Icom radios.


    KC2OBG did not take a photograph :-(
    http://kc2obg.com/s-meter-lamp-replacement-751a-17.htm

    You could also send an e-mail to Scott Malcom at MTS or Matt at SARS Technical Services.
    They likely know and have that spare part to tell you.
    ==

    w9gb
     

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    Last edited: Jan 22, 2012
  3. W9GB

    W9GB Ham Member

    Steve -

    The Icom IC-751A S-meter lamp appears to be a T-1 sized bulb, as shown in photo attached earlier.
    LED replacements suffer from "hot spots" on this radio -- is they are not diffused
    (in other words, don't use high brightness - and you will have to scuff up the transparent lens).

    Icom IC-751a S-meter lamp replacement

    With the IC-751a sitting on the electronics workbench (front panel facing technician).
    Remove 12 machine screws holding the Top cover of the transceiver (6 screws on Top; 3 screws on each side - right and left) DO NOT LOSE.

    With the radio open, on the front panel you can see the backside of S-meter.
    There will be a black rubber grommet/plug mount with two wires on the back of that S-mater. That is the meter lamp.

    GENTLY pull out this rubber mount from S-meter with your hand.
    You will see the meter lamp inside of the rubber extrusion.

    Assure that the lamp is actually bad. Some owners discovered that the wires were pinched or broken
    Check the meter lamp to assure that it is indeed "burnt out".
    The IC-751A schematic will show the Voltage (8V, 12V, etc) to this lamp.

    Replace with NEW Meter Lamp -- assuring that the bulb glass has the same clearance (rubber grommet) as the old bulb.

    Ken, K4EAA sells a package of 4 lamps ($5.00) - 12V, 50 ma, 4mm dia., 6" wire leads for
    Kenwood hybrid radios that fit many other radios of this era.

    Reassemble the radio - 12 screws that you saved.

    ==
    w9gb
     
  4. KB3UWC

    KB3UWC Subscriber

    Thanks Greg, This weekend i am going to my dad's house for his birthday. I know he keeps these in stock. Replacing the bulb looks straight forward enough. I have the factory service manual, so will check the voltage.

    Thanks and 73 Steve
     
  5. WA7PRC

    WA7PRC Ham Member

    Inrush current kills incandescent lamps. The current pulse, typcially several times the running current, causes a mechanical shock to the filament. Eventually, the filament breaks. The answer is to limit the inrush current to no more than the running current.

    For DC-powered GOW lamps, the LM317L (about 50ยข) + 1 small resistor can function as a current regulator (see datasheet). Since it drops 1.25V, use a 12V lamp instead of a 14V lamp. I did that with my olde Kenwood TS-120S and TS-130S rigs several years ago. Zero failures since then.
     
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