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Thread: well, the darn thing is out

  1. #1481
    Super Moderator LarrBeard's Avatar
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    M38A1 Distrributor RFI Capacitor

    Quote Originally Posted by pelago View Post
    Is this cap in line with 24vdc another noise suppressor?
    Yes. On the M38 the coil/distributor noise suppression filter was on the fire wall. On the M38A1 it changed to the coaxial RFI filter on the distributor.

  2. #1482
    Senior Member bmorgil's Avatar
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    Maybe a quick Electronics 101 might help. The capacitor (also called a condenser) will short any stray opposite voltage signals to ground. They have a polarity. Placed on a PLUS DC line and grounded, they will ground out any stray NEG voltage spikes (AC noise) that cause the familiar AC whine in the radio. Placed accost the points they will charge when the points break, preventing an arc.

  3. #1483
    Senior Member pelago's Avatar
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    Quote Originally Posted by bmorgil View Post
    Maybe a quick Electronics 101 might help. The capacitor (also called a condenser) will short any stray opposite voltage signals to ground. They have a polarity. Placed on a PLUS DC line and grounded, they will ground out any stray NEG voltage spikes (AC noise) that cause the familiar AC whine in the radio. Placed accost the points they will charge when the points break, preventing an arc.
    however no where on wiring diagram nor on distributor indications does it even say that there is a cap in the 24vdc line,,,,,, just a curiousity thats all but then my manual is very limited and only covers what we called 2nd echelon maintenance in the corps, i was a electronic tech chief for 2nd mar div

  4. #1484
    Super Moderator gmwillys's Avatar
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    The diagram doesn't show the guts of the distributor. This diagram is small, but it will give you a descent breakdown.

    http://www.surplusjeep.com/M38A1/m38...electrical.htm
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    Last edited by gmwillys; 04-15-2019 at 03:39 PM.

  5. #1485
    Senior Member pelago's Avatar
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    I believe i got it!! Running time so far two hours, not a glitch so far, actually have a video of timing light but cannot do here

  6. #1486
    Super Moderator gmwillys's Avatar
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    Good deal! Hopefully you banished the Gremlins for good.

  7. #1487
    Senior Member bmorgil's Avatar
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    It almost looks like a redundant "points condenser" in your drawing. I can see however that it may have helped with radio noise. Everything was relatively low frequency AM band stuff and short wave in the WWII era. Any stray AC would wine for sure. The points opening and closing definitely would generate a little noise. Maybe the boy's were on to something!

  8. #1488
    Super Moderator gmwillys's Avatar
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    Even in the new military vehicles, you can still pick up a him on the intercom during testing. I don't know about the radios, but they run a narrow band, but that was a dozen years ago. Some of the radios have been upgraded, but that was more the tracking equipment for anti-friendly fire.

  9. #1489
    Senior Member pelago's Avatar
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    WHEN I WAS WITH 2ND MAR DIV We had a issue with the LVT7 (amtrac) the audio for the intercom and radio in the amtrac had a hum this audio system converted all audio to digital for use in the system. the culprit was the shield on the cover, the gasket was a (546.00 GASKET, THAT RIGHT 546.00 US DOLLARS) IT creating a actual antenna that transmitted the audio as rf, was wierd, but new copper infused gasket that actually cost less cured it

  10. #1490
    Super Moderator LarrBeard's Avatar
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    M38A1 Distributor and a Serving of Crow

    Excuse me, Ma’am.

    May I have a large helping of crow – without feathers if you have it. Thanks to everyone who looked at my drawing of the M38A1 ignition and helped me realize that it wasn’t correct. I hope it didn’t mislead anyone.

    To reduce the radio interference in the M38A1, the engineers put ALL of the ignition stuff in a shielded box; the points, “condenser” and ignition coil. To prevent interference from crawling back out of the can onto the 24-volt vehicle wiring, they put the “capacitor” on the input to the distributor – which I believe is Wire # 12 in the harness.

    Hopefully this is a bit more correct.
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