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#2 VHF will not turn off!


Mazurb
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We have a 74 year C-130H1 with a #2 VHF radio that wil not turn off through SCNS. Turning off SCNS also does not shut off the radio. We have swapped IDCUs, RTs and mounts, and have checked the Power on/off signal wire for continutity and a short to ground. Everything checks out. Also, Bypassing Secure voice has no effect.

 

Has anyone seen this before or have any out of the box ideas? We've got a head scratcher here.

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  • 11 months later...

So I finally was able to re-attack this problem. We have pulled pin 54 from the plug at the Co's IDCU and pin U at the radio. both times the radio still turned on, and we could not shut if off! We have verified no shorts to ground at pin 54 and U to all other wires in the IDCU connector. We have changed out the Co's IDCU and pulled the breakers on the other two IDCU's.

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We know two things. There is 28 VDC going to the radio, and the radio is turning on via a ground from somewhere. I don't have the connector layout for J2, but is Pin A anywhere near pin U? Here's what I would do if I was working on the aircraft. Remove the ground terminal lug from the aircraft frame going to pin A on J2. I would think that if you removed the power and chassis grounds lugs going to the chassis and power grounds of J1, pins P & E, that the radio would be dead.  

Having said all of this, it may be worth getting a new plug for J2, and pulling all wires out of the old plug, verifying wire numbers, and repinning into the new connector. Good luck.

VHF #2 wiring.pdf

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  • 5 months later...

Finally fixed it! It was a well hidden problem that existed ever since the plane was modified for ARC-186. (Reference 1C-130H-2-13 figure 4-6 2RF4.00)  The wire RF2018C20 was supposed to have been removed from pin18 at the back of the adapter mount P1 and be spliced into RF2036A20. Both wires are 28VDC from the circuit breaker. Also pin 18 of the adapter mount J1 is connected to P2 pin H at the RT (not shown in the diagram).  Pin H is the 24 VDC switched power developed internally. So when SCNS turned the radio on everything came alive. But when SCNS tried to power off, the 28 VDC at pin 18 provided by RF2018C20 was back feeding through the mount and J2 pin H the keeping the radio active even though the power on signal was not present at Pin U.

So a missed step in a mod back in 1981~1985ish and missing info on a diagram lead to one hell of a wild goose chase. 

ARC186 VHF-FM.pdf

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It was me, we were trying to use it as an AM/FM radio when we first got the planes, before we really understood how these were wired. I had ATIS tuned in and still heard it when I turned it off. Weakly of course because we were pulling it in from just the coax connector on the front of the R/T.

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