Radio and Cabin Lights Failure

[rakesb]

Dear Friends:

On Tuesday, I flew my 1966 M20C Mark 21 from Fort Leonard Wood, MO to Louisville, KY (actually Jeffersonville, IN because I couldn't get into Louisville VFR despite extraordinary assistance from the Louisville Approach controller, but that's another story...). Anyway, I used both radios (King KX 170B and KX 175B TSO) on the way out and most of the way back, but after the final handoff for continued flight following to my home airport, I couldn't contact Center on either radio, but I continued to receive just fine on both radios.

Perhaps unrelated, after sunset, I turned on the twin cabin ceiling red lens spotlights which illuminate the instrument panel, and when I rotated the on/off rheostat knob to the full stop brightest setting, both spotlights flashed off simultaneously. Subsequently, the single white light cabin ceiling light failed to illuminate when I tried turning it on. It worked fine the last time I used it a few weeks ago.

None of the circuit breakers popped. The filament in the single, panel-mounted fuse appears fine. I found three (there may be more) wire-in-line fuses under the instrument panel. Each appears to have a spare fuse taped to a wire adjacent to the fuse in use that it is apparently intended to replace.

I need to return my daughter to Jeffersonville, IN (her car is parked there because of my diversion from Louisville to Jeffersonville) on Tuesday, and really would like to have a least one operational radio for such a long VFR flight. The cabin lights are less critical and time sensitive.

What's the most likely reason for both radios to stop transmitting simultaneously, yet continue receiving just fine?

Are the cabin ceiling spotlights and the cabin ceiling dome light on a shared electrical circuit with a fuse located under the instrument panel?

Thank you,

Bobby

(573) 337-3447

Sent from my iPad

[Hank]

When my C had a total electrical failure, it was a blown component on the circuit board behind the rheostat on the panel that controls the lights. I have the overhead red lights but no ceiling dome light. Finding a replacement board was a challenge! Hopefully you will have resolution quicker than the 3-4 months that it took me . . .

 

Best wishes!

[MB65E]

Possibly the battery, I know it doesn't make sense. I spent $ 220 chasing after the master relay the last time my radio transmissions were scratchy...then another $250 on the

Well we have a new relay and battery now. At night, with a hi load, and poor battery there can be some funny things that happen.

The extra power it takes to transmit draws more current on the system. I'm convinced the issue was the battery. Similar issues it sounds like. Not sure about the cabin lighting. Lights should be on their breaker.

Good luck, let us know.

[larryb]

I'm thinking low voltage due to a charging system problem. Did you notice what your ammeter and voltmeters were showing?

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[N601RX]

What kind of audio panel and intercom do you have?  Do they still come on?  Most of them have a failsafe circuit that will automatically connect them to the #1 radio if they fail.  You might try turning the audio panel and intercom off and see if either radio will work.

Also most intercoms and audio panels have a light that comes on when the mic is keyed, Is it coming on?  If you have a separate intercom the mic switch will key the intercom, the intercom will key the audio panel and finally the audio panel will key the selected radio. You can tell a lot from which lights are coming on.

[HRM]

| What's the most likely reason for both radios to stop transmitting simultaneously, yet continue receiving just fine?

 

 

 

 

 

 

Transmit requires significantly more power than receive, so something is overloading a circuit, but not to the point of failure; i.e., blown CB or fuse. Apparently, the loading is right at a sweet spot (or sour depending on how you want to look at it) where there is enough power for reception, but keying transmit does not cause enough drain to blow a breaker.

 

The fact that turning a rheostat (dimmer) knob cause some things to go out tells me your issue is up in the overhead panel. I pulled mine out yesterday as I am redoing my interior ('66 E) and discovered a shocking, no pun intended, rat's nest of wires up there. The fact that some wayward A&P or PO had safety-wired a RadioShack speaker up in there confirmed my opinion that an electrical wazoo had been at work. Being an EE, I plan to clean that up before putting it all back.

 

So, check the battery, do the audio panel check as suggested above, and if you have no joy, I'd pull that panel and disconnect it just to get home and worry about the repair later--just plan on day VFR 

[rakesb]

Thank you so much to everyone for your comments and suggestions. I replaced a BUSS AGC 30 32 VOLT in-line fuse under the panel on the pilot side and a 2A in-line (wire labeled, "INTERCOM") fuse under the panel on the copilot side with the spare fuses that were taped to their respective wires. Upon startup the ceiling-mounted instrument panel spotlights and and cabin ceiling dome light all work again; I'm think they must be on the circuit with the 30A fuse. There was a high pitched squeal in both pilot and copilot headsets, so without shutting down the engine, I reached under the copilot panel and removed the 2A fuse and just left the circuit open. I used my Icom A23 handheld transceiver connected to another headset to depart, then switched back to my panel-connected headset to monitor weather and traffic enroute, then switched again to my handheld-connected headset for arrival at my destination. On departing for my return flight (JVY to TBN) and switching from my handheld-connected headset to my panel-connected headset, I successfully contacted Approach and picked up flight following for all the way home. The radios work fine with the intercom circuit open! So, must be a problem with my FlightCom 403 intercom and/or it's associated wiring, right?

Thanks again,

Bobby

(573) 337-3447