How to - radio master relay problem

[bradp]

Hi all.  I thought I'd share an issue with my radio master relay in case anyone else has similar issues.  I have a 77J. Supervised work.  Please excuse the lay terms.  This is just how I. understand.  

Master radio was acting up - no power to radios.  After tracking down that it wasn't the switch, we went to the relay.  

The "off" position of the radio master switch actually acts to close the circuit and energize the relay.  This energized relay holds the contacts away from their default position of radio bus line in the "on"/closed position.  Thereby if the switch fails, the contact goes back to the default position and your radios still have juice. 

My problem was the radio power didn't come on.  Verified that the relay was getting juice from the main bus.   Tracked it to the contact that goes from the relay to the radio bus bar.  It had become unhinged from its little plastic frame and was acting like a see-saw but the weight of the 12-ga wire on the other end of it was causing the see saw to be away from the relay switch.  

Shee it. Where am I going to find a 40 year old relay.  Fortunately there's a relay company that sells these things but the part numbers have been superseded (schneider legacy relays)  

Even more fortunately, this model has two sets of contact points.  The fix was to plug the spades into the other side, remove the offending contact see saw, and isolate and insulate the remaining contact.  Good as new after a thorough de-Oxiting   

Whichever Mooney engineer thought that this would be a fun project was a real joker.  My arms are scraped beyond recognition.  

Some pictures to illustrate the process. 

Can anyone tell me what the function of the solenoid behind the relay is?  I'm curious and I haven't looked it up.

take care  -b

 

Busted contact for the wire to the radio bus bars shown at the top left of the relay. 

 

What the radio master switch actually does...

 

 

Internal construction of the relay:

 

Wires for power and to the radio bus moved to the right contact points:

 

 

Victory!!!

 

Isn't it fun to find new and exciting failure modes?

[Hank]

Congratulations! Man, I really, really hate electrical troubleshooting . . . . It's just not something I can hold in my hands, play with and figure out. Maybe that's why I'm a Mechanical engineer.  

[Yetti]

Best I can tell the other solenoid is for the test switch for the warning lights and stall horn.  When you push the test button, it closes the relay and applies power to the lights and horns.

[bradp]

39 minutes ago, Hank said:

Congratulations! Man, I really, really hate electrical troubleshooting . . . . It's just not something I can hold in my hands, play with and figure out. Maybe that's why I'm a Mechanical engineer.  

I'm a physiologist (or at least I have a phd in physiology).  Hemodynamics is like a combination of ME and EE principles.  V=IR works in the human body too.  :-)

[Hank]

19 minutes ago, bradp said:

I'm a physiologist (or at least I have a phd in physiology).  Hemodynamics is like a combination of ME and EE principles.  V=IR works in the human body too.  :-)

Yes, the body is a mechanical system governed through low voltage electrical signals, like much of the equipment at work. The difference is the biochemical controller versus electronic one . . . Two more things that I stay away from. Bodies and electronics run by magic--I have found they work best if you keep the blood in the body and the smoke in the electronics, as letting either escape causes significant difficulties and expense.

So be careful while working on your airplane's electrical system, where you have the opportunity to do both at once . . . Voltage doesn't always operate in ways that are obvious to the uninitiated, and sometimes not in ways that more familiar persons expect.

[carusoam]

Nice work, Brad.  Thanks for sharing.

+1 blocked electron flow and blocked blood flow are bad.  

Best regards,

-a-

Edited May 6, 2017 by carusoam

[Mooneymite]

Great explanation.  Thanks for sharing.