Intermittent Push to Talk

[MBDiagMan]

I have a panel in my Cessna 140 that was custom made in the mid eighties using Narco Navs, Coms, Audio Panel and Transponder. The audio panel is a Narco CP 136M.

Last Summer I had a few issues that got sorted out, but one intermittent issue remains. It didn't present itself over the Winter, but is now back.

Here's the issue. Sometimes when getting in the plane and flipping on the master, the PTT switch does nothing. There is a switch on each yoke wired in parallel and when one fails the other fails.

Almost every time, I can start the engine and move the plane a little and it will work normally for the rest of the flight although SOMETIMES it will quit right after landing although this situation hasn't shown up in quite a while.

I have checked the wiring, and last Summer the avionics shop checked the wiring and it all seems to be in solid shape.

When this problem occurs I hear no clicking at the audio panel. When it decides to work again I can again hear the clicking in the audio panel. I pulled out the panel and sure enough there are two small relays in the unit.

Since this seems to correct itself after engine starts and sometimes also requires a little taxiing to cause some shaking, I seriously suspect the relay in the audio panel. I think this because after the engine starts, the alternator raises the Voltage level to the unit and the vibration would seem to help make the contact arm move.

Although I'm not an avionics guy I have LOTS of electronic troubleshooting experience in my past. From my experience with all kinds of electronics and systems I would give the relay a high probability of being the problem. The fact that it is slightly brown from heat over the years does not lessen my suspicion.

Now, first of all, if I could find a relay, I assume that it would not be legal for me to replace it. If this is the case, I would not even think of doing it myself. Secondly if it's not legal can anyone recommend a good Narco repair source with parts availability.

Also, if there are any electronics or avionics guys or gals out there, I would like to hear your opinion of my analysis.

Thanks to all,

 

[Henning]

Yep, I think you're on the money. The coils in the relay are going bad and require full voltage to activate. Before I went there though, I'd check to make sure my battery wasn't low voltage first.

 

[Capt. Geoffrey Thorpe]

I'm thinking either some corrosion on a connector, a bad crimp, or (most likely) a wire that is broken internally that sometimes makes contact and sometimes does not. It worked OK during the winter when the insulation was cold?

Checking the wiring - did that include wiggling / pulling on each wire in the PTT circuit? At both ends and at each splice?

If it were the relay contacts you would still hear the click. If it were the relay coil, why did it work over the winter when things are cold and probably don't move as well?

 

[Henning]

Yeah, sorry, didn't mention that, I would assume you had already checked/cleaned all connections as step 1.

 

[MBDiagMan]

Yes I've cleaned all the connections and reseated the module. I've wiggled around on wiring until I'm blue. About the only part of the wiring I haven't checked is the PTT switches themselves. Since they both fail at the same time and are wired in parallel that doesn't seem likely at all.

If I can find a relay would it be illegal for me to solder it in place myself? I think I know the answer.

The battery is only a year old and took a charge well when the IA left something on at annual and had to hand prop it. It charged well after he propped it. I got a little over 12 volts out of it after it was charged with engine off and no load. I haven't done a load test on the battery, but last Summer when the battery was new it was doing the same thing. A bit circumstantial, but it seems okay.

Another thing right after the hand prop and recharge, I was worried about a battery drain and put an Ammeter in series with the battery and all switches off. I had no current at all.

 

[Henning]

Yes I've cleaned all the connections and reseated the module. I've wiggled around on wiring until I'm blue. About the only part of the wiring I haven't checked is the PTT switches themselves. Since they both fail at the same time and are wired in parallel that doesn't seem likely at all.

If I can find a relay would it be illegal for me to solder it in place myself? I think I know the answer.

No, not per FCC or FAA regs. How anyone would ever find out...

 

[Skip Miller]

It charged well after he propped it. I got a little over 12 volts out of it after it was charged with engine off and no load.

What voltage do you have when the engine is running? You should have something over 14 volts. This is the voltage that your PTT relays will see when you are flying.

Having said that, it should work at 12 volts as well. Not working at 12v indicates that there is a problem but you already know that. :wink2: I agree with the others that it sounds like the relays are slowly crapping out on you...

-Skip

 

[Henning]

The battery is only a year old and took a charge well when the IA left something on at annual and had to hand prop it. It charged well after he propped it. I got a little over 12 volts out of it after it was charged with engine off and no load. I haven't done a load test on the battery, but last Summer when the battery was new it was doing the same thing. A bit circumstantial, but it seems okay.

Another thing right after the hand prop and recharge, I was worried about a battery drain and put an Ammeter in series with the battery and all switches off. I had no current at all.

You should have 13.2 V sitting with a full charge. 12v is quite discharged.

 

[MBDiagMan]

13.2 or more would be with the engine idling. With the engine off, making alternator inactive, I've always considered 12.5 Volts to be a fully charged car battery. I was seeing something like 12.3 or more so I didn't look at it any harder.

You make a good point though and I want to check the battery a little closer.

Thanks for all the replies. I'm pretty sure that a fresh relay has a very good chance of correcting the problem.

 

[Henning]

13.2 or more would be with the engine idling. With the engine off, making alternator inactive, I've always considered 12.5 Volts to be a fully charged car battery. I was seeing something like 12.3 or more so I didn't look at it any harder.

You make a good point though and I want to check the battery a little closer.

Thanks for all the replies. I'm pretty sure that a fresh relay has a very good chance of correcting the problem.

Check it at the battery, you may have a significant voltage drop to, or I accuracy with, the volt gauge. 2.2V per cell is was resting full charge should be seeing.

 

[Garthur]

My PTT switch went out and I had to replace it. I knew it was the PTT because the hand held mic still worked. Replacing it was such a pain that I decided to put a balloon over it and the yoke. The protection of the balloon protects it from the natural oils in your skin that I think gets into the switch and ruins it. It seems to work, the new switch as been working fine for the last 10 years.

 

[airguy]

*MOST* - but certainly not all - PTT work on a floating voltage that is grounded when you hit the button. If you've got a flaky ground (or a high-resistance corroded ground) for the PTT lead wires, that would affect both at the same time like you are seeing. The engine-start and after-landing scenario sounds to me like it's voltage (engine-RPM) related, which makes sense if you've got a high-resistance corroded connection somewhere. More voltage (engine/alternator RPM) will push enough current to pick the relay, but battery voltage alone is not sufficient.

Try putting ground power on the ship for a solid +14 volts and try it, then pull the ground power and hit the landing lights and try again while watching your bus voltage - it might be "enlightening"...