Stall horn blares when radio is transmitting

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kubark42

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Massachusetts
Yup, it really does. It was quite the fright when the stall horn went off at 80mph.

It started doing this after we fixed a loose ground on the right magneto switch. Since all we did was tighten the screw, and there's nothing else on the switch, it's pretty clear that the problem is not the magneto fix. However, it's also clear that fixing the magneto is directly linked to this issue.

The problem diminishes when I tap on the switch panel. This hints at faulty wiring.

When we unscrewed the panel there was so little slack that it was very hard to get it far enough away from the headliner to get in there with a screwdriver. My thinking is that even though we were gentle, we still upset something in the 20 year old cable-job. Perhaps another ground is loose and at the moments when it becomes fully disconnected the stall horn signal wire becomes an antenna, capturing the EM radiation while I'm transmitting.
 
Yikes. Sounds like a short in the switch panel to me.

One trick: remove the window stop and fold the side window up and strap it to the wing. Then remove the switch panel and let it hang down so you can access it through the open window. Sometimes gives more room to work.
 
Big Ed said:
Yikes. Sounds like a short in the switch panel to me.

One trick: remove the window stop and fold the side window up and strap it to the wing. Then remove the switch panel and let it hang down so you can access it through the open window. Sometimes gives more room to work.
Click to expand...
that's a great idea!
 
Big Ed said:
Yikes. Sounds like a short in the switch panel to me.

One trick: remove the window stop and fold the side window up and strap it to the wing. Then remove the switch panel and let it hang down so you can access it through the open window. Sometimes gives more room to work.
Click to expand...

I'm not seeing how a short would cause this behavior, because the trigger is transmitting from the radio. Does the PTT circuitry run through the panel?

Either open circuit or short circuit, though, the remedy will be inspecting the panel's backsides. If the ground was loose, and then there's this other thing happening just from us working in proximity, who knows what's next to go wrong?

I'll see if I can do what you suggest about the window. Unfortunately, the window stop is riveted on, so I guess I'll have to drill that rivet out. I also recall that the wiring loom is *very* tight to the headliner, almost like when it was manufactured they didn't leave enough slack for the panel to move down by more than a half inch.
 
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kubark42 said:
I'm not seeing how a short would cause this behavior, because the trigger is transmitting from the radio. Does the PTT circuitry run through the panel?

Either open circuit or short circuit, though, the remedy will be inspecting the panel's backsides. If the ground was loose, and then there's this other thing happening just from us working in proximity, who knows what's next to go wrong?

I'll see if I can do what you suggest about the window. Unfortunately, the window stop is riveted on, so I guess I'll have to drill that rivet out. I also recall that the wiring loom is *very* tight to the headliner, almost like when it was manufactured they didn't leave enough slack for the panel to move down by more than a half inch.
Click to expand...

Actually I just looked at mine, and my window idea may not work. That will help access the aft part of the switch panel, but there is not enough slack to pull down the fore end of the panel.

Here is a picture of mine. It may give you some ideas. Note the stall warning horn is the round black object at the red arrow. It is in close proximity to the headphone jacks at the blue arrow. That is where I would start.

switch panel.webp
 
If I ever re-do my electrics, I will make all those cables longer. Were they trying to save wire?
 
Bartman said:
there is probably no room up there to tuck away any excess wire.
Click to expand...

There isn't, but the answer is to make a bigger box, not make the box unserviceable. :mad:

Oh, well, not a lot to be gained by being angry at decisions made 20+ years ago that were likely constrained by FAA certification.

I still don't have a working theory for why the stall alarm would be triggered by radio transmission. Since it's triggered by both front and rear PTT, that means that it's not as simple as a mic short since that would be isolated to only the front seat. Maybe I'll figure out what was wrong when I see the alarm, but I would be happier if I knew the mechanisms which could cause this before opening it up. Could turn a several hour excursion with lots of test flights into a 5 minute adjustment with (relative) certainty about the result.
 
My panel is in the wing root. Plenty of room in there. If my cable was an inch longer it would be trivial to add switches or breakers.
 
this is going to be an interesting one when you're back here explaining what was causing the problem. like an episode of House! lol
 
Check to see if your intercom power wire shares a CB with the stall horn.
Stall horn is stupid...one side of the horn is grounded and the other (hot) wire comes from the stall vane which gets it's power from a CB or fuse.
PTT is ever simpler...one side of the button goes to ground the other side goes to the intercom. When you push the PTT you supply ground to the "key" line to make the mike hot AND enable the transmit function on the comm. As you state both sticks can honk your horn I'd be guessing the intercom (+) line is at fault...
Chris
 
Found the problem. The stall horn had not been reattached properly after we'd fixed the loose magneto ground. In a nutshell, the stall horn is attached through the body of the panel, with a screw which looks very similar to the one which holds the panel to the plane. Thus, when all three screws were undone in order to remove the panel the stall horn was accidentally unmoored. When it was put back on, the screw missed the threaded hole in the buzzer. Because the screw was such a tight fit in the plastic, even though the screw was turning in the air it still had enough bite to make it feel like it had correctly engaged with the horn. Problem solved, no more horn blaring when using the VHF.

Now to figure out why we have to push the PTT harder and harder to make it transmit. I guess that switch is wearing out. No rest for the wicked!
 
Easy fix. I used a Radio Shack switch with great success, but for six bucks you can get aircraft grade.
 
Factory makes a nice PTT switch with 3D printed housing that fits in the top of the stick tube. Of course it is more expensive, but it's got "PTT" printed on it in case you forget.
 
Don't joke. The Stearman back seat has a red button and a black trigger. I am rarely back there - only the first three hours with a new partner. It is trial and error - if the first call to ground goes unanswered, I try the other switch.

I should leave myself a note . . .
 
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