Stall Warning Horn

[skifast13]

Anybody have a serviceable/working stall warning horn that they want to sell/part with? The one in my '52 just clicks now. Thanks all!

[GAHorn]

It may be repairable. I have successfully done so on several…. and the horn itself is a capsule inside the case which might be replaced.

(Carfeful…that BakeLite case is fragile and eaily broken.)

viewtopic.php?p=128210&hilit=wrote+a+sh ... cs#p128210

[Bruce Fenstermacher]

IMAG0674.jpg (56.22 KiB) Viewed 677390 times

Above is a photo GAHORN posted of two horns, one I think original and the other who knows. These horns, not small enough to fit the bakelite housing, were also used in Lionel diesel train engines. (Said clearer I hope, the same style horn is used in Lionel diesel train engines, but they are to big to fit in the bakelite housing thus not a suitable replacement.)

They all work the same. They are an electromechanical affair where a electromagnet pulls a metal diaphragm which also has a set of points. As the diaphragm moves the points are open and the electromagnet is cut off and the diaphragm returns to its resting state closing the points and energizing the electromagnet again. This happening hundreds of times a minute creating sound waves and Voilà, you have a horn. The screw you see on the back of each of these horn in the photo adjusts the point opening.

When these horns set for a while, the points get dirty and do not conduct electricity. So no electromagnet and no diaphragm movement, and no horn sound. Often just shaking or rapping on the horn with power applied is enough to make the points conduct again and the horn starts working. Once that happens blow the horn for a bit to clean the points.

If you can not get the horn to beep with a rap on the side, you can move the point adjustment and accomplish the same thing. What do you have to lose at this point. CAUTION. This adjustment is very sensitive. Mark the exact position of the screw. move it a little and move it back.

Good Luck. I've resurrected quite a few Lionel horns in my day.

[skifast13]

Thx again guys. When I applied 12v across the buzzer it worked, so I troubleshot the switch assembly. Cleaned all contacts, bench tested it - all looked good. When we incorporate the capacitor into the circuit, we have major intermittent connection problems. What is the purpose of the 12v 35w capacitor in the assembly? When we bypass it, the switch works flawlessly. The metal casing suggests it provides heat. Is this a required part or can it be bypassed permanently?

[gfeher]

George, did your article on replacing the stall warning buzzer ever get published in the 170 News? I couldn’t find it.

[skifast13]

Final remarks on this fix: My Lift Detector was Safe Flight Part #148 and included the 12V 35W capacitor that serves as a stall warning horn heater (much like pitot heat). Would have worked fine had the 12V 35W capacitor NOT been wired in series back to the buzzer in the cabin. I still don't know why wiring through the capacitor worked for two years and just now became intermittent, but completely removing the capacitor from the circuit rectified the issue. Wanted to document a solution in the event somebody else comes across this scenario. You are an expert think tank of aviation enthusiasts and I appreciate all your help through the years.

[n2582d]

Final remarks on this fix: My Lift Detector was Safe Flight Part #148 and included the 12V 35W capacitor that serves as a stall warning horn heater (much like pitot heat). …

Thanks for documenting your fix on this. I think you mean “stall warning switch vane heater” rather than “horn heater”.

Stall warning switch heater

[sfarringer]

Are you sure that you are not mistakenly calling a heater by the name "capacitor".

[DaveF]

That's a resistor, not a capacitor.

[n2582d]

... Would have worked fine had (it) NOT been wired in series back to the buzzer in the cabin. ...

As you know, the stall warning switch heater was not originally on the C-170. Here's a screenshot of how Cessna wired it on the the early 172s. It's part of the pitot heat circuit, not the stall warning switch circuit.

Click to Enlarge

[Bruce Fenstermacher]

Guys, skifast13 is talking about the cap found in the bakelite warning horn/lite. See Georges photo.

Only Gary will be able to find a schematic for inside the warning horn.

[sfarringer]

Guys, skifast13 is talking about the cap found in the bakelite warning horn/lite.

You may be right, Bruce!
But the skifast post was so vague that it is hard to tell with certainty.

[ghostflyer]

While I am not endorsing using bogus parts , I had a situation where we in-ported a Piper Archer . It was wings off and placed in a shipping container .
On arrival and assembly was taking place and it was noted the stall warning horn had been crushed . A new one was ordered from Piper with a 16 week wait and and a cost of $286 . Spruce had nothing and a 4 month wait . So one of the guys went down to the local Tandy store and came back with a horn . It was $2.60 . This was the SAME part including the part number stamped on it. As it wasn’t a critical flight part per the regulations it’s still there 20 years later .

[GAHorn]

Let me attempt to answer two questions asked in above posts:

Firstly, I believe Bruce simply made a typographical error when he posted “too small to fit” in the bakelite case….. The two horns (of course) DO FIT inside the case.

Secondly, as to the purpose of the capacitor: Capacitors are nothing more than a “short-life/term battery” and are used in electrical circuits in parallel with contact-points (such as in the case of this horn which uses vibrating-points….and old-type automotive ignition circuits which use ignition points and condenser (a condenser is a type of capacitor)…. and in AIRCRAFT MAGNETO POINT SYSTEMS…. to act as a “shock absorber) for the rapidly operating points ….in order to absorb the spark-energy of that opening/closing circuit to prevent the points from rapidly BURNING.

I.E. Capacitors/Condensers modify the electrical surge and protect the ponts.

BTW: Your main aircraft battery is a HUGE CAPACITOR and protects your various onboard equipment from power surges also…and is why you do not want to operate your system without a battery… and including why you do not want to connect an outside power-source such as a GPU to your airplane without the battery being in the circuit.

[Bruce Fenstermacher]

Firstly, I believe Bruce simply made a typographical error when he posted “too small to fit” in the bakelite case….. The two horns (of course) DO FIT inside the case.

What I said, but not very clearly, was the same style horn is used in Lionel diesel train engines. But the Lionel horns are too big to fit in the stall warning bakelite housing thus not a suitable replacement for the horn found in there.