Pop goes the A/P

[flyboy0681]

So there I was flying today with a buddy of mine who flies 767's for a living. We were cruising along at 7,000ft enjoying the scenery when we both heard a "pop", not very loud but noticeable. I looked down at the KAP 150 and saw a little puff of smoke, and all of the lights off. I immediately pulled the breaker (which didn't pop out) and we checked for more smoke, and worse. 

I'll ship it to A/P Central next week. Anybody want to take any wild guesses?

I fear the worst and am prepared to assume the position.

 

 

 

Edited October 1, 2016 by flyboy0681

[DVA]

You might be surprised. Sounds to me like a power supply capacitor failed, if so, it should be an easy fix. Not many electrical components “pop,” electrolytic caps do. The reason why you didn’t pop a breaker was because the cap failing unloaded the current from the AP buss.

Good luck on this one!

DVA

[carusoam]

Letting the smoke out of the electronics usually isn't a good sign...

I am looking forward to seeing DVA being right on this one!

Get a good photo of the bad capacitor if you can.  

Best regards,

-a-

[ArtVandelay]

You might be surprised. Sounds to me like a power supply capacitor failed, if so, it should be an easy fix. Not many electrical components “pop,” electrolytic caps do. The reason why you didn’t pop a breaker was because the cap failing unloaded the current from the AP buss.
Good luck on this one!
DVA

Ditto this, but it could have been caused by a bad servo causing an overload, you should test the servos before hooking repaired AP back up.

[Guest]

It does sound like the smoke retaining membrane has failed.

Clarence

[Hank]

Glad to see I'm not the only one who believes that modern electronics run on smoke and magic. It's just easier to tell when you've let the smoke out!   did that nice with the circuit board behind the panel light dimmer switch . . . Which caused a total electrical failure.

Good luck with your repair! Simple is always better.

Edited October 1, 2016 by Hank

[flyboy0681]

1 hour ago, daver328 said:

You should've never started letting airline pilots get in your Mooney. Especially don't let them touch anything!  (You asked for "wild guesses" .... ;-)

Live 'n learn.

[Awful_Charlie]

From "Prince of Darkness" Lucas Industries...

[Guest Mike261]

capacitors pop...and they smell different than wiring insulation when they go.

acrid smelling.

hopefully you didn't let all the smoke out.

Mike

[rbridges]

when you do find out what happened, let us know.  

[thinwing]

You know all that smoke you see while soldering components?Thats the magic smoke that causes electronics to do their thing!..When the smoke escapes....

[flyboy0681]

The unit has been removed from the panel and once I pick it up I'll remove the cover and report what I find.

[cliffy]

Make sure that when you handle it you don't break the mirrors inside. They all run on smoke and mirrors. 

[flyboy0681]

I opened up the unit and do think it's the capacitor. If my suspicions are correct, the failure was self-contained and moved the housing down to the point where it hit a nearby transformer.

Thoughts?

[carusoam]

Winner!

It has moved out of its holder.  Looks like the spring clip may have failed to hold on...

Great photos!

Best regards,

-a-

[aviatoreb]

Im sort of surprised to see integrated circuits - I was almost expecting to see relay switches, vacuum tubes, rubber bands and gerbils inside old school avionics.

[flyboy0681]

What shocks me the most about the unit is that all of those components somehow allow the aircraft to fly a perfect ILS or LPV approach. Simply amazing for very early 80's technology.

I was also surprised to see that a lot of the components were made in Malaysia and Mexico. I wouldn't have expected that for the 80's.

[Guest Mike261]

The cap definitely let go, sometimes they just inflate a little and can be harder to find, that pop you heard is usually a dead giveaway that its a capacitor.

The coil may or may not be damaged, depends on how hard it got hit...the windings are very delicate.

Now the issue is why did the capacitor fail? Often the issue is something else that pops the capacitor... sometimes you get lucky and its just the cap.

 

Mike

[flyboy0681]

5 minutes ago, Mike261 said:

Now the issue is why did the capacitor fail? Often the issue is something else that pops the capacitor... sometimes you get lucky and its just the cap.

Well, the capacitor is over 35 years old. Nothing lasts forever.

[FlyWalt]

I thought that smoke means that the Genie is coming out to grant me a wish :-)

[flyboy0681]

Just now, FlyWalt said:

I thought that smoke means that the Genie is coming out to grant me a wish :-)

In this case, I was granted one wish: no fire.

[carusoam]

Did the capacitor move?

It looks like there is a spring holder that holds the can in place. The capacitor may have moved and touched something...

It would be interesting to learn if the spring holder let go to allow such motion?

in the photo, which way is gravity pulling?

Best regards,

-a-

[flyboy0681]

7 hours ago, carusoam said:

Did the capacitor move?

It looks like there is a spring holder that holds the can in place. The capacitor may have moved and touched something...

It would be interesting to learn if the spring holder let go to allow such motion?

in the photo, which way is gravity pulling?

Best regards,

-a-

I have no idea. It's now in the hands of Autopilots Central to determine what happened.

[DVA]

There’s nothing odd going on here; the cap failed.

Electrolytic caps degrade over time and it's been a few decades in this case. As they degrade the capacitance value changes, which can cause the circuit they are in to act in a totally different manner. The change can cause an increase in current draw - or a decrease - amount other things. The dielectric materials dry up over time, which often result in an internal short, creating heat causing expansion and then a “pop.” The good smoke comes out, the circuit fails, you suddenly smell something funny in the cockpit, poop your pants and land quickly. All this due to a $0.35 component that served you well for many many years.

I doubt there is any significant damage elsewhere. These old non IC non ASIC electronics are actually very resilient and reliable, and normally easy to fix. It’s old school PC boards; easy to troubleshoot, easy to solder on, and the components are still made.

There is one rub here: This is an old piece of equipment, and there are a lot of other old components that could easily fail at any time just due to their age. You are likely past any confident reliability curve and are now just running on good old fashioned solid design and a four leaf Clover.

Please let us know how you make out.
DVA

[flyboy0681]

Epilogue. The work has been completed and the following was noted on the work order: Bench tested unit and found blown capacitor C222; broken trace between CR231 and R2134;shorted transistor Q216; need to comply with SA-106 (short locking rod) .Replaced blown capacitor C222; replaced shorted transistors Q216; repaired broken trace between CR231 and R2139; replaced missing labels on eprom microprocessors; re-threaded locking rod as per manufacturer's service AD KC190/191/192-106. Bench tested using manufacturer's service manual 006-5590-00, Rev O, June 1983 as a reference.

If anybody could decipher this, please respond.