Cessna 172 autopilot power supply repair? - Page 1
Forum section: electronics / repair

author: nardev
author_rank: Unknown
timestamp: August 12, 2021, 10:26:37 am
content: Hey guys, we have an issue within the power supply for an airplane autopilot. Pretty old tech, can't understand it completely.Here is the schematics and the picture, we would start from the transistor but we have no idea which one would be good replacement fit. Some n-Chan JFET, but, have no idea where to start.Thnx

author: Gyro
author_rank: Super Contributor
timestamp: August 12, 2021, 11:03:03 am
content: Quote from: nardev on August 12, 2021, 10:26:37 amA TO92 pkg JFET current source fire? That'sreallystrange. I would have suspected a massive surge (lightning) on the 14 / 28V supplies if it wasn't for the fact that the Tantalum bead cap on the supply rail (C2) still looks intact!The nearest non-resistor path to ground from the 14V supply is via Zener VR1, I can't see if that is blackened or just black painted. It would be worth checking whether that is shorted.

author: nardev
author_rank: Unknown
timestamp: August 12, 2021, 11:12:42 am
content: Quote from: Gyro on August 12, 2021, 11:03:03 amShould definitely check the whole thing but that was the most obvious blown part.

author: Gyro
author_rank: Super Contributor
timestamp: August 12, 2021, 11:40:39 am
content: Yes, 14V input through Q1 and VR1 is the only path I can think of. The sequence would have been Q1 developed a short, VR1 went short and then Q1 caught fire (assuming that the 14V comes from something low impedance like the battery. Hopefully a supply fuse blew.It's hard to say what would have happened to the 9V regulator drive (connector pin 8 ) during this process - it might have blown the series pass regulator transistor on another board or heatsink, feeding the main +9V Regulated supply to the rest of the autopilot, (and feeding back to connector pin 2) and / or possibly causing an overvoltage.Hard to say if U1A has been damaged (possibly not as it's powered from the 14V supply rail), but the damage might extend wider in the autopilot if the +9V supply went overvoltage.

author: Renate
author_rank: Super Contributor
timestamp: August 12, 2021, 12:59:05 pm
content: Just replace that FET with a 2700 ohm resistor and bench test.You'll soon see whether the zener or the op amp is blown and it won't shoot flames in any case.For better accuracy you should replace the 2700 with the correct FET (and adjust the 9V).

author: fzabkar
author_rank: Super Contributor
timestamp: August 12, 2021, 06:54:13 pm
content: I would replace the tantalum capacitors, even though are probably OK, because I have seen too many tantalum fires. In fact I saw one beaded tantalum light up like a match head on my test bench. C2 looks like another tantalum, and I have seen those burn a hole through a PCB (in a disc drive).I would also consider replacing the FET and zener with a TL431. You would need three resistors, one in series with the supply, and two to set the voltage (5.6V). The series resistor would prevent a repeat fire in this area.BTW, the designer chose a 5.6V zener because this is the voltage at which a zener has the lowest tempco.I realise that aviation regulations may impose liabilities which affect modifications, but I would hate to think what your liability would be if you restored the original components only to suffer another fire in the future.

author: floobydust
author_rank: Super Contributor
timestamp: August 12, 2021, 10:08:18 pm
content: Original 172's have dirty electrical systems due to the electro-mechanical voltage regulator and from an era where transient and surge protection was not yet mandated.The original N-ch JFET KA1455, 36269-00010 I could not find data for.It functions as a constant-current source for the reference zener ZD1 5.6V so I would put in something hand-selected to have IDSS in the 2-10mA range, and highest voltage I could find at least 35-40V because the part does experience voltage surges.There are many parts that would work but JFETs are less used nowadays and constantly being discontinued. Because JFETs have a wide variation almost 10:1 the usual approach is to buy say 10 and hand hand pick one to work.Repairs on avionics I see guys don't want to change/improve the circuit at all due to liability.Linear SystemsLS190same pinout, most others i.e. 2N4393 pinout different would have to be flipped to work.Digi-Key now selling LSI as a Marketplace product, USA onlyhttps://www.digikey.com/en/products/detail/linear-integrated-systems-inc/PN4393-TO-92-3L/14312337With a shorted JFET it would likely damage other parts such as VR1.I would outright replace C2 150uF 40V, C4 100uF tantalum- simply based on their age 1974 is too old.

author: Bicurico
author_rank: Super Contributor
timestamp: August 12, 2021, 10:24:57 pm
content: Out of curiosity: is such a repair legal?

author: floobydust
author_rank: Super Contributor
timestamp: August 12, 2021, 10:50:04 pm
content: What police are you expecting at the doorstep? The FAA does have grandfathering for such vintage equipment that parts or part number are no longer available.But the repair depot does need certification I believe, to prevent putting in Radio-Shack parts or worse. I would use AEC-Q100 it's stricter stress testing than 1974 and I don't see MIL-rated parts in OP's board.Problem is avionics electronics parts are incredibly expensive. A JFET is $2 and the original if you can find it is 50x that.

author: fzabkar
author_rank: Super Contributor
timestamp: August 12, 2021, 11:49:22 pm
content: After repairing the module, I would inject an accurate 9.00V source into pin #2 and adjust the trimpot (9V ADJ) until the regulator drive (pin #8) just switches state.Edit:An alternative calibration method would be to connect an NPN pass transistor to the circuit. The collector would be wired to 14V (pin #10), the emitter to the 9V input (pin #2), and the base via a resistor to the regulator drive (pin #8). Then adjust the trimmer until you get 9.00V.

author: jonpaul
author_rank: Super Contributor
timestamp: August 13, 2021, 01:05:40 am
content: Hello any repairs to avaition electronics must be done at an approved shop or the mfg.Upon yearly inspection, it could be flagged by the FAA.As an autopilot is capable of causing flight control misdirection it is a safety issue.Suggest to research this issue a bit.Kind Regards,Jon

author: fzabkar
author_rank: Super Contributor
timestamp: August 13, 2021, 01:14:37 am
content: I wish I could show people the repairs done by an "approved" shop in Australia. I could have trained a chimp to do a better soldering job.

author: james_s
author_rank: Super Contributor
timestamp: August 13, 2021, 01:20:04 am
content: Quote from: fzabkar on August 13, 2021, 01:14:37 amI don't doubt it in the slightest. That doesn't change the fact that in some cases it is mandatory that the work be done by the slobs that have the piece of paper to prove they supposedly know what they're doing. It absolutely drives me nuts too when I KNOW I can do as good or better job than the certified tech but that doesn't change anything. Aviation is arguably the most heavily regulated industry in the world short of maybe nuclear, I am 100% confident you could do a high quality repair on that PCB but I am completely unqualified to say what is legal, especially in a place as unfamiliar to me as Bosnia so be sure to check.

author: nardev
author_rank: Unknown
timestamp: August 13, 2021, 10:15:12 am
content: Quote from: Bicurico on August 12, 2021, 10:24:57 pmWell, legit question, in short:"Without getting into much details, avionics maintenance organisation can issue repair documents, claiming that repair is done by approved methods, with also detailed report on functional test. At least in EASA country..."

author: nardev
author_rank: Unknown
timestamp: August 13, 2021, 03:36:55 pm
content: Quote from: james_s on August 13, 2021, 01:20:04 amWell, in Bosnia it's pretty much EU rules. If you want to keep you airplane certified, that's what you have to do. But there is an option to present every case of repair and if the certified company is satisfied, than it's approved as if it was fixed buy them. You pay less as they do only the review.

author: Hamelec
author_rank: Regular Contributor
timestamp: August 13, 2021, 04:29:40 pm
content: i googelt and found the FET is listet here for 8$:https://www.carpenter-avionics.com/wp-content/uploads/2019/05/PartsOffer_CarpenterAvionics_Rev_03-28-2019.xlsx

author: Kerlin
author_rank: Regular Contributor
timestamp: August 13, 2021, 11:56:07 pm
content: Quote from: fzabkar on August 13, 2021, 01:14:37 amWondering if that was an Avionics repair ?As mentioned before they can be more expensive repairs, so you are correct to expect more.

author: fzabkar
author_rank: Super Contributor
timestamp: August 14, 2021, 12:44:04 am
content: I think it was some mob in Canterbury, NSW. My friend had a problem with a landing light, and he was charged AU$700 (?) for repairs to the controller. One of the switching semis had been replaced. The unit still worked on my bench, despite the butchery. It turned out that there was a short in the aircraft's wiring.

author: sokoloff
author_rank: Super Contributor
timestamp: August 14, 2021, 12:53:31 am
content: Quote from: jonpaul on August 13, 2021, 01:05:40 amBy the time the FAA has taken apart your autopilot controller to determine if the transistor replacement was legal, you’re long-ago screwed because they’re obviously targeting something about you or your operation.Quote from: jonpaul on August 13, 2021, 01:05:40 amThis is a fair point and argues to be very sure the repair is done correctly.

author: floobydust
author_rank: Super Contributor
timestamp: August 14, 2021, 04:07:47 am
content: Imagine the autopilot operation while the JFET smokedit's already been a worst case failure. The autopilot has a limiters so the (altitude?) drop will wake you up.

author: CatalinaWOW
author_rank: Super Contributor
timestamp: August 14, 2021, 02:10:52 pm
content: I talked to a licenced aircraft maintainer about the legality of this repair. Turns out that here in the US it's complicated.For any kind of commercial use of the aircraft must be a manufacturer certified repair operation. Repair must be performed to published procedures. (This last can be misleading. Those published procedures can range from things better than the best HP documents to a sheet that says "fix according to industry standard" or most pernicious "return to manufacturer for repair").If the autopilot is included in the type certificate it requires certified repairer. But by his recollection the autopilot is not in the 172 type certificate. Is on the 182.If the repair person is the owner and operator of a non commercial aircraft it is legal, but may be flagged at 1000 hour inspection, which will be aware of the repair if maintenance logs are properly maintained. In this case if something bad happens owner has all liability for the results.Actually there were nuances that made it more complicated than this, the above is the paraphrase of the general situation.

author: NiHaoMike
author_rank: Super Contributor
timestamp: August 14, 2021, 02:46:49 pm
content: The thread was started by someone who is not in the US, so unless the aircraft flies in the US, FAA is irrelevant and what matters is the local equivalent.

author: CatalinaWOW
author_rank: Super Contributor
timestamp: August 14, 2021, 03:18:28 pm
content: Totally agree. Only mentioned because aviation rules are somewhat similar worldwide and may well be equally complex at the OPs location and because Cessna 172s are ubiquitous and US members of this forum might get some value.

author: sokoloff
author_rank: Super Contributor
timestamp: August 14, 2021, 03:25:44 pm
content: Quote from: NiHaoMike on August 14, 2021, 02:46:49 pmIf it’s an N-registered airplane, the FAA rules apply. Many airplanes are based outside the US and operate under N (US) registration.

author: nardev
author_rank: Unknown
timestamp: August 17, 2021, 03:24:15 pm
content: Quote from: Hamelec on August 13, 2021, 04:29:40 pmOMG how did you find this...Thnx

author: Renate
author_rank: Super Contributor
timestamp: August 17, 2021, 04:36:15 pm
content: Quote from: nardev on August 17, 2021, 03:24:15 pmHave you tried to see what else is broken with my resistor suggestion?If you put in a new FET it could go sky high.

author: nardev
author_rank: Unknown
timestamp: August 18, 2021, 04:33:22 pm
content: Quote from: Renate on August 17, 2021, 04:36:15 pmAt the moment, it looks like there are few replacement units on the market that could potentially cost less than a repair. It will take time to get the parts anyway.