PA28-181 Panel light glitch

[MajorTurbulence]

My 1980 PA28-181 has been having a slight glitch of the dash lights that are controlled by the vertical wheel rheostat on the right. Over the years it often would not come on immediately unless I recycled. Later, even after recycle, there would be a slight delay, and then suddenly come on. Lately, with the switch on, after about 10 minutes of flying, the lights would suddenly come on, or immediately after when I recycled during cruise.

It is not a big deal with me to use in this manner, but on the last annual, the A&P predictably noted it inop on the annual report. In fact, the few of the gauges that are affected are supplemented by my independently lit Aspen. My fuel totalizer is hard to read without a flashlight though.

So is there an easy fix for this, like cleaning up the rheostat in some way? Does it need to replaced with a new one if they are even available? Are these rheostats easy to replace or service? Cost is always an issue so no outlandish suggestions please.

Also, I’m making the assumption it is the rheostat. Could it be something else like a ground wire?

 

[Bell206]

I’m making the assumption it is the rheostat. Could it be something else like a ground wire?

Technically they're considered potentiometers but work in a similar way. However, I would get someone to check the basics before you simply change parts to ensure you change the right part. Given there is a delay in when the lights come on I would look to see if your system also has a transistor and possibly a resister in the circuit. It sounds more like you have a bad transistor. All the parts are still available with most considered as standard parts and can be purchase at an electrical supply store. But that will depend on your mechanic. Look for free Piper manuals online and see what you have installed then get a VOM and chase some voltages.

 

[Dan Thomas]

Might just be a loose connection. But potentiometers are well-known for wearing and starting to glitch out like that. A pot has a carbon track that has a brass or copper finger that runs on that track and is moved by the knob. It varies the resistance as it moves. That carbon track wears and the pot gets dust in it as well, and both of those cause the resistance to fluctuate wildly sometimes when the thing isn't even moving. A tiny particle of carbon under the runner will do weird things. Start running electricity though it and it heats up and makes stuff go haywire.

There might also be a bias resistor in the circuit. It's there to electrically raise the lower end of the pot off its ground reference so that the dimming uses the whole travel of the pot, not just the upper half or so. Incandescent lights don't do much with the first 5 or 6 volts into them, so that resistor gets the lamp voltage up to 5 or so volts and the pot raises it from there. That resistor is prone to cracking or breaking if someone is poking around under the panel, and sometimes its soldered connection gets worked loose. Makes problems.

The pot controls the base of a power transistor, which is mounted on a heat sink. It can work loose as well. Vibration and so on and then its contacts get beat up and intermittent.

The typical Piper dimmer circuit. This one has no bias resistor to enable practical use of the entire pot travel. The pot is the thing on the lower left, the wiggly line with the arrow to it. The knob moves that arrow up and down the wiggly line, which is the carbon track. It varies the voltage to the transistor's base, which acts like a water valve.



Old airplanes. Lots of electrical fun.

 

[Lon Stratton]

I have a 1982 PA32 that probably uses the same system as yours. This is what the back of the potentiometer assembly looks like in mine. One resistor and the pot.

edit: there may be a transistor on the opposite side.

 

[Dan Thomas]

I have a 1982 PA32 that probably uses the same system as yours. This is what the back of the potentiometer assembly looks like in mine. One resistor and the pot.
View attachment 115926
edit: there may be a transistor on the opposite side.

Each of those resistors are feeding the base pin on a transistor. That whole plate is the transistor heat sink.

 

[Lon Stratton]

Now I remember. It WAS the transistor that was replaced. Picture was for reference....10 years ago.

 

[Bell206]

At least Piper lists the transistors as separate parts to include the standard number 2N3055 (#18). Better than Cessna that normally would sell the transistors with its heat sink as a "dimmer assembly". A 2N3055 transistor costs less than $2.

 

[Tarrow]

I’d still be concerned about checking for a loose connection or the potentiometer carbon tracking problem. It could also be the on off switch contacts since it goes off randomly. I wouldn’t go for the transistor as the first cause.

 

[Daleandee]

Is it possible to use some contact cleaner on this?

 

[Bell206]

Is it possible to use some contact cleaner on this?

Not really. The potentiometer is basically a sealed unit. Chasing voltage and drops usually takes you to the culprit pretty fast especially with what the OP is seeing. This can actually be one of the more inexpensive things to fix on an aircraft.

 

[Hang 4]

Transistor is a common issue as @Bell206 has mentioned. Another alternative is Maxdim https://seatoneng.com/our-products/#maxdim
A bit more complicated, but has an STC. The actual potentiometer is sealed. The other alternative is a Wentworth or other used parts dealer, but tough call how long it's going to last.
Other thread on same topic https://www.pilotsofamerica.com/community/threads/broken-nav-light-rheostat.120879/

 

[Daleandee]

Not really. The potentiometer is basically a sealed unit. Chasing voltage and drops usually takes you to the culprit pretty fast especially with what the OP is seeing. This can actually be one of the more inexpensive things to fix on an aircraft.

Thanks. I had no idea of the design of it ...

 

[Dan Thomas]

Cessna has the breakdowns of those dimmer assemblies and many other such components. Just have to ask for them. We used to buy the transistors cheaply, even from Cessna.

One should also buy the mounting kit for the transistor. It has a thin hi-temp plastic insulator that goes between the transistor case and the heat sink, and small insulators for the mounting screws, and some dielectric heat-sink compound. Stuff that gets lost when switching the parts. The transistor case is electrically hot, being the collector. Can't have any electrical contact with the sink, unless the sink is insulated from the airframe instead.

 

[Bell206]

Cessna has the breakdowns of those dimmer assemblies and many other such components. Just have to ask for them. We used to buy the transistors cheaply, even from Cessna.

In recent times yes. But 30 years ago or so they weren't as forthcoming. It was then I learned a good number of aircraft electrical parts were off the shelf standard electrical parts. Just need to know the rules on which ones you can replace one-for-one. Probably should have worked on more Pipers since they give the same info freely. Then again I'm not a big fan of low-wings so I guess it was a wash.

 

[murphey]

Transistor is a common issue as @Bell206 has mentioned. Another alternative is Maxdim https://seatoneng.com/our-products/#maxdim
A bit more complicated, but has an STC. The actual potentiometer is sealed. The other alternative is a Wentworth or other used parts dealer, but tough call how long it's going to last.
Other thread on same topic https://www.pilotsofamerica.com/community/threads/broken-nav-light-rheostat.120879/

The Maxdim is what Piper is using in all the new aircraft.

 

[MajorTurbulence]

Thanks for all the comments. So much useful information.

My IA initially said that tracking down the problem was quite an extensive process (without asking, it sounded like $$). However, I’ll give him an initial pass. He did not know the actual symptoms I provided in this thread as I put off the investigation for the future.

Considering all these great comments, I feel motivated to ask my IA’s permission to look for myself. I have not looked under the panel yet, and have not purchased a VOM, but suggestions welcome for a meter not junk but with very flexible usage.

Additional questions:

1) How easy is it to get access to the area that is visible to inspect/test, by me, and later allow easy replacement of parts or solder by my IA? If Lon Stratton’s photo demonstrates the view of the major trouble points, I’m not seeing any big impediments to testing. Obviously, replacement may require getting access to both sides of a mounting panel, whether for mounting screws or transistor heat sink insulated mounting plate.
2) I am an almost complete novice at this, but looking at the schematic, I see a home run wiring structure with 3 branches (L5a, L5b,L5c) using a ground connection for each light(?for the compass). Given my symptoms where it is either all lights on or off, I would guess the problem is probably at/ near the where the potentiometer, resistor(if in the circuit), or transistor is located, potentially making it easier to find. Also, once on, the lights do not flicker at all making me think a very loose connection less likely. The fact that it takes a bit of time for lights “on”may however be indicative of a better contact through expansion on something a bit loose, or a defective solder joint that is improved when the back panel area is warmed up by the radio stack.

My question was anticipated on whether the potentiometer could be cleaned, but since it is a sealed component, it is just replacement then; and ditto for the on off contacts also in the sealed POT. But it would have been a good thing if the contacts could be cleaned.

I did call Wentworth, and they had 3 used pots. They were $295, but I agree with the comment that it must be a crap shoot to know if you are getting something that has no anomalies. One came out of a 1977 Arrow, and that is older and likely more hours on it than my aircraft; I would hope water damage would be obvious, but who knows.

The use of the Maxidim is an interesting possibility. I’m sure the function can be restored with this, but would the aesthetics/cosmetics of the vertical wheel be gone and not look “factory” again. As long as it would not look hodge-podge, that could work. If it is so straightforward, it might reduce labor by my IA.

Lastly, as a novice in using the VOM, I would guess that the negative lead on volts selector scale could be connected to a ground source, and the positive lead with the switch turned “off” should read 12 volts on one of those poles on the POT. And when the switch is turned “on,”there should be a voltage reading on some of the other testing points if getting past the POT. If no voltage past the POT, that would likely isolate the POT or it’s internal contacts. If I wait a few minutes, that might change after panel heating, as noted above in symptoms. I am going to label Lon’s photo, and post here, to help me choose where testing should be done on the POT, transistor and other parts from the experience of you commenters.

 

[MajorTurbulence]

I’d still be concerned about checking for a loose connection or the potentiometer carbon tracking problem. It could also be the on off switch contacts since it goes off randomly. I wouldn’t go for the transistor as the first cause.

It only goes “on” in the beginning 10 minutes randomly, but once “on” it does not go “off” or flicker unless I turn the switch off manually.

 

[sourdough44]

Of course work on it & get it fixed, if $$ allows.

I have a plane from 1976, with panel lighting issues. Over the years instruments come & go, things are rewired.

I have a strip of lights that was installed a few years ago, under side of the glare shield. It is controlled by a rotary switch, all new. Of course, not as ideal as internal lighting, that works. I also take a red headlamp if getting serious.

Just throwing out some lower cost work arounds.

 

[Bell206]

How easy is it to get access to the area that is visible to inspect/test, by me

It will depend. Is the thumbwheel switch at the bottom of the panel or 6 inches up from the bottom? Without seeing a picture of your panel you have a 50/50 chance you'll need to pull both front seats and lay down on the floor to see the back side of the panel. Then the fun will start.

using a ground connection for each light(

FYI: the system uses the panel and airframe as the ground connection.

either all lights on or off, I would guess the problem is probably at/ near the where the potentiometer, resistor(if in the circuit), or transistor is located

Thats a good guess. The key is to see where the power delay is: at the output of the potentiometer or at the output of the transistor. Being physically able to put your VOM leads on the test points is usually half the battle. It might be easier to check the IN and OUT sides of the transister vs the OUT on the potentiometer. If need do a few practice runs with no power applied

as a novice in using the VOM,

Plenty of vids on youtube on how to use a VOM. Same principals used on aircraft work.

. I am going to label Lon’s photo,

Keep in mind it appears his setup is different from what I see in the parts manual. I cant post links at the moment but get you a free copy of the mx and parts manuals online for your aircraft and compare the schematic and diagrams to your aircraft to get an understanding of the system. Then select your test points and give it a try.

 

[murphey]

When I installed the Maxdim, I didn’t care if it matched the original wheel or not. I wasn’t interested in installing something very old from Wentworth. When the “original equipment” arrived from Wentworth, it was dusty and filthy. Sent it back, ordered the Maxdim. It’sa knob by the way, and only needs the A&P sign the installation. because it’s STC/PMA.

 

[Dan Thomas]

The transistor connections:




Lon Stratton's setup. Note the connections:



Now, that collector connection, the transistor's case, is hot when the switch is on. That's why there are insulators between it and the grounded heat sink plate and on the screws. See the top picture. Note that the pot's lower terminal is grounded to the plate.

The wiring diagrams I find for the Archer don't have that 22-ohm resistor between the pot and the transistor base. Might be there for a 24-volt system.

 

[Dan Thomas]

When I installed the Maxdim, I didn’t care if it matched the original wheel or not. I wasn’t interested in installing something very old from Wentworth. When the “original equipment” arrived from Wentworth, it was dusty and filthy. Sent it back, ordered the Maxdim. It’sa knob by the way, and only needs the A&P sign the installation. because it’s STC/PMA.

Buying used potentiometers is a little like buying used engine oil.

 

[MajorTurbulence]

The transistor connections:

View attachment 115992


Lon Stratton's setup. Note the connections:

View attachment 115994

Now, that collector connection, the transistor's case, is hot when the switch is on. That's why there are insulators between it and the grounded heat sink plate and on the screws. See the top picture. Note that the pot's lower terminal is grounded to the plate.

The wiring diagrams I find for the Archer don't have that 22-ohm resistor between the pot and the transistor base. Might be there for a 24-volt system.

Wow, you did the labeling for me!
Obviously, will need to look at my particular wiring configuration, and whether there is a resistor. The schematic calls for a resistor in the late models of the serial number range, which mine was probably in, I think. Although, I do recall however that Archer III was 24 volt so not sure if that is where the resistor was required.

From what you have marked, I would check that the bus voltage is carried to both terminals marked when master is on. Then check that 12v is present at switch out when thumb wheel switch on, and that 12v or some lesser voltage comes out of the Pot out when thumb wheel is spun.

If ok at this point, I think I could skip to see the voltage output at the emitter pin terminal to lights, and see what happens when thumb wheel is spun. Getting output at all would seem to confirm that all is well with the other connections from the POT to the transistor. But if that voltage is very low or absent, then that would require clarifying if Pot output is getting to the transistor, or the resistor, if present, is not defective, along with the wiring connections.

If the resistor and connections are good, then it’s the transistor. Hope that is close to being right.

 

[Dan Thomas]

If the resistor and connections are good, then it’s the transistor. Hope that is close to being right.

If the collector has 12V, and the base has 12V or anywhere near it, the emitter should also have 12V. If the emitter doesn't have anything when the collector and base are at 12V, the transistor is defective or its pins are loose in their contacts in the socket. If the pot's tap (center terminal) has voltage but the base doesn't, it might be a bad resistor, if it's there at all.

The switch on the back of the pot might also be intermittent.

The whole idea of the transistor is as an amperage amplifier. The old dimmers were just heavy wirewound rheostats that directly varied the current by varying their resistance. All the light current went through them. They got hot and tended to roast things behind the panel. And they got expensive after transistorized stuff took over and the demand for them fell off. The transistor uses a small base current to control a large emitter current, and the base current is small enough that a carbon-track potentiometer can handle it without even getting warm. It's the transistor that gets hot, so it's on a heat sink. The Cessna heat sink is a finned aluminum casting that's much more effective than the Piper's aluminum plate.

 

[Dan Thomas]

Oh, and if the transistors in your airplane are mounted like the ones in Lon Stratton's airplane, use the insulators exactly as you find them on the old transistor. His setup will likely have the screws right against the transistor case, carrying the collector current into that strip of metal up the center of the insulator on the backside of the heat sink. If you use insulators on the screws, it won't work.

 

[MajorTurbulence]

I finally got some views of the potentiometer and the transistor. These views look quite different from the Saratoga we were working with before. But because of all your help previously, I was able to find things by reviewing the many views that I obtained. However, I am not sure I see a resistor in the circuit. Although I have some views of the transistor, I did not focus on that as I did not initial recognize it in that location. It seems that the component schematic from Bell206 was closer to the actual views, and that helped me.


Before I begin to actually test, it seems that the mounting side of the transistor can be exposed pretty easily if the overlying KT76A transponder is slid out it’s slot. Does that look correct? Also, taking pictures are one thing, but hopefully putting the VOM leads on the various connections will be easily done since I think I know where everything is now. I can focus in on the connection side of the transistor with more photos if needed. Also, regarding that transistor, is it obvious which protruding foot is the emitter and which is the base without it being removed entirely? Of course, if it checks out, it won’t be replaced in any event.

Also, it is not completely clear which labels in Saratoga picture is equivalent to the connections in my pictures, although I guess checking for voltage with the switch on and off, and with the Pot turning and at the different places on the transistor will locate the issue.

 

[cowman]

The odds that you have the exact same problem are slim but years ago I had an Archer II in the shop for inop panel lights. We looked at transistors and a bunch of other stuff, what it actually turned out to be was a wire pinched in a radio tray where it wasn't visible shorting out.

 

[MajorTurbulence]

Now that I see my own set up using my photos, if Maxdim was needed, could it be that it would fit exactly where the original fit, and the shaft is long enough, and that the shaft diameter could accept the original knob from my panel? Would the already installed transistor be needed and wired as is, if it checked out in good condition. If all questions answered in the affirmative, it looks to be a fairly easy process. What am I missing? ( Does the solid state design mean it eliminates the need for the transistor in the circuit?)

 

[Jim_R]

I've never seen a Maxdim modified to replace the OEM thumbwheel. I've only seen it directly "poking through" the panel with a knob that is oriented 90deg to the original.

 

[MajorTurbulence]

Maybe, nobody has tried. Looks like it could work looking at these photos.

 

[Bell206]

Does that look correct?

Yes. Maybe include the radio above it if it permits more room. Another option could be to remove 76 and swap the transistors and see if the problem follows it. Thats provided the other transistor is good. As mentioned these things are cheap.

although I guess checking for voltage with the switch on and off,

Don't forget your problem has a delay. Just keep that in mind when checking, i.e., no power from the potentiometer in the 1st 10 minutes.

Looks like it could work looking at these photos.

Anything is possible but it would be your APIAs call. I personally haven't seen it done, but have seen replacement potentiometers/rheostats installed using the same slot as the thumbwheel for the adjustment shaft/knob.

 

[Dan Thomas]

The Piper pot might have a flat on the shaft, since the wheel probably just presses onto it, with no retaining screw. The MaxDim pot might not have that flat. Cutting the flat without damaging the pot would be interesting.

 

[Dan Thomas]

Also, regarding that transistor, is it obvious which protruding foot is the emitter and which is the base without it being removed entirely? Of course, if it checks out, it won’t be replaced in any event.

Those two pins are offset from the center of the transistor so that you can't install it backwards. A pic I posted earlier:

 

[MajorTurbulence]

Those two pins are offset from the center of the transistor so that you can't install it backwards. A pic I posted earlier:

Yes, I saw that but they did not look offset to me, that is why there was a question.

 

[Dan Thomas]

Yes, I saw that but they did not look offset to me, that is why there was a question.

Oh, they're offset alright. I've never seen any TO-3 case that doesn't have offset pins.

 

[MajorTurbulence]

The Piper pot might have a flat on the shaft, since the wheel probably just presses onto it, with no retaining screw. The MaxDim pot might not have that flat. Cutting the flat without damaging the pot would be interesting.

There is a tiny screw on the OEM thumb wheel, so maybe yes if the shaft diameter is the same. The pictures do not show a flat, but maybe only seeing one side or not the distal portion of the shaft covered by the thumb wheel.

 

[MajorTurbulence]

But with the Maxdim solid state unit, is the OEM transistor even used when rewiring? The directions probably tell the details.

 

[MajorTurbulence]

Anything is possible but it would be your APIAs call. I personally haven't seen it done, but have seen replacement potentiometers/rheostats installed using the same slot as the thumbwheel for the adjustment shaft/knob.

Could it be that if the crappy looking knob that it comes with is not used, then the installation is considered experimental and not permitted in a certificated airplane. Oh I would hope sane judgement of my IA would prevail.

 

[Bell206]

then the installation is considered experimental and not permitted in a certificated airplane.

"Experimental" doesn't play into this. This would be considered an alteration which is perfectly legit. Now whether that alteration is considered a major or minor will be up to your mechanic. I'm not familiar with the Maxdim but a quick look through their documentation I don't see any big issues even if you try and retain the thumbwheel. However, as I noted this would be your APIA decision.

 

[MajorTurbulence]

I’m about to buy that VOM. I will be using it for this purpose, automotive and household usage including AC current. A quick Amazon search show examples ranging from $6.99 to about $190 for one with a “Craftsman” name. Analog and digital appear as choices. So we what do you recommend?