lighting control

murphey

murphey

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murphey
The panel light pot on the cherokee is shot and I need a new one. Part 67435-02. What I really need to find is the non-aviation version, which will probably cost about a buck ninety-five instead of the $300+ from any of the aircraft suppliers.

Anyone know the specs?
 
BigBadLou said:
Do you have access to the MM? Does it specify resistance range? (I would hope)
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Docs only list the part number. I can find a photo online but there's no other information other than $330 for a pot with 6-8 wires of different colors.
 
I've never known a MM to give much info on lighting sys/control other than description and operation. Maybe the rheostat has other numbers on it that can be cross-referenced? If you have a wiring schematic you should be able to determine what you need.
 
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GlennAB1 said:
I've never known a MM to give much info on lighting sys/control other than description and operation. Maybe the rheostat has other numbers on it that can be cross-referenced? If you have a wiring schematic you should be able to determine what you need.
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Will go hunt for the schematics. But it may be easier to remove it from the airplane and hope there are more details on the pot.
 
ah...mine crapped out but the dimmer side still works. the click on for nav lights failed on me....almost to causing smoke! So we found the wire in the connector for nav lights and I got a CB switch to turn them on manually now and not part of the dimmer switch.

Have you crawled up behind the panel and sprayed the rheostat with cleaner??
 
simtech said:
ah...mine crapped out but the dimmer side still works. the click on for nav lights failed on me....almost to causing smoke! So we found the wire in the connector for nav lights and I got a CB switch to turn them on manually now and not part of the dimmer switch.

Have you crawled up behind the panel and sprayed the rheostat with cleaner??
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Yup....cleaner and also swapped in new transistors. Only thing left is the pot. I think I'll take it out tomorrow and look for a replacement.
 
GlennAB1 said:
I've never known a MM to give much info on lighting sys/control other than description and operation. Maybe the rheostat has other numbers on it that can be cross-referenced? If you have a wiring schematic you should be able to determine what you need.
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Found it in the Service manual. Looks like a 2.5K ohms and 5 watts. Now I need to pull the old one to get the dimensions. Looks like inflation has pushed the non-aviation price from $2.95 to $14.99.

Update: very poor quality printing in the manual. what I thought was many generations of copying with speckles all,over was wrong. Not a 5 watt but 0.5 watt.
 
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Aircraft Spruce
11-06674a.jpg
 
GlennAB1 said:
It is now. ;)
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Yes, but if it really is a potentiometer/rheostat, then if you don't know the resistance value you can pry open the back of the case, see where the break is, measure both sides of the break, add the two together and get a reasonable approximation of the original resistance value.

The better question is why the hell they would put a wirewound pot into a circuit that already has a chunk of silicon in it to handle the heavy current? That seems to defeat the purpose of transistor current gain, as wirewound pots have been notorious over the years for failure.

Jim
 
Who knows. Why did Douglas go with a cable drive slat system on the DC-10 and Lockheed go with a torque tube/jackscrew drive on the Tristar? Engineers...
 
BigBadLou said:
This is what's installed on smaller Pipers to dim the panel lights (and control the nav lights at the same time).
dimming-switch-67435-002.png
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On my cherokee, there are 2 pots...one controls the nav lights and the other controls the overhead light and the panel. Will be digging it out on Friday. I already know it the specs thanks to the service manual. Now to get one with the correct connections and rotary dimensions.
 
BigBadLou said:
That does not jive with the part # you gave: 67435-02.
Could it be somebody altered the pots in the past? (not the first time in history :) )
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Sorry...thr 674 is the one not working. The other one is fine.As for the number, its direct from the Piper PA28 parts & service manuals, circa 2009.

The A&P ( who also happens to be FCC certified) crawled under the panel today and confirmed it.
 
[QUOTE="murphey, post: 2239829, member: 5121"
The A&P ( who also happens to be FCC certified) crawled under the panel today and confirmed it.[/QUOTE]

What does that mean?

Jim
 
weirdjim said:
[QUOTE="murphey, post: 2239829, member: 5121"
The A&P ( who also happens to be FCC certified) crawled under the panel today and confirmed it.
Click to expand...

What does that mean?

Jim[/QUOTE]
Means he's legal to do radio repairs. At least that's how I understand it.Actually, it's irrelevant but he mentioned it today.
 
One size does not fit all with the dimmer circuit in the Cherokee line. The circuit changed over the years so you're going to need to know how the airplane you're dealing with is set up. I'd be worrying less about what the parts manual calls for and more about what is in the plane, although they likely match.

If your normal mechanic doesn't understand electricity (most don't seem to), find one that does to help you out.
 
Aircraft, in general, often have changes throughout its production life, commonly referred to as "effective part" or "part effectivity" for a given aircraft serial number, production number, or line number, with possible interchangeablity. Refer to pertinent Illustrated Parts Manual.
 
GlennAB1 said:
Aircraft, in general, often have changes throughout its production life, commonly referred to as "effective part" or "part effectivity" for a given aircraft serial number, production number, or line number, with possible interchangeablity. Refer to pertinent Illustrated Parts Manual.
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I have both the parts and servce manuals, and the electrical schematics. Everything points to 67435-02. After talking with many parts (non-aviation) shops, the "industry standard" these days is 2.5Kohm 2w and no one is carrying or making the 5w version anymore, other than Piper, hence the $330 price tag.
 
I don't know Cherokees but a lot of airplanes have a power transistor (emitter follower configuration) between the "rheostat" and the panel lights and given the small size of the potentiometer I saw on this thread I'd say that's likely the case there. That's important because one reason the pots fail is the base-collector junction of the transistor opens leaving the full lamp current flowing through the pot. That burns out the resistive element in the pot so it only powers the lamps when the wiper is on the high side contact at the end of rotation. Best to check this out before replacing the pot or you may be replacing it again shortly.
 
gismo said:
I don't know Cherokees but a lot of airplanes have a power transistor (emitter follower configuration) between the "rheostat" and the panel lights and given the small size of the potentiometer I saw on this thread I'd say that's likely the case there. That's important because one reason the pots fail is the base-collector junction of the transistor opens leaving the full lamp current flowing through the pot. That burns out the resistive element in the pot so it only powers the lamps when the wiper is on the high side contact at the end of rotation. Best to check this out before replacing the pot or you may be replacing it again shortly.
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There is a transistor, 2 in fact. According to the shop, they replaced the transistors last week, didn't solve the problem.
 
I've had the FCC approval to fix aircraft radios since I was in college (I got a 2nd Class Phone because I was engineer at the campus radio station). That isn't sufficient, as Jim is alluding. The shop has to be approved by the FAA as well.
 
If that thing has the on-off switch as part of the assembly, and you're getting off and full bright, check the schematic to see if there's a small resistor between one of the pot's outer terminals and ground. Cessna uses one there, and it's to give a full range of control to the incandescent lights. (Without it, and with the pot grounded at that point, you'd get them on dim about halfway through the pot's rotation and then going to full bright.) If that resistor fails, the voltage dividing function of the pot is lost and the base of the transistor gets enough current to keep it full on no matter where the pot is set. The resistor is a lot cheaper than a new pot.
 
gismo said:
I don't know Cherokees but a lot of airplanes have a power transistor (emitter follower configuration) between the "rheostat" and the panel lights and given the small size of the potentiometer I saw on this thread I'd say that's likely the case there. That's important because one reason the pots fail is the base-collector junction of the transistor opens leaving the full lamp current flowing through the pot. That burns out the resistive element in the pot so it only powers the lamps when the wiper is on the high side contact at the end of rotation. Best to check this out before replacing the pot or you may be replacing it again shortly.
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And just think, for lack of a floorsweeping quality PNP supplying that base current (two bits at most) they could have gotten by with a fifty cent volume control size pot that never would have burned out, the PNP/NPN pair being a ground-seeking rather than battery voltage seeking. Sigh.

Jim
 
weirdjim said:
And just think, for lack of a floorsweeping quality PNP supplying that base current (two bits at most) they could have gotten by with a fifty cent volume control size pot that never would have burned out, the PNP/NPN pair being a ground-seeking rather than battery voltage seeking. Sigh.

Jim
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I guess a two transistor circuit to dim the panel lights is just too advanced technology wise for an airplane with an engine designed a significant fraction of a century ago.
 
gismo said:
I guess a two transistor circuit to dim the panel lights is just too advanced technology wise for an airplane with an engine designed a significant fraction of a century ago.
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there is a power transistor for each of the dimmers.. radio lights, and instrument lights
 

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Update...when I had ADS-B installed summer of '17, I also had a solid-state switch installed to replace the dead pot. (http://www.maxpulsemaxdim.com/index.htm) According to the shop (Depot Avionics in KALS) it's what Piper's using in the new airplanes. The shop is replacing a lot of old stuff with MaxDim units. And 30-50% cheaper than one of the old units, assuming you can find one that still works.

The other benefit - it controls LEDs. I've replaced a few of the GE53 lamps behind the panel and the MaxDim works great dimming the LEDs.
 
bluerooster said:
Interesting. Mine has no transistor, and works just fine.
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Yours will have a big wirewound rheostat that generates a lot of heat at anything less than full bright. The heat tends to damage stuff behind the panel, and those rheostats are such old technology that they're rare and expensive.

The transistorized circuits use a power transistor that also generates heat, but it's mounted to a proper heat sink that can be placed where it doesn't cause problems and the heat isn't so focused. Some of the Cessnas have it outside the cabin wall in the wing root.
 
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