Topic-icon ATA 31-16, Master Caution Lights

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5 months 1 day ago - 3 months 3 weeks ago #484 by Mark
Question...... what would cause one set (of two) of the individual Master Caution Lights to fail on the Master Warning Panel?



Hopefully that question will be answered when I have the time to mess with it.
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Last edit: 3 months 3 weeks ago by Mark.

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4 months 3 weeks ago - 4 months 3 weeks ago #487 by Mark
The master warning and test circuits on 757's are somewhat difficult to grasp and follow on the schematic or wiring manuals. I was lucky on this problem that a severe "gutting" wasn't required.

The problem as noted was one bulb of each master warning red light was out. (Both bulbs of the "Cabin Alt" are on in this image. This was shot after the initial problem was discovered.)



From the best as I could figure out, the master warning lights (red) on a 757 receive power from two sources. One being the battery bus. I'm assuming the reason for this is if the aircraft was in an emergency situation using just battery power, one bulb of each light would still function as a warning to the crew. No other examples of this circuit were noted on any of the other cockpit warning or caution lights. Not to say there aren't more, I just didn't bother to look.

The prints for this system lead you to the P26 panel. Cockpit right rear. Numerous cards are found inside, along with plenty of relays.



Some of the prints for this system, 33-16-**, show extra circuit breakers not located on the normal overhead locations. The first problem was isolated down to a power input failure. The extra breakers were located on the upper section of the P26 panel behind a cover. Sure enough..... one was popped.



Resetting the breaker did not return all the lights to a double bulb "on" condition. Upon messing with the lights themselves, some of the failed bulbs would light if the annunciator was held just right. Pulling out the panel and wiggling the wires behind, returned the other bulbs on.



Both of these problems are indicative of bad annunciator assemblies. Specifically, the sockets. It's almost certain that one of the annunciators popped the breaker.

I recommended all six assemblies be replaced. I've never done this type, but it looks like after loosening two locking tabs the socket can be slid forward out of the mount. The wire/pins look to be removed with a red/white plastic removal tool.




757-200
AWM 33-16-51 ASM 31-51-03 AMM 31-51-00
Last edit: 4 months 3 weeks ago by Mark.

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4 months 3 days ago - 3 months 3 weeks ago #494 by Mark
It took some time to procure the replacement parts needed for the Master Warning Lights. It was actually easier to order the whole assembly. Although it is not the purpose of this site to show actual repairs, in this case the Maintenance Manual is lacking in practical steps for actually doing the job.

The mount for the light segments behind the center instrument panel seems to be permanently installed (there was no movement at all when grasping with a hand and there is no noticeable mounting hardware). Individual light assemblies (annunciator, diode block, and base) slide out of "sleeves" (6 sleeves, in this case). The annunciator and diode block come out forward. The base aft.

Some extra wires required removal from the base units of the replacement assembly. The pins came out easily with the red/white plastic tool. I used the Deutsch tool for all of them, but a Amp tool was ready for use. (Rear Release Pins)



The annunciator and diode block come out together. The are some pesky plastic pins on the diode block that take some "convincing" to slide out of the sleeve. I treated all this gingerly to start with, but it was discovered that some firmness was required to remove and install some of these (a bent pick was a great help).



The bases came out by loosening two small set screws from inside the sleeve and bending some tabs on the outside. The bent tabs could be pushed back into position with just a finger. This cannot be bypassed when installing new bases in the instrument panel sleeve (the tabs will "not" catch the raised notch on the replacement base).





Each base has a stamp "A" through "F" on the back, but the bases are physically all the same. Working with the replacement assembly on the pedestal was quite easy...... working behind the panel, not so much fun at all. The first base out was the outboard upper (or "A") "FIRE" light base.
The securing screw and base tab can be seen.


Not every base required replacement. Some of the lights just needed the annunciator/diode assembly replaced. All three of the bases that did require replacement had at least one spring loaded pin stuck down.



This job was really quite a pain in the ass. Finding the right combination of annunciators to diode blocks was frustrating. The pedestal was strewn with chunks of parts everywhere. The stuck spring pins on the bad bases could be seen from inside the sleeve, so the ones requiring replacement where identified quickly.



757-200 AMM 33-16-02
Last edit: 3 months 3 weeks ago by Mark.

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