Hey guys,

Has anyone else experienced a failure in the APM mux?

I've been playing around with my APM and today I was playing with the dancing servo demo with some (really crappy somewhat dead) servos i've have scrounged from crashed planes.

Now I've noticed that the first output has stopped outputting.  For the dancing servo demo it no longer gives a PWM signal on Out0 (it did earlier today when i tried it).

I checked connectivity from the pin to the MUX, Out0 is connected (correctly) to MUX pin 4.  I checked that the input was connected to the ATmega1280, and it was outputting a signal.

I also verified that other channels on the MUX are working, only OUT0 (first channel of the mux) doesn't work correctly.

My questions are:
* Has anyone else experienced a failure similar to this with the mux?
* What could it be (other than somehow i fragged my mux chip)?
* Any advice other than hot-air it off and put a new one on?
* Is it possibly a power supply issue (not enough power)?


 (Also if i do take the chip off I'll put it on a test bed to verify that it failed)

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Hi Alex

I had the same on ch2, sorted out the input and output till I found a replacement chip from Farnell. Easy to change with a standard small tip iron. I cut the legs of the old one to remove it from the pcb, then cleaned up the pads before fitting a new one. Not sure why it failed but working fine now.. If you have trouble getting one (the right pitch) let me now I have a few spare.

Whats the part number on the one you got?
I know pretty much any 74157 chips are interchangeable, but what's the footprint? (I'm trying to figure that out right now)

Looks like its a SO16 or a SOIC16 package.
Found it (i think)

74ACT157 from Fairchild semi (at least the one on my board is) and looks like its the 3.9mm-wide body SOIC 16 package:

full part number (from my search): 74ACT157SC
Yep been there as well 8(
On two different APM board , I had to change the MUX . I fried CH1 and CH3 (throttle) on one board and CH3(throttle) on second board.
What I "suppose" fried it was to badly connect the servo plug from the ESC to the board, I might have connect the +5 to the signal connector.
A friend of mime with magic hands manager to unsolder/solder new mux. Now everything is working again and will be extra careful with this now.
I fried mine too once, shorted output and now everything is ok. New APM will have better protection against +5V to signal pin. That's what burns it.. One pin too up and oops :)
Measure twice, cut once. Works every time for failure free operation.

Just a thought.

Also, now i only switch servos when they're unpowered. No +5V, no muxdeath
How do you protect against that? (as a circuit designer im really curious now)
This is the part I used from Farnell

Farnell Order Code: 1750389
Manufacturer Part No: CD74ACT157M

I have kind of a similar issue, but it may not be the same thing - in manual mode, ch1 will not provide an output, all other channels seem fine, but all output channels appear to work correctly once I switch to stabilize or another auto mode. I'm running a simulation with X-Plane, and the radio inputs seem to work the entire time. I didn't see any bad connections on my first look, but I need to check more closely.

I'm not clear on exactly what the mux does in manual vs. non-manual modes, so I'm not sure where the signal isn't coming through.

I had the same happen on channels 1-4 of my APM (not sure how or when I fried it) but when loading the "servo dance" sketch OUT4-7 control the servos properly but OUT0 makes it swing madly and 1-3 just makes them vibrate...

Could this also be a fried MUX chip (have a few on order in case)

I do NOT have a receiver connected at all (still being delivered) ... would this cause OUT0-3 to behave erratically - I thought perhaps the failsafe is switching to manual due to there being no signal on IN07 (Auto mode switch from rcvr) and there are no channel input connected so causing issues?

Any help appreciated


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