Panel lights fuse

[FlyBoyM20J]

2 hours ago, Yetti said:

I don't think there is a bus for the radio lights.   on the 75F.  The 75F is a testbed for the J.    there is a wire coming off the transistor board that is run to all the radios that need panel light input.   Seemed hokey.

Yeah, the whole panel lights setup seems like an afterthought (certainly, the transistor current limit was added in at a later date and even the transistor symbol is drawn backwards), but there is what looks like a bus strip labeled "radio lite bus" in the schematic.  The transistor output feeds the depicted bus strip via the left-most signal.  Here's the excerpt. 

[flyboy0681]

Boy dos this thread bring back (bad) memories. We had the identical problem about five years ago, after replacing the fuses, turning on the panel lights would eventually blow them. Sometimes it would be instant and sometimes during a flight. The latter pointed to a short which occurred during vibration. It took months of trial and error and finally when we brought her in for some panel work the shop isolated a dangling wire located on the right side of the panel. Man are those fuses expensive.

[FlyBoyM20J]

49 minutes ago, flyboy0681 said:

 It took months of trial and error and finally when we brought her in for some panel work the shop isolated a dangling wire located on the right side of the panel. Man are those fuses expensive.

I bet I have the same kind of thing.  This could have been an issue for years since the previous owner never flew it at night and perhaps didn't even turn on the panel lights.  Ever.

I'm using cheap fuses on a temporary basis because the spec'd part is about $50/each.

But now I think I'm going to wire in a little 5A circuit breaker in place of the fuse just for debugging purposes so I can avoid the time required to test and replace even the cheap fuses.

Or rather, I'll put it on the list for the hangar elf and see what he comes up with.

[flyboy0681]

1 hour ago, FlyBoyM20J said:

I bet I have the same kind of thing.  This could have been an issue for years since the previous owner never flew it at night and perhaps didn't even turn on the panel lights.  Ever.

I'm using cheap fuses on a temporary basis because the spec'd part is about $50/each.

But now I think I'm going to wire in a little 5A circuit breaker in place of the fuse just for debugging purposes so I can avoid the time required to test and replace even the cheap fuses.

Or rather, I'll put it on the list for the hangar elf and see what he comes up with.

Had we not gotten to the bottom of it, our plan was to do the same thing, wire in a circuit breaker.

[jaylw314]

2 hours ago, FlyBoyM20J said:

I'm using cheap fuses on a temporary basis because the spec'd part is about $50/each.

What is the spec'd part?  I just assumed it was a Bussman-style 5 A fast-acting fuse?  They are about $11 each at Newark...

[FlyBoyM20J]

46 minutes ago, jaylw314 said:

What is the spec'd part?  I just assumed it was a Bussman-style 5 A fast-acting fuse?  They are about $11 each at Newark...

It's an FM01-5A like these at Mouser for $36 each...

I was thinking it was a slow-blow part, though.  I'll need to check my notes because I researched this thoroughly when I first started on this project a week or 2 ago.

I ended up buying 10 of these cheap ones for $0.64/each for debug purposes.

[jaylw314]

20 minutes ago, FlyBoyM20J said:

It's an FM01-5A like these at Mouser for $36 each...

I was thinking it was a slow-blow part, though.  I'll need to check my notes because I researched this thoroughly when I first started on this project a week or 2 ago.

I ended up buying 10 of these cheap ones for $0.64/each for debug purposes.

The Mouser datasheet says the FM01 is a fast-blow fuse.  My plane came with the GMW-5's installed, so I just assumed those were the correct ones.  I don't know if the maintenance manual actually specifies the specific fuse type, though

[FlyBoyM20J]

2 minutes ago, jaylw314 said:

I don't know if the maintenance manual actually specifies the specific fuse type, though

Just this in the sch for my serial # range:

 

But the part # for the actual FM01-5A that blew the first time I had this problem had some more data on it and when I looked it up, it was a slow-blow at about $50 replacement.  I'll need to find that blown fuse to get the full part # again. 

I'm using the Littlefuse fast-blow 5As and they don't blow at turn-on, so I don't really see a need for slow-blow.  I'm not going to worry about it either way when I'm done with this debug.  Any 5A that fits should be fine, especially an FM01-5A.  And I got the idea to buy the Littlefuse parts because that's what I found in the fuse holder for the glare shield lights circuit...so whoever had to replace the FM01-5A for that (prior to my time) was fine with this solution.