mursch123

Hy

My Msd 6AL broken again. The Engine was running suddenly - dead and smoke coming out of the MSD Box. I took the box apart and discovered that a tyristor was burned. I replaced it. But same thing again. Donīt know wht to do.

I use the box with a MSD Pro Billet Distrubutor and MSD Sparkwires.

The Engine was running fine for about one year.

Here some pics.

http://img101.imageshack.us/img101/218/dsc01440u.jpg
http://img808.imageshack.us/img808/4858/dsc01442f.jpg



I think with the wiring is everything ok. I checked it twice or more.

So whats wrong?

Thanks

jlg2002

Smoke out of an electronic box usually means there's a dead short on the component that's smoking. Unless you have the affected circuits wiring, you're never going to isolate the issue.

mr_velocity

Gotta say this is the first time I've seen inside a 6 box, normally I run 6M boxes that are potted and sealed. I'm surprised the components are attached to the case as a heat sink. The 2 with the shrink tubing on them (one burned) could be attached to the heat sink which will provided much needed cooling (need to use insulators). If of course there wasn't a dead short that burned it up.

BuckeyeDemon

i wonder if a snubber or something isn't working causing a breakdown of the transistor? what was the part number of the component you replaced? Was it a FET? generally i've seen FET's become shorts when they are damaged.

it does seem strange that they used to be both an electrical and thermal isolator on the power transistors.

any chance you have a schematic? i'm curious to see what's really in one of those designs.

i would be inclined to replace the unit.

Gun Jam

yes that is a FET.

not sure what caused it to burn im guessing the FET failed secondary to some other issue..I think you are pretty much screwed unless you're hobby is IC related.

mursch123

Hy

Thanks for your answers. It is a MOSFET. Part Number: FS70KM-2 (from mitsubishi)(http://www.datasheetcatalog.org/data...on/mXrqvuq.pdf)

I replaced it with this one here: http://docs-europe.origin.electrocom...6b80d27032.pdf

The shrink tubes on them are not original. I put them on the FET. Maybe he just overheats. My fault. But why does it got burned the first time? The FET has a metalside (bottom side). I thought it shouldnīt contact with the case. So i put the shrink tube on it.


Maybe in the first time the contact between FET and case was not good enought. I will replace the FET again and tight it down to the case. Hope it will work.

Is there some way to test the Box with an multimeter or so?

Thanks for you help. And happy hollidays!

BuckeyeDemon

if you want to use that FET as a replacement you need something like the following to electrically isolate the electrically hot tab. the pad is thermally conductive and will get the heat out like the original FET's do.

http://search.digikey.com/scripts/Dk...name=BER182-ND


So, if you choose to try and replace the transistor again with a new FDP3651 then use the heat pads. don't put that heat shrink on them.

power transistors need to get the heat out.

i don't think anyone will be able to debug why it's going bad given the info you have provided. without a schematic, i feel blind...

mursch123

Hy

Thanks for your help. Here is some kind of schematic: http://www.molla.org/DIY-CDI/SC-DIY-...icle-hires.pdf

The problem is Iīm not sure if the FED is really a FS70KM-2 It is melted so iīm not able to read all the letters.

Thanks

BuckeyeDemon

can you read the neighbor transistor part numbers? if it's similar to that schematic there will be pairs.


to really debug these things, you would need to trace out the circuit, and get an o-scope on there to see what's happening. i have had to do that before (tracing out a design because the correct schematic didn't exist) and it can take several hours.

there could be a number of failures that could cause the FET to suffer it's death. there can exist high voltage transients in the path of a high speed switching device. failures of any of those components can slowly stress a device until it dies.

mursch123

Hy

Here is another schematic. http://web.tiscalinet.it/giordy/ECU/msd6a_02.pdf

Itīs from the 6A but the are the same (except the rev limiter). I think the transtistor is this one: Q14 (MJE3055) but Iīm not sure. On the printplate are the numbers Q240. Maybe some one can send me a pivture of a open 6AL where i can see the transistor.

Thanks