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Burnt transistor in 'Joes better spark output circuit'. Cause?

Old 06-23-2017, 04:04 AM
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Question Burnt transistor in 'Joes better spark output circuit'. Cause found but not working..

Hi all,
I'm still working away at my MS2DIY install and got her started and kind of idling today

In the process of troubleshooting the wideband reading 20+ when idling and the smell of the car running super rich I smelled the smell of magic smoke around the megasquirt when I was poking around near it.
Popping the lid off and I see a scorch mark on the top of the case and a cracked/burnt transistor in the modified spark output circuit:


I can't see any other obvious damage.
I have no idea when that happened precisely or why. Any suggestions of what could have done it and how to avoid it happening again?

Last edited by Okisub; 06-26-2017 at 03:14 AM.
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Old 06-23-2017, 04:52 AM
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Another related question, would a spare 2N3904 transistor I just found as a leftover from the MS2 build be OK to use as a replacement?
I see in the circuit mod thread an NPN that can handle 50ma is needed. 2N3904's are good to 200ma.
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Old 06-23-2017, 11:19 AM
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yes, a 2N3904 is just fine. Did you solder the old transistor from the underside? One leg appears to have no solder on it.
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Old 06-23-2017, 02:50 PM
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Originally Posted by WestfieldMX5 View Post
yes, a 2N3904 is just fine. Did you solder the old transistor from the underside? One leg appears to have no solder on it.
Thanks. I'll try again in the morning when I can start the car and pay attention to when it heats/blows if it tries again.
I might have accidentally changed a setting at some stage rather than something being physically wrong.
Yes it was soldered from underneath, I just didn't use enough to get a nice cone of solder on the top.
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Old 06-23-2017, 02:52 PM
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yeah lets see that circuit from below please.
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Old 06-23-2017, 03:30 PM
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Above, below and closer of the prototype area:


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Old 06-23-2017, 03:39 PM
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the top transistor, that blewed up, is missing its 270ohm pull-up.

everything else looks good, clean build.
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Old 06-23-2017, 03:53 PM
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Originally Posted by Braineack View Post
the top transistor, that blewed up, is missing its 270ohm pull-up.

everything else looks good, clean build.
Fantastic, thanks. No idea how I missed that one.
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Old 06-24-2017, 12:38 AM
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Tried it out just before and the replacement transistor isn't even warm. Thanks again.

The car is still idling like it's trying to shake itself to pieces and showing AFR past maximum but I'll keep reading/tinkering before making another hand holding thread.

Last edited by Okisub; 06-24-2017 at 08:32 AM.
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Old 06-25-2017, 11:35 AM
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OK, I'm still having an issue here.
A full weekend of reading and trying everything I can think of and I'm back to ask again...

The car started but was only running on 2 cylinders.

I'm not getting anything from spark A, cylinders 1 and 3.
Using the test mode with the spark plugs out and resting on the rocker cover spark B ticks plugs 2 and 3. Swapping the trigger wires on the coils and 1 and 4 will spark. So from that I assume my coils are ok.
Measuring the voltage on the MS2 with a multimeter I see about 3v on IGN but 0v on IAC2B(I might need to double check if I have that the right way around, can't remember 100%).
I have replaced all 4 transistors- Q6, Q8 and the 2 on the proto area. All the resistors around that area measure correctly.

One thing I have noticed is that the 2 outer legs of Q6 show as a closed circuit but Q8 shows open. Also D16 will faintly illuminate when I'm poking it with the multimeter measuring impedance but D14 will not (when the board is disconnected from the car and the daughter board is removed). Both LEDs illuminate when the MS is powered up)

I have been over all my solder joins with a 10x loupe and there are no bridged pads or anything like that.

Where do I backtrack from there? Or any more advice?

Edit: I'll also add that this is a new build and has never run before, so the missing spark might not even be related to the burnt transistor...

Last edited by Okisub; 06-25-2017 at 12:02 PM.
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