Dead End Laika

I dug this one out last night because I really want to get it working, but I somehow made it worse. Build Doc here: https://drive.google.com/file/d/1t9UkjtPVxwSyIMK1RBP7tvHhX9SKaRui/view

Initially, I got a sound that just sounded like it was completely voltage starved, but at least all the leds were working and there was sound coming through. I reflowed some solder joints - there are admittedly some pretty ugly ones in there - and reconnected the power, input and output. Now I get no sound at all and no led indicators. What's weird is I tried tracing the circuit and hit what seems like a problem right at q1 (BS250 mosfet), I'm reading 8.83 at the Drain and 0.56 at the Source pin. I've reflowed, checked the pinout, used an xacto knife between the joints and legs, but keep getting that reading. Everything after it checked at random reads around .4-.4v.
 

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Updating this thread, because I'm almost certain the problem was q1. That was supposed to be a BS250, but since they no longer make those I had a BS250P in and I've read a few things about them being very different re: voltage drop. Can anyone recommend a good mosfet substitute for BS250?
 

benny_profane

Well-known member
Pretty sure you have Q1 installed backwards. That's being used for reverse polarity protection. I'd suggest double-checking the pinout of the device you're using and ensure it's installed correctly.
 
Pretty sure you have Q1 installed backwards. That's being used for reverse polarity protection. I'd suggest double-checking the pinout of the device you're using and ensure it's installed correctly.
Definitely not backwards, although I can see why it looks that way in the pics. It matches the shape and pinout on both silkscreen and build doc.
 

benny_profane

Well-known member
The pinout, top to bottom, is S-G-D. For the BS250P, it should be oriented such that the flat side is facing left (a contradiction of the silkscreen). The picture is very grainy, but your voltage readings at the various pads point to the device being installed backwards.


522-BS250P.png
 
The pinout, top to bottom, is S-G-D. For the BS250P, it should be oriented such that the flat side is facing left (a contradiction of the silkscreen). The picture is very grainy, but your voltage readings at the various pads point to the device being installed backwards.


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F**k me, you're right. Wow. Thanks so much. Will reply to this thread and mark it accordingly if this solves the issue.
 
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