Fried heatermeter board?


 
Personally with the history you have had burning up the rPi etc I would have run the HM with the rPi and no LCD board connected for a while to be sure that part is stable. I have looked over your base board pics again and nothing strikes me a bad, I can't spot any potential shorts anywhere. I haven't seen the LCD board, though since you said it didnt work then it magically worked that would seem to point to an issue on that board.
I would boot the HM with no rPi or LCD board and measure my voltages in the power supply... 12V in, 5V after the OKI and 3.3v after the MCP1700-33 (transistor looking device near the OKI). Having smoked an rPI (or two) you definitely want to verify your supply voltages are proper.
 
Nothing jumps out at me on the button board either. I have yet to build my HMv4.3 button board so I am not sure how close the LCD board mates to the main board and the rPi? You might assemble it (without power) and look for any place where one board may be able to touch the other and short things out? To fry a rPi is odd, particularly at random after working, seems kinda like a momentary short somewhere.
 
and you are sure you didn't set the HM on anything metallic that could have shorted things out while powered up? I am at a loss here due to the intermittent nature of the issue.
 
nada. was on a wood table or cork board.

the pi shorting happened suddenly; if i plug the pi in now it only takes seconds before the chip is burning hot to touch.

checked the voltages to the pi from the hm and they read out correctly per the voltage diagram.

could a bad transistor or other piece cause something like this? electronics is not my forte...obviously.

again, thanks for the help and assistance.
 
The BS170 on the button board is in control of the background light, it uses pin 15+16 to connect to the LCD, pin 15 is +5v supply, pin 16 is the control signal.
Here is the schematic for the button board:
HeaterMeterLCD.png


The 10K contrast pot would be one area that seems it could short out fairly easy and the polarity of the LED's could be a problem as well, other than that the board is pretty simple. Do all the LED's work?
 
Here is the base board schematic, note the rPi header on the bottom with very few connections to the HM board really... I would follow those leads from the rPi header and inspect the solder work and component values of the parts they connect to.

HeaterMeter433.png
 
The SCK and MO leads appear to be the points where the rPi connects to the LCD board, going to pin 10, 11 and 14 of the shift register IC. I would check that circuit closely.
 
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