Quoted from ChipScott:
I will keep the old board and try and repair this burnt-appearing area. Anyone know exactly what part/piece that is that looks bad?
Some time ago, I published the following documentation which describes the functionality and components of the I/O board:
The burned transistor Q12 on your board is switching the current for the Coil 2 Output. It is a n-channel MOSFET PSMN039-100YS (100V, 28A). Unfortunately these transistors shorten when they melt, causing constant current at 70V through the attached coil - which is not healthy.
As the general 70V coil power is supplied by the transistor Q1 (an FQB22P10TM p-channel MOSFET), this one also becomes (too) hot and melts.
I saw (and replaced) some melted coil transistors on I/O boards. Sometimes the protection diode (here D18) is also dead/shortened.
The cause is unknown - due to the circuit design, various options are possible:
- Shortend output, i.e. the wiring from the board to the coils is damaged
- Unsuitable transistor, there are similar models which can manage higher currents and voltages
- Bad control circuit, leading to unhealthy transitions during on/off switching (which happens a lot as the coils are pulsed by PWM), see post above.
- My guess is the fact, that the transistors are fully controlled by the I/O board CPU, so finally by software. If the I/O board code stops working as intended (for whatever reason) the transistor may be switched on too long. There are different security mechanisms to prevent this, like a "whatchdog" which restarts the CPU if the program code freezes.
And there are hardware overcurrent protections for the coil transistors and also for the 70V general power implemented on the board. Both do not work as they should, as your board shows. Ironically the coil transistor protection was disabled with the bridging wires (and 0 Ohm resistors) - obviously it made things worse!
Lots of unnecessary/wrong components - what's so bad about good old fuses? There are even nice small SMD types, JJP pinballs are using a lot of them...