First post, by Mu0n
Foreword: I don't *need* this gamepad, as I also am equipped with an Advanced Gravis GamePad as well as both the Advanced Gravis Joystick (one button on top of the stick) and the other with two.
Goal: repair its bizarre behavior just as a fun electronics diagnostics/repair project.
Using Joycheck.exe to test this out:
Blue works as regular button 2 in single shot mode (bottom switch to the right)
Green works as auto-fire button 2 in rapid shot mode (bottom switch to the left)
Yellow works as regular button 2 in rapid shot mode (bottom switch to the left)
Dpad left and right worked in some rare tests I did and did not work most of the time (currently don't work)
No other button/directions worked.
The underside of the PCB where the single/rapid shot switch is located, there was some FAIR amount of crusty electrolytes in there. It's weird because the nearest isolated capacitor is placed in such a way that it'd be hard to imagine it was the culprit. It is possible that there's a tiny capacitor inside the switch assembly (really?) that has leaked, but I haven't opened it up yet, waiting to see if there are opinions about it here first.
The pinout to the male cable connector is *really* strange and doesn't make sense. There's potentially several disconnected wires. Only one 5V pin and only one GND pin are used. Only button 2 is continuous.
The voltage levels kinda reveal why most buttons don't work. The blue button gets to 4.90 V facing ground, so that feels right. The red button (presumably button 1) only has 2.08 V which tips down to around 1.90 V and gets back up to 2.08 V. I've tried slightly coloring the main traces for the 4 buttons without overwhelming this top of the PCB.
And finally, the underside of the PCB.
The transistors are all MPS3904.
The autofire 555 chip is a NE555N.
R1 = 4.63k
R2 = 4.64k
R3 = 6.77k
R4 = 66.1k
C1 is marked 1uF 50V
C2 = ?? looks like a 10 nF, it's right between pin 1 (GND) and pin 5 of the 555 chip.
According to this 555 astable calculator https://ohmslawcalculator.com/555-astable-calculator I should be getting around 18 Hz, but joycheck gives me between 7 and 8 presses per second. Dunno if this difference is explainable or indicative of something (probably not)
What component should I test out first, possibly out of circuit?
Should I directly connect the pins of the db15 connector of my vintage PC to a breadboard, without using the included cable to make sure each button can communicate to the outside world?
I could be willing to bypass the autofire subcircuit entirely with bodge wires if that makes everything way easier - what would be needed?