PCEngine GT control pad faulty

[Quagmire]

Hi folks I've got this PCEngine GT that appears to have a joypad issue, only up, left, and run seem to work   Is this a common problem with these? any ideas what areas to look at

I've managed to get the sound working and the screen seems ok, but playing games on it is impossible up, left, up again, left again. not good

[Necromancer]

I wouldn't say that's a common problem.  I'm no help in the electronics department, but I'll give some advice on the mechanical side: clean the contacts thoroughly if you haven't already and try rotating the rubber contact thingy to see if it's the culprit.

[Quagmire]

I was hoping that's all it would be but alas no I've traced the pads all the way back to the IC in the centre of the control pad board, some of them even get 5v but still don't work

I'll have to investigate further

[Duo_R]

u didn't notice any corrosion on the PCB? I had a system that had a corroded II and Start button.

[Turbo D]

Hello Quagmire, welcome to the pcenginefx forums! The problem that you have described is a simple one with a simple solution. You or whoever performed the sound fix on your PCEngine GT may have accidentally lifted a pin or multiple pins on one of the chips that happens to control the buttons not working. This may have happened when the shielding was being removed or maybe just by clumsiness. I've enclosed a picture that illustrates an area in which your problem may lie. If this is infact the problem you are facing, simply resolder the pins back to the pcb. I hope this help you out.

[Quagmire]

Thanks TurboD I'll check that out, There had been a sound mod done before I got it as there was a cap floating around in side the case (bad soldering skillz by someone). When I got the GT the control pad didn't work and the sound was dead, so it looks like the person who did the mod first made a right hash of it

Cheers for the advice peeps I'll let you know what I find.

[Charlie]

For those interested in the technical aspects of TE JoyDisc and TurboButtons, that is IC700, a Hudson custom ASIC #HUC6280. 

If I remember correctly, it works like this:

The JoyDisc/TurboButton pins are 22,24,25,26,31 and 32. 

Pins 22,24,25, and 26 are the 4-bit multiplexed inputs to IC700 from the JoyDisc/TurboButtons.  Because there are 8 sources of data (Left, Right, Up, Down, Run, Select and the two combined Turbo switches), the data is input in two groups of 4.  Left, Right, Up, and Down are the second group, while Run, Select, and the two combo's are the first group.

The implication here is that the JoyDisc/TurboButton PCB has it's own "smarts", and it does, which you will see (or have already seen) when you remove it. 

Pins 31 and 32 are the outputs from IC700 to the JoyDisc/TurboButton that control the timing (WHEN the data is to be sent to IC700), and data selection (WHICH GROUP of the 4 data bits is to be sent to the IC700). 

Charlie

[Quagmire]

Cheers Charlie that may help

[Quagmire]

Finally found time to take another look at the GT and I've fixed the control pad issues by reflowing the IC on the control pad pcb.

It's now undergoing leaky cap replacement.