Stuck bit at port 1FE Bad Controller IC ?

[CrazyBoss]

Hi.

I was testing an Intellivision Model 5370 - PAL, mainboard is from 1981

 

There is a stuck bit at port 1FE, which is the Right Controller. No madder what this bit (2) is set to 0, if I press keys the other bits changes, but bit 2 remain 0, Also while the controller is not plugged in.

Normally if nothing is pressed Port 1FE returns 255 (11111111 binary)

 

But now it returns 251 (11111011 Binary).

 

I assume its some logic IC that when bad or is it the CPU ?

 

Thanks.

 

[mr_me]

Controller signals goes through the sound chip ay-3-8914.

[CrazyBoss]

8 hours ago, mr_me said:

Controller signals goes through the sound chip ay-3-8914.

ohhh yes thats correct,  also the fact for MSX computers. Hmmm could it be a bad ay chip. I might try to swap it out to check.

[CrazyBoss]

ok seems like the stuck bit issue was solved after removing the ay-3-8914 and using some electronic cleaner at the socket and put back the ic again.

 

Everything seems to work find for the Right Controller now.

 

But i see another strange behaviour, when using the left countroller, pressing some buttons, actually will affect bit2 at the Right controller. 

 

Especially if i push the buttons hard. Also the pressing the disc, in some directions,at the Left controller will affect then right controller, but only bit2.  

[CrazyBoss]

ahhhhr now its back again to the first issue. With the stuck bit2 at Right controller, and the issue descriped where the left controller affects the right one is none existing.

So it have to be ay-3-8914 that is unstable in some way. 

 

I see from the schematics, then some 0.01uf capasistors are connected to some of the pins at the controller pins, maybe they are bad or shorted,  would it be causing this issue ? 

c34,35,36,37,38,39.

Edited April 16 by CrazyBoss

[+nanochess]

Capacitors aren't used for joystick pins.

 

I would suggest measuring the joystick pin against the chip pin with a multimeter set to 20 kohms to see if the signal reaches the chip.

 

Also clean with isopropilic alcohol and a brush the connector solder points, over time solder paste can become conductive.