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Author Topic: A4000 joyport glitching  (Read 1450 times)

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Offline Mr.ToastTopic starter

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A4000 joyport glitching
« on: November 17, 2018, 01:38:33 AM »
Having trouble chasing this problem down: A4000 with previous battery damage (lots of traces repaired and damaged mitigated). Got it at least back to a serviceable condition from the dead. Long term plan is to relocate the parts to the Acill A4000 re-spin as I am sure there's hidden dangers lurking from the battery damage. Its also been recapped, although missing banks 1-4 due to damage. I don't see a point in ordering simm sockets until I am ready to transplant to the re-spin.

Problem seems to be in the joystick port lines. All axis work, but when the joystick is held in the up position, the up registers-a half second later, all 4 axis on both joy and mouse register as full on. This has been verified using multiple DB9 joysticks. See the video below.

https://youtu.be/_g7utehhPew

All traces (checked with http://www.amigapcb.org) buzz out properly. No apparent shorts.

Has anyone experienced this before and solved it?

Thanks for any insight.
Past is prologue -=A1000|A500|A4000|A1200]=-
 

Offline rkidd7952

Re: A4000 joyport glitching
« Reply #1 on: November 17, 2018, 03:30:19 PM »
Hi,

I'd look at U975 and the pull up resistors R981-R988.  (See page 16 in https://www.amigawiki.org/dnl/schematics/A4000_Rb.pdf)  Check for corrosion or junk on the pins, and see if there's any contamination that might cause shorts.

U975 is a parallel to serial shift register.  All eight directional lines from the ports feed to this chip, then Lisa reads the values out over a single line.  The resistors set the voltage on the directional lines to 5V when no direction is selected on the stick.  When you move the stick, the corresponding direction line is connected to ground.

I'd check the voltage on one of the directional pins of the other port while you do your test.  The pin should be a steady 5V as you hold the joystick on the other port.  If it drops below ~3V, I'd look for damage around the pull up resistors.  If the other port is OK, look for damage around U975, and perhaps replace that chip.

Robert
 

Offline Mr.ToastTopic starter

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Re: A4000 joyport glitching
« Reply #2 on: November 17, 2018, 07:07:26 PM »
Thank you for the comprehensive reply. I am reading low on joy up (port1) on R985. All others hold 5v clean. Swapping to port 0 is nominal. Continuity checks on U975 and all surrounding traces look great with no obvious shorts. This area received the worst of the battery leakage damage so it is possible that U975 is compromised. I'll look for a replacement-maybe there is a modern equivalent?

I just realized ER985 is going low-can't easily check r983. ER985 should be where r983 exits inbound to U975 (edit)

Everything else about this machine looks pretty rock steady so far-even though I have plans to transplant.

Thank you very much for the help.
« Last Edit: November 17, 2018, 07:15:47 PM by Toast|FPV »
Past is prologue -=A1000|A500|A4000|A1200]=-
 

Offline Mr.ToastTopic starter

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Re: A4000 joyport glitching
« Reply #3 on: November 17, 2018, 08:22:42 PM »
Poking at U975:

pin 10 4.93v - joy0 up 0v
pin 11 4.93v - joy0 left 0v
pin 12 4.93v - joy0 down 0v
pin 14 4.93v - joy0 right 0v (edit)

pin 2 4.93v - joy1 up 0v
pin 3 4.93v - joy1 left 0v
pin 4 4.93v - joy1 down 0v
pin 5 4.93v - joy1 right 0v (edit)

all inputs look good going into u975. ordering 74HCT166 from https://bit.ly/2DKab8j

« Last Edit: November 17, 2018, 09:28:08 PM by Toast|FPV »
Past is prologue -=A1000|A500|A4000|A1200]=-