Apple IIGS - RESET line stack on 0V

3 posts / 0 new
Last post
Offline
Last seen: 1 month 3 weeks ago
Joined: Jan 7 2024 - 08:50
Posts: 2
Apple IIGS - RESET line stack on 0V

Hello,

Weird problem here:I bought an Apple IIGS (in non-working condition), ROM01. The motherboard is in bad shape, the battery has leaked everywhere and damaged the traces.

The previous owner fixed the wiring and replaced some ICs.

As far as I can tell the repaired tracks are OK, I've checked them all.

The computer wouldn't start, so I downloaded the schematic and soon realized that the RESET line was constantly at 0V.This brought me to the ADB Controller (UI14). UI14 should start the system by pulling pin 49. UI14 has its RESET pin (16) connected to the RC circuit - a few seconds after powering up it has 5V on pin (16) - so it starts the ADB controller, but nothing else.I even tried bringing the RESET line to 5V by connecting (via a resistor) to 5V, but it got stuck at 0V.Can somebody help me? Any idea in which direction to check next is welcome.

Note: I thought about desoldering the ADB, but removing small chips from the motherboard is way beyond my soldering skills.

Offline
Last seen: 2 hours 43 min ago
Joined: Feb 27 2021 - 18:59
Posts: 490
look for clocks

A few comments on your troubleshooting:

 

The RC delay on the RESET.L line to the ADB microcontroller charges a 1µF capacitor through a 200kΩ resistor, so it should bring RESET.L high within 200ms, not "a few seconds". You can probe test point TP74 to see this waveform which should look like an upward logarithmic curve when stored in scope memory. The CMOS microcontroller will register RESET.L as "high" (and begin running) when the voltage on that test point passes around 3V.

But the microcontroller still won't do anything unless its clock input is running! Pin 17 (CREF) is driven by pin 79 of the MEGA-II chip, UJ2. And that is divided down by the MEGA-II from its pin 76, 14M, from the VGC UH2's pin 10. And that is divided down by the VGC from its pin 11, 28M, from the crystal oscillator OS1. Every bit of synchronous logic needs to have a clock from somewhere.

It also needs to have power to operate, so the Vcc pin (46) and grounds (15, 20, 45) on the ADB micro should be good.

Offline
Last seen: 1 month 3 weeks ago
Joined: Jan 7 2024 - 08:50
Posts: 2
Thanks, I'll check it up.

Thanks, I'll check it up.

Log in or register to post comments