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AppIe IIGS logic board troubleshooting

groink

Experienced Member
Joined
Mar 3, 2014
Messages
171
First off, are there any step-by-step guides to troubleshooting a IIGS logic board in a somewhat dead state?

I have a logic board, ROM01, part number is 607-0713D.

With no cards installed and no keyboard connected, I power it on and it displays the checkerboard pattern, with white and black boxes. There is also no beep out of the speaker.

I have a multimeter, logic probe, and oscilloscope. I have some knowledge of 8-bit CPU troubleshooting, but I'm very unfamiliar with the IIGS logic board.

Voltages are all there. No short-to-ground on anything detected.

Probing the 65816 CPU, I do detect a RESET signal on pin 40. With the probe on pin 40, I turn the logic board on, and for a short time it blinks LO, and then it switches to HI and it stays there. There's also clock on pin 37 - I didn't measure the frequency of this signal, but I presume it is getting a clock signal.

I also probed address lines A12 thru A15, with A12 fluttering extremely fast, while A15 flutters the slowest of these address pins. I also probed all the data lines D0 thru D7, and they all appear to have digital signals running through them. So it appears that I have a good CPU, the CPU is resetting fine on power-up, it is trying to access address lines, and it appears that there is data going across the data lines.

At this point, does it appear to be bad RAM? There are different sets of RAM on this logic board - standard RAM and fast RAM. And, because of the current state of the logic board, I can't run any diagnostics.

Thank you!
 
Logic board is actually in pristine clean condition. I replaced the battery two years ago.
 
And you are positive the power supply is putting out correct voltages? What about when its plugged in and powered on?
 
I tested the power supply using an adapter I made to power up a 3.5-inch IDE drive, in order to give it a load. I also tested it while it was connected to the logic board.
 
black and white boxes is something youd see on a mac not s iigs so i have never seen that one. you have made sure each socketed ic and socket is cleaned as well? can you send photos of the screen?
 
I'm targeting the standard RAM right now. I desoldered all four chips and installed sockets. I tested all four sockets and made sure everything checks out in accordance with the schematic diagram I pulled down from AppleFritter. I re-arranged the four modules in different arrangements, and each arrangement produces different patterns. In one case, it produced @ and "p" characters, so I think I'm on the right track as this board can at least generate characters. I've ordered new memory from Jameco, so they should be here in a few days. In the meantime, I have a TL866II programmer, so I'll pull the ROM and compare it with a known good binary file dump. As for sockets, this board really doesn't have much socketed. I've already checked the sockets for the CPU and ROM.
 
Wow, reading the ROM in an EEPROM programmer isn't easy. Does anyone have the pinouts to make a 531000 ROM adapter so that I can use the programmer to read it as if it is as a 27C1001?
 
I wouldn't worry about the ROM, its a mask chip and usually doesn't go bad.

FWIW, I get the checkerboard on my machine when my Transwarp GS is having issues. One time it was a bad clock crystal oscillator on the TWGS of all things.
 
Finally found some time to troubleshoot further on this board. Apparently, A2 on ABUS is dead. All the other address lines check out okay for the most part. I did a continuity, short-to-ground and resistance check (0.2 ohms) between A2 pin 11 on the 65c816 and AD2 pin 41 on the FPI, and both check out fine, so don't think it is a break or corrosion. I'm presuming this is an input-only line, from the FPI to the CPU, so it may be the FPI that is bad. Even if the CPU is bad, I'd at least expect a signal from AD2. It seems that the FPI handles memory management between the CPU and the fast memory (UH13 thru UH16.) So I checked the resistance between RAx on the FPI and UH13 thru UH16, and they are all reading 100 ohms as indicated on the schematic diagram. To rescue this board, I'll more than likely need a parts board to get the FPI. Or if someone has the FPI available for sale, the part I need is 344S0050-A.
 
The 65C816 A2 signal, pin 11 is the address bus output from the CPU. If this is not working correctly your CPU chip is broken.
 
The 65C816 A2 signal, pin 11 is the address bus output from the CPU. If this is not working correctly your CPU chip is broken.

Ah, A2 is an output pin? I guess it is the CPU. I'll have to purchase one.
 
Board has been repaired! Yes, it was the CPU. Good learning experience!
 
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