Classic checkerboard screen

[donbroc]

Some time ago I bought a Macintosh Classic. I checked online and people said to recap a non-functional logic/analogue board, so I cleaned and recapped the analogue and logic boards fully.

"Okay" I thought, "maybe I should turn it on." So I did. what I got was what I'd describe as a flickering pulsating checkerboard pattern with weird sounds coming from the speakers. I checked online and a post here said to replace the Optocoupler and some diodes on the analogue board.

So I did.

It stabilized the Image and got rid of the flickering and speaker problems, but that still left me with a checkerboard pattern. Did some more snooping around here and a lot of posts said that the checkerboard pattern tends to be a RAM issue, due to the memory mapped display these old Macs have.

So I bought a multi-meter and went to town on the ram chips. All of them seemed to be fine when doing a continuity test and checking for short circuits.

I also have thoroughly cleaned both boards with alcohol after removing the original caps and re-seated the ROM chip.

I've tried searching extensively across the web for info on this topic but most of the info I could find was relating to the Classic II.

I wanted to know what kind of issues a checkerboard pattern could indicate, just so that I can eliminate some procedures from the list. Any help would be appreciated, even if its just a link to an old service manual or a PDF of a more up to equivalent of the dead mac scrolls.

 

[dochilli]

Did you measure the 12 & 5 V voltages at the floppy port? 5 V should be higher than 4.85V for booting a classic.

 

[donbroc]

Yeah, the 12 & 5 pins are fine. I did notice however that there is some green corrosion on the cable which gives power to the logic board so I will see if I can clean that up.

 

[dochilli]

Did you reseat the ROM in the right way? There must be two pins free at one side of the rom holder.
A checkerboard can be a result of a faulty rom.

 

[donbroc]

Yes, the ROM is the right orientation with the pins in the correct place. I do not know if the ROM chip itself is faulty though as I haven't got any other parts to test with and parts are quite hard to come by where I am.

 

[donbroc]

I don't know if it helps or not, but the checkerboard pattern wasn't entirely consistent. By that I mean its as if some pixels on the screen were the opposite colour. Here's a picture of it:

 

[donbroc]

after doing some digging online I think it may still be a ROM issue. Another thing that happened is when I left it on for a bit most of the chips were at an acceptable temperature, including the ROM chip, but the CPU was very hot.

About the ROM chip, in case it is the problem, I just wanted to know what type of eprom I would need to buy. Most of the ones I've seen have said either 512 x 8 or 256 x 16. I was wondering what kind the Macintosh Classic uses. Given that the ROM file is 512kb, I'd assume it was the 512 x 8 but I'd like to hear a more experienced opinion on what kind of eprom to buy if it comes to me buying a new ROM chip.

 

[bibilit]

Can be, but to my knowledge they don't go bad easily, so chances are thin a replacement will cure your issue.

 

[dochilli]

 

[dochilli]

Macintosh Classic compatible EPROM

What EPROM parts are compatible with the Macintosh Classic logic board?

68kmla.org