Thanks for your reply!
The problem I have is that some of the RAM (8chips) are soldered onto the logic board. I probably should have mentioned that I have tried with the RAM daughterboard removed and I don't get the checkerboard, just fine horizontal lines.
I can rework the RAM if required, I just want to be damn sure it's that before breaking out the hot air gun and buying chips. Based on the RAM chips I think it is 1024KB of onboard RAM.
I think I'd better break out the logic analyser and scope, see what the RAM and address/data lines are doing too. I just need to figure out a way to get it easily accessible as the power lead from the analog board is short, so getting to where I need is hard, even with the logic board out of the machine.
Ok guys update, was able to fix the LC 520 Board and work like a chap...put it in on the Color Classic, Speaker work, everything works...now, original CC board recap, but still no chine...the unit boot and with headphones you can hear it...sometimes after shutdown, will no boot back...any idea on that board what can be the issue.
Hi All, and thanks for having me here.
It seems I'm yet another person with a Mac Classic that is dead.
It was working fine up to about 6 months ago and would happily boot from its internal hard disk. I put it to one side as I had other projects on the go, as I'm mainly to be found on ST forums as a mod and a repairer, but despite their similarities, my knowledge of the Mac Classic is limited.
I have done a lot of research though, and spent a lot of time trying to get a handle on this problem, which is this screen at boot time:
No bong or sad Mac, just the checkerboard.
So far this is what I've done.
1. Removed the 7 capacitors that are known to leak on the logic board, and replaced with tantalum equivalents. Note the picture of the mobo shows C5 and C9 missing, these were taken out to troubleshoot a non-existent short, but have been subsequently refitted.
2. Cleaned the area around the caps with IPA and thoroughly washed the logic board in soapy water and dried for 48 Hours.
3. Replaced the leaking PSU capacitors on the analog board.
4. Possibly killed the ROM. It has a 42 pin socket, and for 40 pin ROMs should be plugged in starting at pin 2. I mistakenly plugged into pin 1 after cleaning the pins on the ROM, and it got rather warm.
I should say that despite the leaky caps on the analog board, none appears to have got down to the logic board, and neither has the battery leaked. A very small amount of electrolyte was present around the 7 electrolytics, but no corrosion was visible and it has been thoroughly cleaned.
Here is the logic board:
Whilst I suspect the ROM because of my hamfistedness, I'm not sure it's bad, as I tried a ROM emulator today, both with normal and byteswapped images just in case, but I get the same image on screen. I also get the same image on screen with no ROM installed, so I think this is something more fundamental than the ROM. Also, the /CE signal at the ROM is high, suggesting it's not active. I am wondering if RAM is the problem.
I'd even be inclined to say that it isn't even getting to the stage where the ROM is accessed. CPU has a clock of just under 8Mhz (7.834Mhz to be exact) and the /HALT & /RESET lines are high. Checking online the CLK speed should be 7.8336Mhz, so that's 100% fine.
I've been over the PCB with my microscope, and can't see any shorts or corrosion and I don't see any corrosion on the underside either.
I should also note that after changing the PSU caps on the analog board, voltages were low under load, so 4.77v and 11.6v respectively, so I have tweaked and now get a stable 5v and 12.2v. The HDD spins up at power on too.
Having done everything suggested that I can find on the web, I'm a bit stuck as to what's next. The only things I haven't yet done are to re-flow (re-solder!) the power lead at the analog board and change the optocoupler on the analog board, and I have some of those on the way, but other than that, I'm at a bit of a loss.
If there are any other possibilities and suggestions out there, I'd be glad to hear them as I want to get this machine back up and running. The trouble is, unlike with the STs, where I have experience, and diagnostic tools (Plus I own a stack of them), I know very little about the Mac Classic, other than the schematics I have.
Thanks in advance!