exxos's Atari, Amiga & retro forum

I've been the owner of several Macs over the years. A couple of Macbooks (G3 & G4), a couple of Performas (I still have one and it runs just fine) and a 17" Powerbook G4, which I bought for £40 as spares or repairs, and had it fixed and running OSX within a day of it arriving.

The other machine I've owned for quite a while is a Mac Classic. Up until around 6 months ago, it was running ok, but I recently powered on and it's not so well any more. Since its a retro machine and 68K based, I thought that I would pull it out of my more general blog and into one all of it's own, for hopefully better coverage and assistance.

The symptom is that when the machine is powered on, I get a checkerboard pattern on screen. It also has a wobble/ripple to it, but that does seem to stabilise after warming up a bit. Removing the RAM daughterboard gives a different pattern on screen of close horizontal bars.

Aside from joining 68kmla and watching the entire Mac Repair-a-thon series on Adrians Digital Basement, here's what I have done up to now:

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.
2. Cleaned the area around the caps with IPA and thoroughly washed the mainboard 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 and it got rather warm.

Here is the logic board:
And you can see that there appears to be no corrosion visible around any of the chips. I have checked very close up with a microscope. Luckily the battery didn't leak, and if the logic board caps did leak it wasn't much at all. Although the analog board caps did leak, it wasn't much and was restricted to around the capacitor bases.

The current situation is that I am working to replace the ROM (Possibly with one of @cmorley's ROM emulators. That's a task for this weekend.

I still get the checkerboard display, and with the logic board connected, the voltage is dropping from 5v to 4.7v, which is below the threshold for the machine to run from what I've been told. I can adjust this, so will do so under load to see if it makes a difference, but I am also going to replace the PSU optocoupler as a thread on 68kmla suggested it's common for them to fail.

I'm grateful so far for everybodys suggestions over on my Stuff and Things thread, but I thought it time it had a thread of it's own.

Any help and suggestions gratefully received as these things are new to me.

First stage of getting a temporary ROM setup until I have a replacement.

Step 1, bodge a USB connector onto the ROM emulator:
Step 2, figure out how I get the rom image installed.
Positive start though!

Looks good

Tom

Tomswork wrote: ↑

Sat Aug 01, 2020 9:07 pm

Looks good

Tom

I will program a byteswapped image as well in case I need to flip it over as well.

Testing tomorrow

@rubber_jonnie

Just looking carefully at your board pic capacitor c3 is a small one with a trace above it that may need further inspection.

Tom

Tomswork wrote: ↑

Sun Aug 02, 2020 9:09 am

@rubber_jonnie

Just looking carefully at your board pic capacitor c3 is a small one with a trace above it that may need further inspection.

Tom

Thanks, I'll double check it, but fairly sure I did after soldering in the tantalum replacement and when I was checking for shorts.

@Tomswork I checked C3 and it was fine,it's just my messy soldering on quite a small component. +tive and -gative connect to where they should elsewhere, and there are no shorts. Whilst I was in the area I noticed one side of C4 didn't look right, and it hadn't soldered correctly on one end, so that is fixed, and no shorts there either, though I am going to check the schematic as I haven't been able to trace +tive and -gative for C4 for some reason. I only see .5v across C4 too, whereas the rest I see 5v, -5v and 12V.

Below you can see the checkerboard when the RAM daughterboard is plugged in.
No bong or sad Mac,just the checkerboard.

I'm not sure the ROM is bad, as I tried the 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.

I'd even be inclined to say that it isn't even getting to the stage where the ROM is accessed, though on checking /CE it was high, and it should be active low, so I suspect it isn't even getting to the accessing the ROM stage. 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 again, and can't see any shorts or corrosion and I don't see any corrosion on the underside either.

Off to the internet to look for more info!!

I wonder if there is a problem under socket for the memory board. Another thought if your looking at the schematics take a look at the diodes they do turn into resistor sometime rather than blowing or going closed

@rubber_jonnie


Edit on the case side how is the contector hat connacts the board

Tom

Tomswork wrote: ↑

Sun Aug 02, 2020 7:37 pm

I wonder if there is a problem under socket for the memory board. Another thought if your looking at the schematics take a look at the diodes they do turn into resistor sometime rather than blowing or going closed

@rubber_jonnie


Edit on the case side how is the contector hat connacts the board

Tom

Connector looks fine, but when I replace the opto coupler I am going to check the other end.

Can't say for the diodes but will have a look.

@rubber_jonnie

Spent the day with a Macintosh expert he said to measure the 5v rail at the floppy port if its down around 4.55 your psu is not working correctly or there is a short
This is consistent with the memory card and the pattern
Also he said to check all connectors for cracks or tears
aways from previous openngs

Tom