Mac Classic PRAM battery carnage

[GnatGoSplat]

I bought a broken Mac Classic for a Mini ITX project. I plugged it in just to see what it would do. Blank screen, DOA. I could hear something knocking around inside. What could it be?

Oh, I see what it was, a really nasty PRAM battery! Its juice had dissolved the entire holder off of the board and it also did some major corrosion damage.

Yuck! What a mess!

I cleaned the board, many vias were corroded out as well as most of the traces near them. A diode and resistor were missing, corroded off. Another resistor hanging by a thread fell off and got lost in the sink during cleaning. After a thorough cleaning of the board, I plugged it back in and the Mac powers up! But... no sound, just a checkerboard pattern.

I hadn't intended to save this Mac, but it looked like it might be fun to try. What better thing to do with a free afternoon? Looks like all the corroded vias connected the chip labeled "VIA" to the CPU, and also connected CPU to the ROM chip. I've read an inability to read the ROM results in the checkerboard, so that could very well be the cause. They will need to be repaired.

I didn't attempt to replace the missing resistors on the bottom... don't know what values they are, and I think they are related to the serial port circuits. The missing diode appears to be related to the PRAM battery. Should still boot without them, I think.

Checked all the repaired vias/traces with my multimeter to make sure they're soldered well enough and no shorts. All checks out A-OK.

Turned on the Mac and it beeped! Shows a disk icon with the question mark! HDD sounds like someone is using an angle grinder to kill a screaming cat. I guess it's shot. I try Command+Option+X+O to boot into ROM.

FUUU!

Oh well, I tried! Missed a trace or bad via maybe?

So looks like this Classic has:

- Bad motherboard (has bad caps too)

- HDD shot

- Analog board has vertical hold problem until it warms up (bad cap?)

Looks like it's a good candidate for a Mini ITX project after all.

 

[chuzzum]

That was an exciting story! Too bad it didn't have a happy ending, but at least we got to see the gory details.

 

[LCGuy]

That battery looks like a Maxell battery. There's currently a thread in 68k LC and Performa discussing these batteries. Very interesting.

 

[GnatGoSplat]

Yes indeed, it was a Maxell battery. I thought the carnage here was bad, but the pics in that thread look much worse!

Just a slight update... the HDD now intermittently works. It's still loud as an angle grinder, but the screeching/screaming sounds have quit. I guess the bearings have settled. Every time it boots when the HDD works, I get this error:

Is this a control panel setting to enable AppleTalk, or does the system automatically detect it? If the latter, I'm thinking this may be related to those missing resistors on the underside of the board.

However, it does boot into the Finder.

I have no ADB mouse, so I'm not able to do anything in it though since it seems early Mac OS doesn't support very many keyboard shortcuts. I accidentally put a bunch of extra folders on the desktop trying to find keyboard shortcuts. I believe it has System 7 installed on the HDD, but not completely certain as I can't get into the About window using the keyboard.

When the HDD does work and I do Command+Option+X+O to get into System 6 in ROM, now I get this error instead.

I've read this can be due to AppleTalk, so maybe this is also related to those missing resistors? Anyone know the resistor values for R88 and R89?

I do still get the ID = 02 error IF the HDD isn't detected and if the HDD is disconnected, so there's definitely something still not right. I've double-checked the traces and they seem to be okay, but one possibility is it looks like the Classic has a multi-layer board. Granted it seems a fairly simple multi-layer board, even shining bright light, I can't see that there are complex traces in the middle layer, but there is definitely at least a ground and also some voltage traces in that middle layer. It's possible one of those vias that I repaired needs to be grounded or tied high. Bad caps is also a possible culprit of the instability since bad caps make computers do weird things, although I haven't heard of these particular problems associates with bad caps.

Is there a motherboard schematic out there somewhere?

I doubt this motherboard would ever be stable, but it's fun to tinker!

 

[uniserver]

This post is great... its like you are bringing it back from the dead

 

[techknight]

BOMARC has a catalog of Apple schematics, but since they went out of business, nobody can obtain the catalogs anymore.