Resurecting a battery damaged Macintosh Classic Logic Board

[davidg5678]

I recently purchased a battery leak damaged Macintosh Classic, and to no surprise, upon opening the machine, I discovered that the battery had leaked and corroded the inside of the computer. As a challenge, I am attempting to repair the broken motherboard and get the computer working again.

Aside from a damaged motherboard and metal chassis, the computer was in fairly good shape and cleaned up nicely. I was able to sand down and repaint the chassis, so it is no longer an issue. The logic board of the macintosh was initially covered in lots of residue from the leaky Maxell battery, but I have since cleaned it off. Many components were so corroded, they fell right off of the PCB and I had to throw them out.

(The forum's image uploading seems to be down at the moment, so here are some links to pictures.)

(Before cleaning)

https://imgur.com/DSoSj8c

https://imgur.com/WViqnj2

(After cleaning)

https://imgur.com/9lbCmOl

I desoldered every component in the vicinity of the battery and discarded everything that was beyond recovery. I also soaked several of the worst looking chips in vinegar, which removed the corrosion from their legs. Without components in the way, it was easy enough to clean off the PCB.

Now that the board is clean, I now need to figure out how to repair it. --Assuming it's possible of course.

 The issue I have is finding the right parts to buy, but as of now the following components are missing or damaged:

• Headphone Jack
  
• Programmer's and Reset Switches
  
• SCSI Port
  
• Battery Holder
  
• L1 (Inductor?)
  
• Y1 Crystal
  
• J2 Serial Port
  
• D3 (Diode?)
  

If anyone has suggestions about fixing a logic board as damaged as this one, I'd be very interested to hear them!

 

[archer174]

That's some dedication. I personally would call this one dead, but here is a collection of schematics:

https://archive.org/details/Macintosh68kSchematics

By the time you locate the parts/pay for shipping you may be better off finding another classic board. 

 

[BadGoldEagle]

How is the RTC doing?

It’s an Apple part so you’ll never find a replacement unless you get one from another Mac...

If it’s doing fine, then you can go trace hunting. Oscillators and diodes should be referenced in the schematic.

 

[davidg5678]

How is the RTC doing?

Luckily, the RTC looks to be in reusable condition, but I suppose I won't know if it still works until after I put the motherboard back together. I was able to remove most of the corrosion from the pins of the ICs by soaking them in vinegar for a few days. I'm concerned that there will not be enough remaining metal that I can still solder to, but I won't know until I have tried.

 

[LaPorta]

I had a beast of a time getting my Classic board to tell time, and that was with only a little bit of cap goo under a diode. I understand the drive: there are only so many of these things left! How about some photos of the project so we can better assist?

Having just gone through this, D3 is part of the RTC clock circuit...so you won't be able to see if RTC works unless the D3 is in place and functioning.

 

[PotatoFi]

Your efforts here are admirable. Best of luck to you!

 

[jjclay]

Wow - dedication indeed! How are you getting on? I ask, because I took my Classic out of storage yesterday to recap the board, only to discover a similar scene since I accidentally left the Maxell time bomb on the board (my rule, which I must have missed, is to ALWAYS remove the PRAM battery from an old Mac...). Have pics and will share once image upload is working again on the forum. Going to drill out and re-tin the via, correct any missing traces I find, recap, and cross fingers. Don't have high hopes of fixing mine, but will give it a try. If it doesn't work I like the idea of ripping the guts out and inserting a Mac mini and colour screen, as done here: https://www.youtube.com/watch?v=nguN392TH-g

 

[davidg5678]

 I have not done any additional work on the motherboard since my last post, but I have been doing lots of research about how to fix the computer and what I need to buy.

Despite the battery leaking acid all over the board, it looks like all of the traces are still fairly intact. Unfortunately, the condition of the pads on the board for many of the chips is quite poor, so I will have to run a lot of very thin wires from the traces directly to component legs.

After re-reading @LaPorta's SE FDHD Restoration thread, I decided to purchase a very small drill bit set (the smallest bit is .5mm) so that I can attempt to drill out some of the destroyed vias on the board. Most through-hole components had become so corroded that I was completely unable to get the solder for them to melt, no matter how much flux I used. In theory, I should be able to drill out the corroded pieces of legs and proceed with installing replacements.

Having just gone through this, D3 is part of the RTC clock circuit...so you won't be able to see if RTC works unless the D3 is in place and functioning.

It looks like the diode D3 is completely destroyed (the metal contacts were completely eaten through). Does anyone know the part number for a replacement? I also still need to find replacement parts for the programmer/reset switches and the headphone jack.

Here are some better quality photos of the motherboard: (It looks like the forum's image uploader is working again!)

Thanks for your help! :smiley:

 

[LaPorta]

Let me look this over later, and I’ll try to help. As for the corroded legs, the only solution I’ve found is to get some flat-edge cutters and snip off all the legs and remove the chip. A replacement is of course needed. Once you do that, I keep the iron hot (700 F), and make contact with the now-exposed pin, which should transmit heat inside and melt the solder from the inside out. Tricky and takes patience, but seems to work.

Ill see if I can get the D3 rating for you.

 

[LaPorta]

Wow, I was lucky...that thing was flat-out decimated by that battery. That looks like a huge project to make sure all of that is intact. Do you need pictures of a good board for comparison?

 

[davidg5678]

Do you need pictures of a good board for comparison?

Sure! Those could certainly come in handy.

Wow, I was lucky...that thing was flat-out decimated by that battery. 

Hopefully, I'll be able to identify and fix everything that is broken on the left side of the board, but the only way to know whether or not I can fix this machine is by trying. -Either way, it will be a fun endeavor.

When you made the 14 pin extension cable for your SE FDHD project, did you need to buy crimping connectors along with the Molex socket? I was able to find this part on Digikey, but I wasn't sure whether or not to order 28 of these contacts too.

 

[LaPorta]

I got the socket, crimp pins, and crimping tool from a local electronics store. I’m actually traveling but I can tell you what the pins are when I get home on Thursday. I’ll also get you photos of a good board I have.

 

[davidg5678]

Does anyone know what a suitable replacement for the D3 (diode?) on this logicboard would be?

Thanks!

 

[davidg5678]

Does anyone know what a suitable replacement for the D3 (diode?) on this logicboard would be?

While looking at a spare SE/30 motherboard, I noticed a similar-looking diode which was also labeled D3. In addition to this, it was located near both the SCSI connector and a fuse in the same way the diode on the Mac Classic was.

I was not able to find a part number for the diode in the Classic's schematics, but the SE/30 schematics list D3 as a 1N4001 Diode. The circuits seem to be slightly different from each other, but I am hoping that they are similar enough to both use the same part.

My plan is to try using a 1N4001 as a replacement for the Classic. I am not 100% certain it is the correct component, but with any luck, it should at least be close enough to work.

 

[BadGoldEagle]

Sorry for the late reply. 

D3 is indeed not listed on the schematics but there are a few instances of DP3 and DC3, which are also diodes, and more importantly they're of the same type: 1N4148.



I'd go with this one instead of the 1N4001, but I'm no electrical engineer. 

 

[cheesestraws]

Does it look like it's doing a power-related job or a signal-related job?

 

[davidg5678]

D3 is indeed not listed on the schematics but there are a few instances of DP3 and DC3, which are also diodes, and more importantly they're of the same type: 1N4148.

Does it look like it's doing a power-related job or a signal-related job?

Thank you both for your responses!

After further examining the schematics, I was able to find the 1N4148 diodes DP3 and DC3; however, these diodes are located in the computer's power supply. The diode that I am looking to replace is on the main logic board, next to the SCSI port, so I think these may be unrelated components.

The 1N4001 diode located near the SCSI connector on the SE/30 board looks very similar to the diode located near the SCSI connector on the Classic board. Because the SE/30 schematic identifies the part as a 1N4001 diode, I still think that it is a more likely match.

I'm not sure whether the diode deals with power or signal, but I found a picture of the section of the schematic with the diode I am trying to replace. It looks like it is tied into the SCSI bus on pins 25 and 26 as well as ground.

 

[Bolle]

It’s there to supply termination power to the SCSI bus. A 1N4001 is the correct choice here.

 

[cheesestraws]

25 and 26 are termination power, so a 1N4001 is probably a better choice being a power diode.

edit: aaand I was beaten to it.  Oh well!