Classic II Logic Board +8V rail

[Jamieson]

Just received a Classic II and started to clean the logic board. The Maxell PRAM battery has not yet bombed, but the cap juice has claimed one victim at least. U2 fell off the board with the lightest touch. I found a schematic for the Classic II and concluded that U2 is a linear reg to generate +8V from the +12V rail. It's a little 8 pin SOIC, so there probably isn't much current draw from the +8V rail, just curious where else it goes. (That "Bomarc" schematic is, well, frankly, god-awful.)

 

[Jamieson]

The only schematic I found was for the Rev-B logic board. I've got the Rev-A board (with 4 EPROMs). Most of the resistors and capacitors match up though, at least in the areas of the board I'm looking at.

Anyway, the +8V rail goes to:

U10, MCU/EGRET -- is the micro that handles the ADB keyboard/mouse?
U12, LG1170. Maybe PRAM related since this chip also has the battery connection
U4 labeled "D.F.A.C." maybe sound related

The pic shows U2, an 8 pin SOIC. All 8 pads are GONE. There are vias pretty close by pin 8 (VIN) and pin 1 (VOUT) and some grounds nearby for pins 2,3,6,7. Pins 4 and 5 are no connect. So a new SOIC-8 could be put in there with a few short jumper wires. Or I could find the TO92 three pin version of the 78L08 and find a few pins on tantalum caps to tie into.

 

[SuperSVGA]

Are you sure the pads are gone? It's hard to make out in the picture, but it looks like they might be covered with oxidation or something, similar to the legs of nearby components. I've had similar happen a few times and the pads were mostly fine underneath.

 

[mg.man]

Are you sure the pads are gone?

^ what he said. Do you have any solder wick? If so, try dipping the wick in flux and "scrubbing" the pads with your iron pushing the wick down. Alternatively, you can try gently scubbing them with a glass fiber scratch pen - like one of these:

Amazon.com

The above is just an example, there are sets with different material bristles. You can also get refills. FWIW, these are also GREAT on edge connectors if you're into old PCs or Apple II stuff... I've even very very carefully used one to clean the pins of TTL chips... obviously, YMMV!!

 

[Jamieson]

I checked and unfortunately the pads are gone. There are eight small craters where the pads where used to be.

 

[mg.man]

the pads are gone

Bummer. Should be repairable. Good luck!

 

[Jamieson]

Well I'll be darned, the pads ARE still there! It took a ridiculous amount of IPA, flux, and scrubbing with solder wick to get through the crud, but most of the pads have come back shiny. The +8V regulator is chip is toast, but a new part is on the way. BTW the +8V is only used by the D.F.A.C. sound chip.

There is an error in the Classic II logic board schematic. The +8V power symbol is shown going to the EGRET U10 and battery monitor U12, this is not correct. Those pins are for an alternate +5V rail.

 

[mg.man]

Well I'll be darned, the pads ARE still there!

Great! It did look from the photo that they would be, but you're the one with eyeballs on... I would still invest in one of those pens - a quick "scratch" would likely have given you hope.

Good spot on the schema's... and good luck getting going again.

 

[joshc]

With dull pads like that, I scrape the pads with a surgical/scalpel blade such as a Swann-Morton, it will get the worst crap off, and then the remaining can be done with flux/fresh solder that you then wick away.

 

[Jamieson]

Installed a new 8V linear regulator U2. This chip supplies +8V that goes ONLY to the DFAC chip. Now sound has returned to my Classic II. Since it doesn't look like I can edit old posts, I want to stress again that the schematic has a mistake. The +8V power does NOT go to EGRET (U10) or the reset/battery chip (U12).

 

[nukem]

Just a clarification:

Pin 1 of U12 is 5V when the power is ON and then switches to the battery voltage 3V when the Mac is off. They should have used a different symbol on it.

 

[shaker68]

Hi, I also just revived a classic II that was soaked in cap juice. It now seems to be working as it should except it doesn't save changes to pram or keep time when shut off. I checked that battery voltage is present at pin 7 of U12. Is this IC the PRAM/ clock or just the voltage controller for the PRAM/clock. Thanks