Macintosh II Restoration

[SlateBlue]

Today I received a Macintosh II in the mail. It was gifted to me, and the previous owner stated that it didn't boot up. Knowing that this is likely attributed to dead PRAM batteries, I thought I might be able to revive it and have another toy to add to the collection. Unfortunately, as I got to inspecting the logic board, I realized I had much more work on my hands.

It's a given that these old Macs fall victim to leaky caps. Indeed, every SMD cap on the logic board was leaky. Throwing that aside, I wanted to investigate the battery situation. Both batteries had been removed from the board before I ever took ownership. While this isn't exactly a big deal, what I did find was a damaged via and lifted solder pad. Someone was pretty careless in removing the old batteries.

To remedy this, I took some 30 gauge wire, stripped it back, and then inserted it through the damaged via before adding a bit of solder to hold it in place; I ended up doing this for all four connections. I think I can salvage this mess with a couple of SMD battery holders. I'll adhere them to the board with some double-sided tape, and solder the small gauge wire to the respective terminals on each battery holder. I've also removed all the SMD caps in prep for replacement. The axials are still in place currently, but I will be removing and replacing those as well.

Compared to the capacitor list contained on the 68kmla site, there is a bit of a deviation with my board. Instead of 12 x 47uF 16v caps, my board has 13. And, I also have a 10uF 16v axial cap next to the fourth NuBus slot.

I'll post a couple of pics to show the current rework, and will update this thread as time permits. Hopefully, I'll be able to revive this old Mac II.

 

[SlateBlue]

With the board recapped and new PRAM batteries installed, the Mac II will still not attempt to power on. Using a multimeter, I probed all the power supply pins. According to this web article, pin 1 is +12v, 2 through 6 are +5v, 7 through 12 are Ground. All of the positive voltage pins read 0V. It appears the power supply is dead.

So.... anyone have a spare Macintosh II power supply?

 

[nyef]

Having my own II-series startup issues (IIfx in my case), I have to ask... have you tried "jumpstarting" the PSU with a separate +5v supply?  That's step one, and should tell you if you need to focus on the PSU or the mainboard.

 

[CC_333]

I might have one. I don't know if it works, though.

I'll check it out and let you know.

c

 

[nyef]

Three or four AAA batteries in a holder plus some wire might also do the trick (you'll be at 4.5v or 6v, I don't know how close you need to get to 5v here). Or if you have another machine that uses 5v (say, a PC?) you could bridge the grounds and then tap the 5v from that for a second. Or wire up a 9v to a 7805, or strip one end from a USB cable, or... Anyway, a 5v supply is easy to come by these days.

 

[SlateBlue]

Having my own II-series startup issues (IIfx in my case), I have to ask... have you tried "jumpstarting" the PSU with a separate +5v supply?  That's step one, and should tell you if you need to focus on the PSU or the mainboard.

I have not. Truthfully, I don’t know what you mean by jump starting the PSU. Why wouldn’t there be voltage present when plugged into an outlet? I would think there would be 12v or 5v standing.

And, last question, do the PRAM batteries have anything to do with this? I know without them, the Mac won’t power on. I installed two fresh 3.6 batteries into the newly installed holders and it wouldn’t attempt to start up. Thanks for the help.  

 

[nyef]

The Mac-II era startup circuit is weird. There's no standing voltage from the PSU, but if you supply +5v (apparently +3.6v is enough?) to one of the pins it powers up.  If you then ground that pin, it powers down.  The reason why the II has two batteries when one is sufficient to hold the PRAM state and clock is that the second brings the overall voltage up to about 7.2v, and then there's a drop over a couple of diodes to get about 7v.  That's enough to charge a fairly chunky capacitor plus run a flip-flop attached to the power button (and ADB lines) to detect a power-up request. If there's not enough oomph in the batteries or if something else is wrong with the circuit, then it won't power up...  But if you have an external supply you can supply the startup current to the PSU anyway.

Video of jump-starting a Mac II

Detailed explanation of the power control circuit

 

[SlateBlue]

I haven't looked at the links you provided yet. However, I did jump start the PSU with a single 3.6v battery and the computer did power on. I hear an audible "clunk" or "click" coming from the speaker, but there are no startup or death chimes. I reseated the CPU and MMU. It seems the MMU was not inserted into the socket correctly. Still, neither of these tasks made any difference. I suppose a bad MMU (perhaps destroyed by improper installation) may prevent the Mac from booting up. Anyone know of a way to test it? I should also note that there are no drives connected to the logic board. Does this matter?

At least there are two things I now now:

1. I still have a problem somewhere in my power control circuit (likely thanks to the person who ripped the PRAM batteries out of the board).

2. The power supply is good, but the Mac fails to boot.

Thanks again for the help!

 

[techknight]

well if its full of electrolytics I am sure it needs recapped. 

If it has been recapped, then you have bad traces to work through. 

if it wont turn on with batteries installed you definitely have bad traces. I have several patch wires in my IIx. 

 

[SlateBlue]

I’ve already abided by the first rule of vintage Mac repair: Recap, recap, recap!

All of the caps have been replaced. I don’t disagree that it’s a broken trace, but I can’t find any good schematics for the Mac II board. The one originally from Gamba is a little difficult for me to follow. Anyone have Apple or Bomarc schems for the Mac II logic board?