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tattar8

Advice on repairing Mac IIx

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Hey 68KMLA,

 

I recently got my hands on a Mac IIx, with the understanding that it wouldn't turn on; I assumed this was likely due to the PRAM batteries needing to be replaced since this machine won't turn on without them.  I replaced the PRAM batteries, and recapped the board.  However, I still have no activity when I turn on the machine; no chime, no disk activity, and no video.  There wasn't any significant rotting of traces due to leaking caps, and the batteries hadn't leaked.  Interestingly, once I press the power button to turn the machine on, it won't turn off again until I pull the power cord.  Anything else I should look for as far as things that could go wrong?  Any test points I should probe?

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I replaced all the caps already.  Board still does nothing at power-on, and won't turn off until I pull the power cord.  

 

EDIT:  To clarify, pressing the power button does cause the PSU to turn on, and the fan to start spinning.  Nothing else happens after that.

Edited by tattar8

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Follow-up question:  A bit of common advice I've found on this forum is to stick the board in the dishwasher as part of the troubleshooting steps.  I'm willing to do this, but had some questions on the details since I can't find any comprehensive info on it.  I'm assuming I take off all the RAM, the ROM stick, and the batteries.  Do I remove the capacitors first?  Do I remove all the socketed chips?  Do I run the dishwasher with or without detergent, and with or without heated dry?

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I've had no issues when leaving socketed chips in and then letting it dry sitting at an angle with a fan blowing on it.

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You got broken traces or components somewhere if it turns on and does nothing afterwards. Take a closer look at the startup circuit as that area is likely to go bad from cap goo. The fact that it won’t turn off points into that direction as well.

https://www.downtowndougbrown.com/2015/03/explanation-of-the-macintosh-iiiix-power-onoff-circuit/

 

 

Check if all relevant voltages are present as well just in case and to be sure your PSU is still ok.

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You're right, there is a dead trace near there, one near the RAM (but it goes to the farthest RAM slot so it shouldn't keep the machine from booting), and one between the two smaller DIP chips near the FPU.  I'll try to bypass them when I next have a chance to go to my local makerspace next week.

 

I checked the PSU, all the necessary voltages are measuring as expected: 12v, 5v, 0v, -12v.

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C6 - R3 to R19 continuity has been reported "critical" in IIx startup (R18 in the original II).  If no chime i would also try to reseat chips, memories and rom. 

 

I would also measure resistance between +5 and ground. 

Edited by Macbuk

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I fixed a trace going to the power switch, as well as a trace near the batteries that looked like it was important, though I was having trouble tracing it.  I measured 15 ohms of resistance between +5 and ground.  Is that considered normal?  I'll post detailed photos of the board in a bit.

 

Will the IIx chime without any RAM?

Edited by tattar8

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On 3/25/2019 at 11:58 PM, tattar8 said:

I measured 15 ohms of resistance between +5 and ground.  Is that considered normal?  I'll post detailed photos of the board in a bit.

I do not know if it Is what what apple meant in the original project of the the Macintosh II family but It seems fairly normal.

 

All my - fully functioning - Macintosh IIci, IIfx and my II (all "47 mfd only" recapped) have 16 ohm between +5 and ground.

 

Curiously my LC475 has 160 ohm and my SE30 nearly 170. Do not know why. 

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I fixed a bunch of traces, but now have a new problem -- the Mac now shuts down immediately after being turned on.  I suspect that some wiring around U1 is to blame, but I don't see anything wrong.  Any ideas?

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So I don't know what I did, but it powers up and stays on most of the time now, and the power button can turn it back off.  As I understand it, C7 is responsible for making sure the poweroff circuit doesn't get triggered while I'm trying to power on the machine; its behavior makes me suspicious that C7 should be a higher value?  Would it help if I swapped C7 with a higher capacitance, to help increase the time before the poweroff circuit gets triggered?

 

With this working this much, I think I can move on to the next problem -- there's no signs of life once the machine powers up.  No chimes of any kind, no activity on the floppy drive.  I fixed any broken traces I saw near the ROM SIMM, is there anything else obvious I should look for?

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If you saw broken traces then there will be more broken ones that you can’t see.

 

Also did you repair the traces just by following them or did you take the schematic and actually buzzed them out?

That is what you’re probably going to have to do anyways to get any further.

No chime means you have broken lines between ROM and CPU most of the time.

Edited by Bolle

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I don't have a schematic for a IIx; I have schematics for the II and the SE/30, which I believe is somewhat similar.  I'm just following traces and testing continuity with a multimeter.  Does this board have more than 2 layers?  

 

Turns out there was a broken trace between a capacitor and the sound output of one of the two sound chips near the FPU.  I fixed it and now I have a sad mac.  I'm trying to find my MacII-VGA adapter so I can see if I have an error code on the screen.

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In the meantime, there's a dead trace on the GLUE chip at Pin 14, which according to this schematic is "VIAIRQ1N" which seems like it might be what's causing the Sad Mac.  Am I right in assuming there's no issues with the memory bus since I'm getting a sad mac chime?

 

On a side note, is there no way to edit my own posts here?

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Editing is only possible for a certain time after making a post.

 

Patch the trace up and see if it boots. If not go on looking for more bad ones.

 

Sad chimes also occur when the machine can’t access RAM so it’s still possible there is another issue with RAM. Does the chimes change when you start it without RAM installed vs. when there’s RAM in the sockets?

Death chimes play back slower on some machines when they can’t access RAM so that might help figuring out if it’s memory related or not.

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Update:  I got a startup chime and the machine seems to be booting from its hard drive.  I got a USB-Serial adapter and used the diagnostic mode to determine that the least significant bit of the rightmost RAM stick's databus lines was stuck at 0.  I patched that pin to the matching pin on Bank B and the chimes of death went away.   

 

Next problem is no video.  I don't have a monitor handy (I'll have one next week), but I connected an oscilloscope to the Hsync and Vsync lines and got nothing, and I think that's the fault of the video card since it also has some bad traces and other capacitor damage; a bunch of the pins on the main chip seem to be loose.

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I'm still not able to get any video.  I'm pretty sure the machine is booting, since it chimes and then starts reading from disk.  I tried two different video cards (a Macintosh II High Resolution and a Macintosh II video card), at least one of which is known working.  Is there any way to narrow the problem?  I'm guessing that if there was a NuChip error, the machine would sad mac and not continue booting.  Another thread here had a problem with one of the transcievers (UC7) missing a pin; I had the same problem, and I solved it by scraping at the IC until some metal was exposed and then soldering to it.  Is there any way to narrow the problem?  Perhaps from the diagnostic mode?

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Just a quick update, there was one more bad trace between the NuBus slots and the batteries; after patching it up (that's the 13th patch on this board) I get video out and the machine is working like a charm!  It has a physically enormous hard drive (5.25in half-height, the size of a CD-ROM drive) that still works and has a copy of System 7.1 on it.

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