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JNizzle

Repairing a very damaged Mac Classic

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Hey guys! 

Recently I managed to pick up a Macintosh Classic in very good nick, however, it wasn't working.

The previous owner told be that he was using it one day, and the Mac made a bang and stopped working. He said that the fuse had gone, and that I would simply have to plug in a new fuse.

 

I took it home and tried to power it up again just to see if I was lucky, but sure enough, it was broken. I didn't hear a hard drive spin, no beep, no CRT noises, nothing.

I cracked it open and sure enough, the fuse was missing. I bought an equivalent fuse and plugged it in, I powered it up, but still, nothing happened. Just silence.

There had to be a deeper issue. I completely disassembled the Mac, and took a look at the analogue board and found the true culprit. It seemed a track had completely blown off the board.

 

post-9408-0-31796100-1507016066_thumb.jpg

 

I tested some of the components, mainly the MOSFET and the capacitors to see if they were failing. I noticed some leaky residue underneath the capacitors, so I began re-capping the board. I also noticed a dead-short across the MOSFET, so I scoured digi-key for a replacement. Here's a list of what I replaced:

  • ALL capacitors on the board (I had to buy from two separate brands, Nichon and Saturn)
  • TDA4605 - replaced with TDA4605-3
  • IRFBC40 - replaced with IRFBC40L
  • CNY75GB - replaced with CNY75B

I didn't have any wire, so I bridged that broken track with a lead I had snipped off a capacitor.

 

When I plugged it all back in and tried it again, I was greeted with some jittery checkerboarding. At least it was actually turning on now. But, the CRT was the only thing working at this point, still no beep, still no hard drive noises.

I washed all the sticky capacitor residue off with some isopropyl alcohol and a toothbrush, replaced that battery in the motherboard (I replaced the 3.5v battery with a 3.6v of the same type, does this matter?), and replaced all the capacitors. As my local electronics store didn't stock SMD capacitors, I used through-hole ones that I just soldered to the board.

 

post-9408-0-34354600-1507017182_thumb.jpg

 

However, when I started the Mac, I still had checkerboarding. I noticed that when the RAM module was removed, the checkerboarding turned to straight vertical stripes. Does anyone know what the problem might be?

 

post-9408-0-43825500-1507017302_thumb.jpg

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I tested the voltages, I discovered I'm getting about 4.4v and 11.4v. I adjusted the potentiometer, and I can nail 12v, but I can't get 5v. It just seems to top out at 4.6v and I can't get it any higher. Thoughts?

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Did you check that the pins between the MOSFET and the other components have a connection? Maybe it burned up traces further behind, so check the surrounding components.

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I have the same problem :

 

https://68kmla.org/forums/index.php?/topic/31245-low-voltage-classic/

 

You won't get rid of the checkerboard pattern until your 5 volt line will be close from that figure, as soon as the figure will be around 4.8 or close, your screen will get the usual grey and the logic board will chime.

 

The 12 volt line is less crucial here.

 

The 5 volt line starts at the Schottky barrier MBR 1045 (DP6) and capacitors are CP6/CP7 and CP 36

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Disconnect J4 and clean all contacts. Perhaps you could resolder the pins. You could do the same for J1.

R56 could be defect. Turn it right and left sometimes when the Classic is off.

Test voltages again. I think, turning it to the left increases the voltages.

Edited by dochilli

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I tested the voltages, I discovered I'm getting about 4.4v and 11.4v. I adjusted the potentiometer, and I can nail 12v, but I can't get 5v. It just seems to top out at 4.6v and I can't get it any higher. Thoughts?

 

I think your problem is on the regulation side, CNY75 being new... you can rule this one out.

 

regulation is taken from the 5 volt line on the secondary side, you can replace the following parts DP3 (which is a 1N4148 diode) DP11 which is a Zener Diode 5,6 volt (BZX79C5V6) and if that doesn't work, the DP14 (LM431) the zener schunt regulator.

 

Haven't had a look at mine, but probably will look at these parts first.

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I think your problem is on the regulation side, CNY75 being new... you can rule this one out.

 

regulation is taken from the 5 volt line on the secondary side, you can replace the following parts DP3 (which is a 1N4148 diode) DP11 which is a Zener Diode 5,6 volt (BZX79C5V6) and if that doesn't work, the DP14 (LM431) the zener schunt regulator.

 

Haven't had a look at mine, but probably will look at these parts first.

 

Thank you so much for your help, I'll definitely try this when my exams are over!

I met someone who will lend me a Mac Classic in perfect working order, so I'll be able to test most of my components in there and hopefully rule out some issues.

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(...) you can replace the following parts DP3 (which is a 1N4148 diode) DP11 which is a Zener Diode 5,6 volt (BZX79C5V6) and if that doesn't work, the DP14 (LM431) the zener schunt regulator.

 

 

Where do you find the exact part number of these items? It would be nice to know where to look when you want to replace or troubleshoot something ;D

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Thank you so much for your help, I'll definitely try this when my exams are over!

I met someone who will lend me a Mac Classic in perfect working order, so I'll be able to test most of my components in there and hopefully rule out some issues.

 

thats not so good of an idea. This is how you end up with 2 dead machines if your not careful. 

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thats not so good of an idea. This is how you end up with 2 dead machines if your not careful. 

 

I agree with Techknight, items are so cheap, better replace as a matter of fact.

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I know this from experience, When I was young and just learning electronics and starting out, I did the very same thing, and I learned some very expensive mistakes too!

 

These days, I do it smarter. I compared the working one against the bad one in measurements form. Not physical swap components form. 

 

So I take diode, resistance, and voltage measurements then compare. I find alot of faults that way especially if you dont have a schematic. 

Edited by techknight

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