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View Full Version : Mac Classic low voltage on 12 V rail



afaiello
June 7th, 2011, 07:41 AM
I have a Mac Classic that is only getting ~11.3 V on the 12 V rail leading to the hard drive. It is not enough voltage to get the hard drive to spin. I hooked the hdd to a desktop 12 V rail and it spun up perfect. My guess is the power supply, but I'm not sure where to begin. I don't see any visible damage (bad caps, burned elements, corrosion, etc.) on the PSU board.

Anthony

NeXT
June 11th, 2011, 10:06 AM
If it's the original drive I would be weary of stictation. It's possible the moving of the drive from the classic to your PC for testing might of jarred it loose for the time. Have you tried spinning up another disk on the classic?

afaiello
June 11th, 2011, 12:01 PM
NeXT

No, I have not tried another drive. I don't have one freely at the moment unfortunately =(

Anthony

afaiello
June 11th, 2011, 12:32 PM
The +12 V supply at the HDD and motherboard is +10.79 V and the -12 V supply is exactly -12 V. The +5 V supply measures +4.46 V. The +5 and +12 readings are out of spec based on this source: http://developer.apple.com/legacy/mac/library/documentation/Hardware/Developer_Notes/Macintosh_CPUs-68K_Desktop/Mac_Classic.pdf
I am going to test the loading and try to find a bad element in the circuit. May my circuit analysis skills be strong!

Anthony

twolazy
June 11th, 2011, 12:33 PM
Well since you were able to test the drive on another psu, you hopefully ruled out the drive being bad. Have you tried the drive on the mac, while just the bare hdd itself powered by another psu to make sure it works correctly? This should remove some strain, and hopefully bring voltages back up enough for operation. Not a perm solution by any means, but gotta see if it works lol. Hate to see you troubleshoot to find out the drive is toast. LOL.

If it tests ok, then next check voltage measurements at any large caps. For electrolytes, uF range or larger, you should be able to see the cap charge when you use a high ohms scale with the proper polarity, the resistance will increase until it goes to infinity.

If the capacitor is shorted, then it will never charge. If it is open, the resistance will be infinite immediately and won't change. Keep in mind this will only check for shorted caps, not ones that are fatigued. For that, you will need a capacitance meter. Also, any that you doubt, just replace. Most times you can scavenge the caps out of old psu's, so no need to spend a ton of money here on repair. 8)

Another possibility are the voltage regulators. Take measurements from the center pin of each (ground), and each side. Note the voltages. This will be a huge help in finding if the low voltage is in the caps, regulators or just a worn out/leaky transformer. Also if it has any fuses, try cleaning / testing resistance.

Good Luck!

afaiello
June 11th, 2011, 12:49 PM
twolazy

Yes, I've checked the drive by hooking external power to it and booting up the Mac. It works fine. I was afraid I'd have to test those caps. Unfortunately I do not have an ESR meter, so I'll have to desolder the caps one by one. But thanks for the advice.

Anthony

afaiello
June 11th, 2011, 08:12 PM
Okay, here's what I traced. The +12 V supply traces to an electrolytic cap, then through inductor LP5, then to another electrolytic cap CP8, through a diode DP7, then to the transformer. Two things caught my eye without removing components. One, the in-circuit resistance across the diode measured 48 Ohms forward and reverse bias. Reverse bias should have read much much higher. Second, the transformer measured a voltage of -0.7 V at that pin. I'm thinking that diode is bad, but also thinking what if something caused that diode to fail.

Anthony

twolazy
June 11th, 2011, 08:56 PM
I bet you found the problem! I've had many problems with old motherboards and zener diodes... Mostly ones for keyboard ports though. Nice diagnosis! Less then a buck to fix. ^_^

My guess its just many many years of electrostatic discharges took its toll on the poor diode.

afaiello
June 12th, 2011, 11:58 AM
Pulled the diode and tested it to be perfectly fine. Next, I pulled those two caps in circuit for the +12 V supply. From the outside, they looked fine; there was no bulging, no corrosion, or visible leakage. But, underneath, there was plenty of leakage and corrosion on the leads, along with that fishy smell. I tested those under a multimeter and the resistance would not reach infinity. In fact, it reached a peak, and then started decreasing slowly. So I pulled the other nearby caps. All of them were also bad, but looked good visually from the top. Anyone having similar issues, check those caps!!!!! They may seem good on the outside, but pull a couple and check underneath and then test them electrically. They are the cluster of brown caps, 6 total: 1000 uF 35 V, 2200 uF 16 V, 1000 uF 16 V, two 2200 uF 10 V, and a 1000 uF 10 V. The other caps on the board were surprisingly fine, even the low temperature ones right under those 6 bad caps. Problem solved.

Anthony

twolazy
June 12th, 2011, 06:40 PM
Wow no brown goo or bulging even? O_O

Guess moral of the story is, always replace the old caps in macs... Last few years hearing a ton of brown goo stories, but nothing like yours, having no visible signs of failure... Good work on the diagnosis and repair btw!
Any chance you took any pics or could provide the areas your replaced? Love to get some sort of guide together, with some sort of feedback to which caps/mosfets/diodes etc were replaced/repaired. Been kicking the idea around for a while. Nonetheless any information gathered would not be bested by none whatsoever.

Eh hopefully I can devote some time to it, when I get my Mac Classic 1mb. I've been in talks with a gentleman for one , with a CMS external storage. Hopefully if things go well, I'll have it soon for a REALLY good deal...

Chuck(G)
June 12th, 2011, 07:21 PM
Old electrolytics on switching power supplies of that era often went bad (without exploding). "Low ESR" was pretty new and many electrolytics simply cooked dry with the high-frequency ripple currents.

Anthony12
June 12th, 2011, 11:08 PM
If it's the original drive I would be weary of stictation. It's possible the moving of the drive from the classic to your PC for testing might of jarred it loose for the time. Have you tried spinning up another disk on the classic?

like ur post i was facing the same problem 1 month ago i did the same thing and after that i put the previous one it was workin............

afaiello
June 13th, 2011, 04:22 PM
The brown capacitors were the only caps to test bad. They could not be visually identified as being bad unless removed and inspected underneath and electrically tested. Location and Capacitor Type: CP2 1000uF 35V, CP8 2200uF 16V, CP12 1000uF 16V, CP36 2200uF 10V, CP6 2200uF 10V, CP7 1000uF 10V, and CP11 220uF 25V. The CP11 capacitor is located near the step-up transformer and these are all the brown, high temperature type caps that went bad. Refer to pictures for details. Note that I had replaced CP12 in the picture.

Anthony

afaiello
June 15th, 2011, 02:07 PM
Important: After repairing, if you find the voltages at the rails to not be within spec, adjust the variable resistor PP1 on the power supply analog board. It is small and can be found at about the center of the board. Clockwise will increase output voltages. Be careful, it is sensitive. Only use this if bad elements have been repaired/replaced on the board, otherwise you risk damaging the machine.

Anthony

Chuck(G)
June 15th, 2011, 02:47 PM
The brown capacitors were the only caps to test bad. They could not be visually identified as being bad unless removed and inspected underneath and electrically tested. Location and Capacitor Type: CP2 1000uF 35V, CP8 2200uF 16V, CP12 1000uF 16V, CP36 2200uF 10V, CP6 2200uF 10V, CP7 1000uF 10V, and CP11 220uF 25V. The CP11 capacitor is located near the step-up transformer and these are all the brown, high temperature type caps that went bad.

Wonder if the failure has anything to do with Mr. Jobs' insistence that there be no fan in the Mac?

afaiello
June 15th, 2011, 03:17 PM
Ha ha, this one has a fan fortunately. Wasn't it the SE model that didn't have a fan?

Chuck(G)
June 15th, 2011, 04:10 PM
Macs before the SE didn't have fans. The SE was the first Mac (IIRC) that used a fan. Things were getting pretty hot in that box. Obviously, the fan in the Classic is under-specified.

The Apple Museum has this interesting tidbit on the SE fan: (http://applemuseum.bott.org/sections/computers/macse.html)


The relatively cheap cooling fan was very annoying to several SE users. Some did anything possible to get rid of it. There were several better cooling fans for the SE on the market but my favorite product was the Mac Chimney. Literally, it was a chimmeney that allowed cool air to enter the SE into the CPU. A soon-to-be-scanned picture I saw of the product makes the SE look like a little Mac house!

twolazy
June 20th, 2011, 01:25 PM
Many thanks again for fulfilling my request of detailed repair. I should be picking up my Mac, and I get the choice of nabbing a Classic or Classic II, this weekend. Plans fell thru this past Sunday due to fathers day and rain. Deal even includes an CMS external storage device! =)

I'll be following your footsteps in the next week or 2. Once it arrives I'll be sure to add to this thread or start another, and start work on the webpage I promised. 8)

pep_one
March 22nd, 2015, 08:40 AM
Hi to everyone, I know this is an old thread but I have the same problem and I do not know what to do.
I changed all the browns capacitors e the big CP1, but the voltage rails remains low. Then I changed the diodes: DP5 and DP7 with MBR360 (but I'm not sure of these model) and the DP6 diode with MBR1045.
Now as soon as connect the power supply, the fuse blown. I double checked the welds and seem to be ok.What can it be?
Can you tell me the exact code of DP5 and DP7 diodes?
Thank you very much! Bye

bibilit
May 25th, 2016, 11:37 PM
Now as soon as connect the power supply, the fuse blown. I double checked the welds and seem to be ok.What can it be?

Check the Mosfet, can be shorted, is ref IRFBC40 and located at position QP2, if Ok can be the Bridge rectifier.