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Macintosh Classic issue

demonlg

Experienced Member
Joined
May 22, 2016
Messages
51
Hi to all, again with a questions and sorry for my english.

I have buyed a Mac Classic computer (probably 512k because it have a little ram board connected on the logic board but without sim inserted and jumper setted on SIM NOT INSTALLED). The system not boot up and the screen turn on with a garbage static image and square have a fast flickering, nothing bong!
I have disassembled all because logic board have capacitor leaking (little but present) and very poor state, battery is without leak but 0 volt, i have removed this and at this moment the system don't have a battery.
Then i washed the logic board and recapped it with new capacitor.
Then i have reassembled all and turned on, the screen appear with full horizontal lines but after 2 minutes the system bong and boot up from SCSI hdd (OS7.1) . After 10 minutes of working the system going to continuosly resetting, infinite, and in any case it try to boot then reset again or screen stays with garbage image or freezed.
I have measured 5v rail and 12v rail, and with multimeter i have an oscillation from 4.5v to 5.1v on 5 volt rail and 11.5v to 12,7v on 12 volt rail. At this point i have decided to check the analog board in switching power supply section.
At first look all of output electrolytic capacitor have a leaking and it have corroded part of bottom wiring side, i have replaced only all output capacitors and cleaned board and all damaged solder point, and cleaned all connectors.
Try to turn on, system boot up normally, load OS from disk, and...... again after any minutes the system going to reset continuosly!
On the service manual i have readed section of votlage regulator trimmer resistor, i have turned on system and regulated it to have stable 5,1volt and 12,5 volt on output, the computer seems to work...........but..........after one day i have returned on it and again horizontal lines and same issue!
Now if i measure 5v lines and 12v lines after 2-3 hour of stop i have noted a time increasing voltage, when the system is cold, and stabilize it when hot, the system try to boot up when i have 4,9volt on multimeter, if it is down of this value the system is hanged, or resetting continue. Also i have noted the SCSI disk not spin every time computer turned on, seems spin when the voltage is ok and, i have noted also the voltage drop down when system going to continuosly reset, both 5 volt drop rapidly from 5 to 2 volt and 12 volt drop rapidly from 12 to 9 volt.
Finally, in any case the systen boots up and working for about 10 minutes, or other case i have the issue at first boot time.
I think so the voltage regulation and checnking circuit have a problem, or other capacitor need to be replace on analog board.
Can any one help me?

Thanks
 
Last edited:

Mr Fahrenheit

Experienced Member
Joined
Dec 28, 2020
Messages
78
Location
North America
Hello!
Your English is fine. I understood everything.

Unfortunately I don’t know the technical details of what could be wrong. I can tell you if it is a Macintosh Classic it does not have 512kb. The base model shipped with 1MB on the logic board, and 3MB was added with a module on the side of the board. Likely what you are seeing.

Your Mac must have at least 2MB to be booting into Mac System 7.1, as that is what’s required.

If you don’t see any good response to your technical questions for diagnosing your issues, try the folks over at 68kmla forum group. There’s a lot of tech savvy Mac-only people over there that might be able to help.

Not to say there aren’t tech savvy people here, but posts in the Apple section are few and far between. Hopefully someone has the answer for you.
 

Timo W.

Veteran Member
Joined
Nov 25, 2014
Messages
1,557
Location
Germany
All I can say from my experience with my own Mac Classic: do a full recap of both the logic and power board and clean both pcbs very well. On mine, only three caps in total did not leak.
 

demonlg

Experienced Member
Joined
May 22, 2016
Messages
51
OK, thanks at all, i try to replace all electrolityc capacitor remaining on analog board, the flickering on the display is probably for another not good power rectifiing, or a big filter capacitor on line input voltage, the logic board is fine because it booting when the power problem is not present or machine hot.

For ram questions yes if i remove the memory card expansion the os7 won't boot and say not enough memory on system to boot up this system.......... this when the system booting correcly!

I don't know why the hdd won't spin evetyting, probably not enough power to initial disks spin.
If for you is not a problem i try to send little video or photo of the problem, and i try to 68kmla.

Thanks, i write here if i resove the problem.

Emanuel
 

GiGaBiTe

Veteran Member
Joined
Apr 30, 2015
Messages
2,813
Location
Austin, Texas
I don't know why the hdd won't spin evetyting, probably not enough power to initial disks spin.

Old drives tend to get stuck heads and dry or gummed up bearings. If it's a miniscribe, the servo motor can have the shaft relubricated, which sometimes frees it up enough to work for awhile.

If the heads are stuck to the platter, sometimes you can remove the drive and hard tap it sideways on a desk to break the head armature free. Though there's no guarantee that a portion of the platter won't go with it and cause carnage the moment the drive is spun up.
 

twolazy

Veteran Member
Joined
May 22, 2011
Messages
1,566
Location
Chicago, IL
Old drives tend to get stuck heads and dry or gummed up bearings. If it's a miniscribe, the servo motor can have the shaft relubricated, which sometimes frees it up enough to work for awhile.

If the heads are stuck to the platter, sometimes you can remove the drive and hard tap it sideways on a desk to break the head armature free. Though there's no guarantee that a portion of the platter won't go with it and cause carnage the moment the drive is spun up.
Classics come with an Quantum Prodrive. Believe it is 80mb.

If OP wants an original drive, I have a few around here. 👍
 

Mr Fahrenheit

Experienced Member
Joined
Dec 28, 2020
Messages
78
Location
North America
@demonlg
The Macintosh Classic is a unique machine, in that you can boot the system from the ROM. No hard drive required to boot up a test.

Simply hold down these keys when you switch it on, and keep holding until “Welcome to Macintosh” is displayed:

Command Option x o

“X-O” was the code name for the Classic. When the developers were making it, they had room in the ROM for a startup disk so they secretly embedded one.
 

demonlg

Experienced Member
Joined
May 22, 2016
Messages
51
Hi to all, thanks for support, the mac now is perfectly working, i have replaced capacitor C40 (.022uF) with wrong value when hot, then i have replaced all electrolityc capacitors on analog board. For my prevent, i have replaced LM317T and TDA4605 and zener DP3 1n4148 (in the vicinity of the C40) and then big input capacitor 220uF 400v + bridge rectifier (this one with a little corrosion).

Now the screen not flickering and i have very stable image, hdd spin up, and both line 5v and 12v don't have a slow fluctuation from 4.6 to 5.2 volt or from 11,5 to 12,5 volt, but start immediatly to 5,2 and 12,5 volt stable, finally i have startup bong after 1 second of poweron system and the horizontals lines on screen not appear when booting.

Note: the C40 capacitor is connected to CNY17 but in my case the value of its capacity changed holding it between two fingers while I measured it, the value dropped drastically when it warmed up.

Nice job.

Thanks at all.

Emanuel
 

GiGaBiTe

Veteran Member
Joined
Apr 30, 2015
Messages
2,813
Location
Austin, Texas
Classics come with an Quantum Prodrive. Believe it is 80mb.

If OP wants an original drive, I have a few around here. 👍

Those drives suffer from gummed up spindle bearings. I have a pile of them, and half of them won't spin up without being run upside down. The other half are getting there.
 
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