68kMLA Classic Interface
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| Mac Classic Troubles | |
| Posted by: paulwilco on 2022-09-13 14:03:20 Hi everyone, Im currently trying to fix up a Mac Classic for a museum i volunteer at, but am not having a huge amount of luck. I will describe my process so far. When the Mac was given to us it wouldn't boot from the hard drive, it would only produce a question mark icon. It did, however, boot from the internal OS. I should also say that when switched on from cold the screen would be just lines and very wavy. After a few minutes the picture would be steady and turning it off and on would bring up the exclamation mark icon/boot to internal OS by pressing the key combination. I had the logic board recapped and initially the hard drive span up and it booted from it, everything on the hard drive could be read. This lasted about 24 hours. After this it would boot from the hard drive and not allow access to the files, then on boot the hard drive would spin up and i would get the exclamation mark again then finally on boot there was no sound from the HDD and just the exclamation mark. At this point it would still boot to the internal OS. I had the HDD tested and it was found to be working on a different machine. At this point i decided to recap the analogue board. This is where things have taken a turn for the worse! when removing the old caps i forgot to initially make a note of the voltage of each one i removed. When i realised i worked out which was which through process of elimination, i think i got the correct ones but there is a bit of room for potential error here. Now when i switch the computer on i get a wavy screen as before but every few seconds it sounds like its resetting, the screen goes blank momentarily and the internal speaker makes a very short blip. Obviously at this point there is no booting at all. I think in the first instance i need to make absolutely sure i have the correct replacement capacitors. I haven't been able to find any conclusive list (this is also a UK240v machine). If there is any advice on what could be causing the resetting issue it would be very gratefully received! | |
| Posted by: Phipli on 2022-09-13 14:21:09 Not the new museum in Derby is it? | |
| Posted by: paulwilco on 2022-09-13 14:28:25 Hi Philip, No it's in Hertfordshire, we have a small vintage computer room in the museum which we are working on at the moment. I've heard about the new one in Derby, looks great! | |
| Posted by: cheesestraws on 2022-09-13 14:31:28 Oh, that sounds nice. Have you checked the voltages coming out of the power supply, and that they're stable? All of what you've said kind of sounds to me like the power supply is struggling... (you can check these at the floppy port at the back: https://old.pinouts.ru/HD/MacExtDrive_pinout.shtml) | |
| Posted by: paulwilco on 2022-09-13 14:36:02 I haven't checked the voltages yet, I could find the right schematic. Didn't think about the floppy drive connector! I'll check tomorrow and report back, thanks for the quick replies! | |
| Posted by: cheesestraws on 2022-09-13 14:44:37 Keep watch for a while, to see if they're fluctuating, rather than just taking momentary measurements. It feels a bit like you're getting into a situation where the power lines occasionally wander into spec by accident, causing the machine to reset, but otherwise are not behaving properly. It might also be worth looking at the 5V line with an oscilloscope, if you have access to one—obviously don't poke any of the higher voltage bits with it!—but at least I've had a couple of machines with wibbly screens where that's been caused by power supply ripple. Do you have the AB schematics? | |
Posted by: Phipli on 2022-09-13 14:45:56Hi Philip,Shame, I'd have popped over if it was Derby. I'm not far away. This any help to check you got the caps right? 
Macintosh Classic I + II Analog Board (220V - 240V version) - Recap-a-MacThe Macintosh Classic and Classic II share the same analog board, and they share the same little cluster of leaky capacitors. There are...
recapamac.com.au
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Posted by: paulwilco on 2022-09-14 04:57:47Keep watch for a while, to see if they're fluctuating, rather than just taking momentary measurements. It feels a bit like you're getting into a situation where the power lines occasionally wander into spec by accident, causing the machine to reset, but otherwise are not behaving properly.I don't have the schematics, i couldn't find them. Do you have a link? I do have an oscilloscope so i will have a look at the 5v supply line Shame, I'd have popped over if it was Derby. I'm not far away.Thanks for the link, the only difference is the cap at the top of the cluster on my board was 1000uf 35v and on that picture it is 470uf 50v. I know that one is correct, not sure why its different to the one illustrated on that picture. Other than that it look like i have all correct caps. | |
Posted by: cheesestraws on 2022-09-14 05:14:23I don't have the schematics, i couldn't find them. Do you have a link? I do have an oscilloscope so i will have a look at the 5v supply line Here's the big dump of bomarc schematics: 
Macintosh 68k Schematics : Bomarc Services : Free Download, Borrow, and Streaming : Internet ArchiveMacintosh 68k Schematics========================Bomarc Services reverse engineered several Apple Macintosh computers in the1990s and sold the schematics to...
archive.org
I've attached the Classic ones here below from that dump if you don't fancy downloading the whole lot. Bear in mind that there are two revisions of the Classic analogue board: the PDF includes schematics for both, make sure you're looking at the right one. | |
| Posted by: paulwilco on 2022-09-14 09:06:58 I have tested the outputs on the FDD connector. The results are as follows; Pin 5 -11.75v steady Pin 6 +4.33v fluctuates when it resets Pin 7. +11.07 fluctuates when it resets Pin 8. +11.07 fluctuates when it resets I haven't had a good look through the schematics yet, will do later. | |
| Posted by: joshc on 2022-09-14 09:13:26 This should cover the caps for your machine, they are for the 220-240V analog board: https://recapamac.com.au/macintosh-classic-analog/ | |
| Posted by: paulwilco on 2022-09-14 12:42:10 Thanks for that, I'm pretty sure I have the correct caps in the correct places now. Not sure if I should desolder them and check the polarities, I was very careful putting them in though! | |
Posted by: paulwilco on 2022-09-14 12:57:37Here's the big dump of bomarc schematics:Thanks very much for that, gonna take a while to get my head around them! | |
Posted by: mg.man on 2022-09-14 13:16:30The results are as follows;"+5" and "+12" look really (too) low. IIRC, there's small potentiometer [PP1?] which you can adjust - I *think* there's a small hole on the backside of the 'board you can get a [NON-CONDUCTIVE, small screwdriver like] tool through. *Edit* - nope, looks like you need to get to it from "inside". Be careful, it affects both voltages, so monitor the 12V pin while adjusting. | |
| Posted by: mg.man on 2022-09-14 13:22:13 Here's what you're looking for --  | |
| Posted by: paulwilco on 2022-09-14 13:31:54 Any idea where on the board pp1 is located? I'm looking now and can't see it | |
| Posted by: mg.man on 2022-09-14 13:35:52 😀  | |
| Posted by: paulwilco on 2022-09-14 13:37:07 Brilliant, found it thanks. I'm gonna give it a go..... | |
| Posted by: paulwilco on 2022-09-14 13:55:08 Ok having slowly moved the potentiometer from side to side there is a bit of an improvement. There is no resetting now, just a wobbly screen. Whereas before the wobbly screen would steady after a few minutes now it is staying wobbly. Could it maybe be a diode problem? Taking the board out a cleaning may have disturbed another component and pushed it over its limit. | |
| Posted by: mg.man on 2022-09-14 15:05:56 The typical approach to "wobbly video" is to check for cold / weak solder joints - reflow those for all the connections to the A/B - such as those for the flyback, neckboard, etc. Since you've re-capped, it may / may not be that, but worth checking. I also recently ran across the following (though I've not had opportunity to check this out on any of my "wobbly" A/Bs yet) : Mac Classic with a wobbly screen after recapI have done this already. But do you now you get higher voltages if you turn PP1 counterclockwise and lower voltages if you turn it in a clockwise direction - just like it is described in the Repair Notes? I believe it's the other way around but yes. I had this already but my 12v would go up...
68kmla.org
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