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Mac Repair Resources and suggestions requested.

cj7hawk

Veteran Member
Joined
Jan 25, 2022
Messages
1,058
Location
Perth, Western Australia.
Hi All,

Yesterday, I got my first Mac. In fact, my first "Apple" product ever. Unfortunately, the 128K Euro model had been internally modified, partially by what looks like it might have been a factory upgrade to 512K and part of which was performed by the rain, as the whole unit looks like someone found it on a junkpile after the rain had gone.

Presently, it switches on but the display is just a series of white lines ( left to right ) which are relatively thick - I think around 70 or so of them. They appear to have some pixels missing in places and are stable, so I suspect the monitor itself is fine and it's the logic board. I read that the boards are pretty bulletproof somewhere and only the memory fails, so I removed one of the 256K banks and upon post-removal checking of the RAM chips found that they all worked perfectly, so I assume it's something else.

Present plans are to remove and test the second bank also, then clean the board up with some alcohol and solder in sockets for the RAM, and I found some schematics for the 128K, which also appear to show the additional 253 chip (MUX) for the memory upgrade that someone soldered on top of one of the existing 253 chips, but this is my first MAC repair, and if anyone has any suggestions for resources, I'd be highly appreciative.

Also, if anyone has a suggestion for any common faults that would reduce the central Mac screen to a series of horizontal lines, I'd also be highly appreciative ( My current guess is that it might be Mux related, but that's just a guess at this point ). I have the usual hobbiest repair equipment - Oscilloscopes, Logic Analyzers, Desoldering tools, Prior experience in repair work etc. But at the moment I have less than an hour experience on the Mac at all, so thought I'd ask quickly for suggestions to help bootstrap me into the world of repairing the 128k/512k I ended up with.

Thanks for all and any suggestions and comments -
David
 
@Ioncannon , Thank you that was a better result than I was expecting, and yes, Page 59 and 60 are both relevant, though Page 60 might be closer. It's times like this I wish I had another early Mac to use for testing so I could quickly work out where to focus. I completed removing and testing the RAM last night, and all of it was good, so I'm going to socket it and clean up the memory and put it all back in tonight.

I'm very impressed with the quality of the PCB for the Apple 128s - it is a very good quality PCB and it was relatively easy to remove the rams without damaging any of them. Not a common occurrence with older computers. Many had cheaper PCBs as a cost-cutting mechanism. There's more I need to check yet and the book link you provided is of exceptional help - thank you. It does provide a clear direction in which to look.

Thanks
David
 
An update - I have desoldered the switches, wondering if there was a problem with them ( I was getting some strange voltages on one ) and also desoldered the serial chip, which was getting very hot to the touch.. If I replace the RAMs with OKI rams, I get a checkerboard pattern, but the rams I took out all worked, and if installed, I get horizontal lines.

There are 85 double-lines on the screen, with a double-line width spacing between, which corresponds to 86 blanks and 85 lines, or 342 scan lines in total, so the CRT and the display circuits are working ok. Also removing any DRAM leaves that pixel missing, while inserting it gives the image, so I think the DRAM is working OK. At least it is displaying according to the data bits.

The right hand most pixels always display a common element. The top line shows solid, and the lower sixteen pixels show a pattern, with varying numbers of clearly defined pixels present.

My current thinking is that the multiplexers might be broken, and resulting in the CPU being unable to see the display. I can't tell the status of the serial chip I removed. Power consumption at 5v is 1.5A.

Are there any more detailed Mac repair books anyone can suggest? I'd like to know more about the Mac's startup process so as a guide to know where to look. Most of the signals I'm expecting to see are there, but it just doesn't seem to be doing anything, and without knowing what the Mac is supposed to do when it starts, it's more difficult to infer what is happening.

Any further assistance is appreciated -

Thanks
David
 
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