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#1 2016-07-10 22:04:48

Hoppend
Member
Registered: 2016-07-10
Posts: 4

Classic Mac 512k ED Split screen showing twice

Does anybody know what the problem with my Macintosh is? I took it out of storage and at first the display was not working at all. The display came back after re soldering the J1 and J2 connector.

I hope it is fixable.


-Hoppend

IMG_8268.jpg

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#2 2016-07-11 10:16:59

uniserver
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From: Sf, Mi
Registered: 2014-05-15
Posts: 956
Website

Re: Classic Mac 512k ED Split screen showing twice

very interesting. well atleast the picture is nice and bright!  did you go through the dead mac scrolls and see if there is anything that matches your issue here?


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#3 2016-07-11 10:30:19

uniserver
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From: Sf, Mi
Registered: 2014-05-15
Posts: 956
Website

Re: Classic Mac 512k ED Split screen showing twice

first off the analog board needs all new caps... all the standard stuff... and you are going to want to adjust r53 cut off.
verify your 5v is correct. you adjust that at r56.

The stripe tells me there could be a bad ram chip on the mb.
but then again it could be your 5v rail way out of adjustment.


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#4 2016-07-11 10:31:59

uniserver
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From: Sf, Mi
Registered: 2014-05-15
Posts: 956
Website

Re: Classic Mac 512k ED Split screen showing twice

also pop your rom's in and out, make sure they have good contact.

might even be a bad chip related to the video on the mb.


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#5 2016-07-11 10:32:24

Hoppend
Member
Registered: 2016-07-10
Posts: 4

Re: Classic Mac 512k ED Split screen showing twice

uniserver wrote:

very interesting. well atleast the picture is nice and bright!  did you go through the dead mac scrolls and see if there is anything that matches your issue here?

Yes, very interesting. Close to impossible to get the same result if you wanted to accomplish something like this.
I did look through the dead mac scrolls... nothing really matches.

My reasoning after reading the scrolls:

The screen must be generated twice from the logic board, the analog board seems to be working correctly.

I have not examined the logic board lines yet.. however. I would like to know what area I need to focus on.

-Hoppend

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#6 2016-07-11 10:40:43

uniserver
Member
From: Sf, Mi
Registered: 2014-05-15
Posts: 956
Website

Re: Classic Mac 512k ED Split screen showing twice

yeah also you should go through the whole analog board.  as just " re soldering j1 j2 might be ok for a shot gun fix... but if you really want to properly service it, you want to get your wick and spend some time and carefully desolder many pins on the ab...  there are many more then just that one that cracks.. its the kind of thing you need to take it out... and take your time... get your magnifying glass... get your wick, and your flux and scrub the burnt up pins as they carbon up from flexing and burning, its not a wham bam thank you kinda thing... maybe you have a glob of solder somewhere that is touching something that it shouldn't as well... but do the caps on there it needs it.


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#7 2016-07-11 11:11:46

Hoppend
Member
Registered: 2016-07-10
Posts: 4

Re: Classic Mac 512k ED Split screen showing twice

Ok, than it will have to wait. Currently enlarging my workshop, once that's finished I will get it on the bench and inspect it thoroughly. I will get my wick, new solder and magnifying glass out.

Happy to hear that you think it will be fixable.

-Hoppend

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#8 2016-07-11 15:56:13

Eudimorphodon
Member
Registered: 2014-09-02
Posts: 525

Re: Classic Mac 512k ED Split screen showing twice

My guess as to what's producing the effect is there's a bit stuck in the address generation circuitry for the display. A toaster Mac's display is 512 pixels wide, which is 64 bytes, or 32 16 bit words. (These machines have 16 bit wide memories formed from two eight-bit wide banks.) System RAM is timeshared between the video circuitry and the CPU, and when the video circuitry is in the driver's seat the addresses for the memory reads are generated by a counter-driven state machine mostly residing in a few custom PAL chips on the motherboard. and if the addresses generated by that system always had the 5th bit stuck high then the first half of each display line would be replaced by a copy of the second half. Does the machine actually finish booting and otherwise behave normally? The other possibility is there's a stuck bit within the memory subsystem itself, but if that were the case I suspect it never would have made it past a Sad Mac because RAM would be corrupted from the CPU's point of view.

I'd suggest cleaning the motherboard carefully, removing any potentially conductive dust or goo. A short could cause a stuck bit like that. Alternatively there could a crack in a solder joint or trace, that could also be allowing a line to float and stick.


Flap Different.

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#9 2017-09-03 13:22:35

Hoppend
Member
Registered: 2016-07-10
Posts: 4

Re: Classic Mac 512k ED Split screen showing twice

Ok, so I have had some time to spend on the unit. Was not able to cure it. I have reflowed all contacts on the motherboard and cleaned it thourghly.

I was hoping any of you might have other suggestions that I can try...

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