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Mac Plus Screen Wobble (Yes like a jelly :) )

Manxmann

New member
Hi Folks,

First post :) I'm no electronic engineer, only a keen hack, so I apologies in advance if I ask or say something stupid.

I'm having a problem with a 240v international AB Mac Plus and need some help/advice. When I first got hold of the machine it had no keyboard / mouse meaning other than look at it I couldn't do anything with it. I did however turn the machine on and verified it mostly worked, it had a sad mac memory error, but was alive.

Roll on several months, replacement ram, keyboard etc and now when I turn on the computer the PSU would go into protection, merely clicking. Checking everything over I found that every now and then the PSU would start however the screen continously wobbles horizontally down it's entire length like a jelly in an earth quake!

The AB hasn't had the easiest of lives having obviously been in a damp environment for some time, a fair amount of under mask corrosion is present on the tracks and at some point a battery had leaked destroying the holder but little else.

To tackle the issue I've done the following so far:

/ Cleaned the board

/ Re-flowed pretty much every joint

/ Replaced all the electrolytic caps, using a poly replacement for the NP

/ Continuity checked all potentially bad taces

/ Tested the mainboard in another Mac Plus, OK

With these steps done the Mac now powers up 100% of the time and works with no issue, the screen is bright and focused but is still a wobbly jelly. I've tested the voltages, 5V is spot on 12v is at 12.2 and -12v is at -11.48 and all are stable. The wobble could be @ 50Hz, hard to tell though.

I'm now at the end of my limited knowledge on where to look next. Some reading suggests a possible FBT issue, others suggest the opto coupler and someone kindly suggested an issue with 'clamp' diodes, which doesn't mean a lot to me.

Any advice, pointers greatly appreciated.
 

cheesestraws

Well-known member
Does the quaking reduce over time as the machine warms up or not?

Do you have access to an oscilloscope? Can you see if there's wibble of the same frequency on the voltage lines or not?

I had a similar problem (also on a 240V AB), documented here:


Which may or may not be useful

(as a footnote: just reflowing those joints wasn't enough: it looked like oxides had somehow crept down the legs of a couple of the components. I had to remove the old solder, clean the pads and the leads thoroughly, and apply new solder.)
 
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Manxmann

New member
Does the quaking reduce over time as the machine warms up or not?

Do you have access to an oscilloscope? Can you see if there's wibble of the same frequency on the voltage lines or not?

I had a similar problem (also on a 240V AB), documented here:


Which may or may not be useful

I've only run the Mac for shortish periods, up to 5 mins, so as not to stress an already flaky, or should that be wobbly, board.

What I can say is that at no time does the behaviour change during operation.

I do have access to a scope yes.

Thanks for the link, I will read it now.
 

Manxmann

New member
Does the quaking reduce over time as the machine warms up or not?

Do you have access to an oscilloscope? Can you see if there's wibble of the same frequency on the voltage lines or not?

I had a similar problem (also on a 240V AB), documented here:


Which may or may not be useful

(as a footnote: just reflowing those joints wasn't enough: it looked like oxides had somehow crept down the legs of a couple of the components. I had to remove the old solder, clean the pads and the leads thoroughly, and apply new solder.)

Ok looking at the video that is EXACTLY what I see, so I'm guessing 10Hz not 50Hz :)

As I mentioned the board does have a fair amount of trace corrosion so with that info to hand I'll spend some time clearing out and re-flowing all the joints around the psu section of the board.
 

cheesestraws

Well-known member
I've only run the Mac for shortish periods, up to 5 mins, so as not to stress an already flaky, or should that be wobbly, board.

Ah, that's fair. That probably wouldn't be long enough to check whether warming up would do anything. I think it took mine about 20 mins to fully stabilise.

From what you've said, given that mine too seemed to have had a bad attack of damp, I'd be looking not just for cracked joints, but black oxidisation on component leads towards the bottom of the AB. Damp-induced grot. But I may well be wrong.
 
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