• Hello, Guest! Welcome back, and be sure to check out this post for more info about the recent service interruption and migration.

Mac Classic: Swap analog board with Classic II

dochilli

Well-known member
5V should be 4.85 to 5.15. 12V can be up to 12.8 V. Look at both when turning and measure again after a few minutes!
Sometimes it is useful to turn pp1 from one end to the other before you turn on the classic.
 
Last edited:
5V should be 4.85 to 5.15. 12V can be up to 12.8 V. Look at both when turning and measure again after a few minutes!
Sometimes it is useful to turn pp1 from one end to the other before you turn on the classic.
Okay, so I have the following results:

1/4 turn clockwise:

12v: 12.06v
5v: 4.57v

1/2 turn clockwise

12v: 13.11v
5v: unmeasured, turned it off due to excessive 12v

1/3 clockwise with small adjustments:

12v: 12.74v
5v: 4.81-4.85v


The final few tests, this was the best result I could get on the 5v without exceeding 12.9-13v.

The screen settles on the "interlaced with waving" final picture.

Does this provide any insight?

I still have not replaced the white rectangular capacitors in the picture, would t his have any impact?

Thanks
 

joshc

Well-known member
What are the symptoms with it set to 1/3 clockwise? Is the picture stable?

If the picture is stable and displaying garbage on the screen, I'd refocus your attention to the logic board. If the picture is unstable, there is a problem with the AB.
 
What are the symptoms with it set to 1/3 clockwise? Is the picture stable?

If the picture is stable and displaying garbage on the screen, I'd refocus your attention to the logic board. If the picture is unstable, there is a problem with the AB.

Sadly, none of these produce a stable picture. The picture is always slightly "wavy" (like low tide at the beach). Behind this, there is always the "plain" screen (like a desktop without title bar & icons, but with interlacing - line across screen is from camera picking up refresh:

2.jpg


The following on ("garbage") or similar can come up after a slight "pop" or crackle. Noticed it was solidly on that before AB recap, however after, it usually starts on the other one but can randomly appear at this:


1.jpg

Whatever is on the screen, the whole picture distorts/oscillates slowly ("low tide") all the time, no matter what.
 

cheesestraws

Well-known member
Those white safety/line filter caps are probably OK, and even if they weren't they wouldn't be causing this.

If you leave it on for a while (a few minutes), does the oscillation get worse or better or the same?
 
Those white safety/line filter caps are probably OK, and even if they weren't they wouldn't be causing this.

If you leave it on for a while (a few minutes), does the oscillation get worse or better or the same?

Thanks. Looks like they have a big crack down the middle, but good to know they wouldn't affect the current issue.

Leaving it on for 5-10 minutes, the oscillation stays the same. At the final value I turned PP1 to, 12v stays at 12.74 and 5v climbs from 4.80 to 4.85 and stays there. No difference to the picture.
 

cheesestraws

Well-known member
Leaving it on for 5-10 minutes, the oscillation stays the same. At the final value I turned PP1 to, 12v stays at 12.74 and 5v climbs from 4.80 to 4.85 and stays there. No difference to the picture.

OK. So this tells you it probably isn't temperature-related, which means it's less likely to be mechanical. I'd still cast an eye over the back to look for dud solder joints after replacing caps, but it's less likely if it's stable regardless of temperature. Do you have an oscilloscope or similar? Can you see if there's ripple on the power lines (perhaps at similar frequency to the screen wobble?)
 

CircuitBored

Well-known member
Those white safety/line filter caps are probably OK, and even if they weren't they wouldn't be causing this.

I would have agreed with this until last year when my Classic started showing a wavy image on the screen. All the electrolytics had been replaced already so I took a punt on the filtering caps. When I first replaced the RIFAs it made things a lot worse and the image actually looked similar to the jumbled nonsense @ClassicMacintosh is getting. It turned out I had forgotten to clean one of the pads and a cold solder joint was making a bad connection. Upon resoldering, the display was perfect again.

I doubt that these caps are your only source of trouble but they are worth replacing, partly because of the above but also because they go up in smoke quite spectacularly when they fail.
 

bibilit

Well-known member
Probably still an issue with the AB board, 12v should not be above the 12v range, while the 5v rail is ok around 4.85 / 4.90

Doesn't mean the LB is ok though, if you have another working AB board, you can try a swap.
 
Thanks @bibilit , @CircuitBored .

I have ordered brand new filter caps - PME295 pcb mount cap,4.7nF 440Vac . Will see how this goes. Is there a particular orientation for installing them?

@bibilit , much appreciated, toning down to under 12v currently produces ~ 4.45-4.5, will re-test after installing filter caps.

I will also try recapping the LB with downrated tantalums (currently it has identically rated recap).

Have a couple of other Mac Classics awaiting work in the hope of making one from many, however the other AB is not responding (awaiting resistors, as earlier in the topic) and the Classic 2 LB I have is battery bombed & appears way beyond hope. Was hoping to ask the community one-by-one, however even repairing one is proving to be a challenge! Thanks for your help
 

CarterSchmelz61

New member
Thanks @bibilit , @CircuitBored .

I have ordered brand new filter caps - PME295 pcb mount cap,4.7nF 440Vac . Will see how this goes. Is there a particular orientation for installing them?

@bibilit , much appreciated, toning down to under 12v currently produces ~ 4.45-4.5, will re-test after installing filter caps.

I will also try recapping the LB with downrated tantalums (currently it has identically rated recap).

Have a couple of other Mac Classics awaiting work in the hope of making one from many, however the other AB is not responding (awaiting resistors, as earlier in the topic) and the Classic 2 LB I have is battery bombed & appears way beyond hope. Was hoping to ask the community one-by-one, however even repairing one is proving to be a challenge! Thanks for your help
If your Classic II LB is battery bombed I would remove the SMD caps, wash it in the dishwasher, and recap it.
 
Top