Macintosh Classic II - Dim screen after logic and analog board recap

[chrisdaun]

The Classic II was completely dead when I got it.
I cleaned and recapped the logic and analog board and it booted right up but with no sound and the screen seemed a little dim.
I tested resistance on the speaker and found it was open loop and replacing with one from a SE/30 (same specs as printed on the Classic II speaker, 63ohms .25w) fixed the sound. Its nice and loud now and I get the startup chime/bong.
The hard drive is dead but booting off a BlueSCSI works fine.

On to the issue....
The screen is pretty dim even with the Brightness control panel turned all the way up.
The brightness pot on the back of the analog board makes almost no difference when turned fully up or fully down.
The contrast pot will really brighten up the screen but blows out the image at a decent brightness level.
Also the focus pot seems to do nothing.

I have seen a ton of posts about unstable voltage and wobbly screen image but thats not the case with mine.
Has anyone seen the issue(s) I've described about about the dim picture?

 

[pocketscience]

Check the pots are still operating correctly with your meter. I can't recall the specs off top of head, other than PP1 which is 220 ohms - but they should have it written on them.

 

[chrisdaun]

Check the pots are still operating correctly with your meter. I can't recall the specs off top of head, other than PP1 which is 220 ohms - but they should have it written on them.

Great suggestion @pocketscience
Do you know if they can be tested in circuit?

 

[pocketscience]

Yeah, you should be able to put your meter across the pot legs and see the resistance change as you twiddle the adjustment..

 

[shaker68]

I just revived a classic II a couple weeks ago and had this issue. I cleaned and recapped the motherboard but never touched the analog board as there didn't seem to be any seepage anywhere. When I got it running the screen was really dim even with the control panel maxxed out. I tried fiddling with the pots but didnt help. I figured it was just a tired tube as it had alot of burn in. So for the next couple weeks I decided to give the machine a workout to test stability. used it to transfer alot of files to and from zip disks and external HDs for hours. As of today the tube seems to have made a recovery. The brightness is totally acceptable at 75% control panel

 

[chrisdaun]

Yeah, you should be able to put your meter across the pot legs and see the resistance change as you twiddle the adjustment..

Great! I'll give that a shot when I get some free time to dig back into this project

 

[chrisdaun]

I just revived a classic II a couple weeks ago and had this issue. I cleaned and recapped the motherboard but never touched the analog board as there didn't seem to be any seepage anywhere. When I got it running the screen was really dim even with the control panel maxxed out. I tried fiddling with the pots but didnt help. I figured it was just a tired tube as it had alot of burn in. So for the next couple weeks I decided to give the machine a workout to test stability. used it to transfer alot of files to and from zip disks and external HDs for hours. As of today the tube seems to have made a recovery. The brightness is totally acceptable at 75% control panel

Interesting..... mine unfortunately isn't getting any brighter.
I had to recap the logic and analog boards as mine was dead when I got it.
But both boards did clean up nice.