Color Classic CRT vertical fold over

[christie]

Hey all,

I’m having trouble sort something out. I’m finishing up a nice color classic - stock logic and analog board, nothing crazy.

with the case closed there are one or two scan lines at the top that are folded over. When I pop the case, shifting the CC forward, the crt slowly corrects itself. Then it’s fine, open or closed as long as I don’t turn off the power switch.

if I run the cc without the back, using blocks to support the back of the chassis, no fold over. When I put the back on, fold over.

ive spent a few days and nights trying to  isolate different grounding contacts, trying to figure out what alignment difference takes place with the back on vs off.

has anyone encountered this? Any advice?

 

[christie]

I’ve now isolated the issue to the fan. If I pop the fan out of back, there’s no fold over, even with the back on.

I’ve tried insulating the metal frame of the fan in case it was touching anything. No effect - fold over when the back was on. I then disconnected the fan wire, but leaving the fan in place. No fold over. 
 

I’ve tried adding grounded foil shields in between the fan and the crt neck, or between the fan and the analog board components below it - no real impact. 
 

I’m guessing the motor is putting electrical noise back into the 12v line, or generating EM interference that’s affecting the crt or the coils just below the fan motor. 
 

i don’t have this issue in my other color classics. Next diagnostic would be to swap fans from my other CC.
 

Has anyone else encountered or worked around this issue? I wish the CC ran cool enough to not require the fan but I don’t think that’s possible.

 

[cheesestraws]

I would probably at that point be trying a couple of things, which I'm sure have occurred to you but I'll stick them here anyway in case they're useful

1. Try with another fan, as you mentioned.  There's not much to go wrong in those, admittedly, but if that fixed it I would probably just shrug and run with it
   
2. Does it still do it if the fan is powered from somewhere else?
   
3. Look for dodgy noise filtering components on the 12V line; if you've recapped the machine, check for dodgy solder joints in stuff that looks like it might be absorbing motor-related cruft.  It may be the filtering and not the fan that has gone funny...
   

 

[christie]

I’m going to try a fan swap. I had limited success (Improvement, but not elimination) with a foil shield between the fan and the analog board.

i may also try adding a suppressor cap across the fan terminals.

i didn’t try external power for the fan, but I did try using clip leads to power the fan keeping it far away from the Mac - my thought at that point was that the load of the fan was throwing off the analog board. There was no fold over in that configuration.

at this point I know it’s the powered fan in place that causes the problem. But it’s unclear whether it’s electrical noise on the line or electro magnetic interference.

 

[superjer2000]

One thing to check is that all of the grounding tabs under the motherboard are clean and making contact with the corresponding pads. Lack of contact has been shown to induce various weird video issues which can be made worse when the unit is together due to flex.   I would imagine poor logic board grounding could be exacerbated by the fan.  

 

[christie]

You’re right - this was a big issue on my first color classic - I had all sorts of video problems before figuring that out.

on this one I went over all the grounding points when I first discovered the problem.

 

[christie]

Fan swap cleared the problem.

I then tried a modern replacement, an Arctic F8 silent. While I didn’t have fold over, there was a general constant  flickering of the crt. Again everything looks great and solid with no fan.

So going back to the original fan, I added a snubber cap across the fan terminals and this stops the fold over in a warm boot. It takes 5 minutes with the power switch On for me to be able to boot from adb without fold over.

I thought this was an analog board I recapped - I might have botched something or switched it out a while ago. Who can remember?

Good enough for now I guess...