Mac Classic video waving and boots after 2 min

[amedeo_68k]

Hi,
I have fully recapped a Mac Classic, it worked fine for about 1 months, suddenly last Sunday it started having this issue at power on:
No chime
Video have waves
After 1 minute it chimes and starts booting
The video starts to stabilize slowly, and after 3 minutes everything is good.

I have found the main issue, voltages are low at startup, the +5V and the +12V are initially +4V and +10V
then voltages starts raising and stabilises at 4.99V and 12.10V.

If after warm up I turn it off and back on within few minutes there are no issues.
After it colds down completely the issue start again

I guess there is some issues at switching return line, any ideas ?

Thank you
Amedeo

 

[cheesestraws]

Look at the solder joints, especially the ones you touched when recapping (since this has started misbehaving post-recap). My immediate "suspicious components" for thermal misbehaviour are capacitors and solder joints, and you've already replaced the caps

 

[bibilit]

Common issue on Classic AB , leaky diodes at DP3 and DP4 + nearby capacitor, can also be the Optoisolator, worth replacing all 3.

low voltage Classic

Not lucky with Classic ABs. I have a Classic at home, Logic board has been recaped and working great, but the AB is not delivering the proper voltages. New caps in the usual area and new Optoisolator as well. The 12 v is around 11.4 at best and the 5 v is around 4.5 v. the screen is wobbling...

68kmla.org

 

[amedeo_68k]

Hi, I have replaced the Optoisolator, the issue is not gone, but there is much less noise and for less than 10 seconds. The two 1N4148 diodes has been already replaced, but I could replace again, may be I want replace again the caps around the area, may be they have no ideal ESR, other toughts ?
Thank you

 

[bibilit]

Probably some cap goo still present.
if you have removed all the AB caps, you have noticed that the area is covered with fluid, so probabaly still present around.

 

[joshc]

other toughts ?

Did you check over all solder joints like @cheesestraws suggested?

 

[amedeo_68k]

Did you check over all solder joints like @cheesestraws suggested?

Thank you all, yes I checked them, removed old solder and put new, the board has ben cleaned, no fluid, to be 100% sure, I could remove the main transformer and clean under it, I could remove the flyback as well, but it looks like to be clean

 

[amedeo_68k]

Issue still ongoing, I'm going to replace the TL431 and both QP3 and QP5

 

[amedeo_68k]

Hi, I'm still having issues, I took a thermal camera picture of the analog board within the first seconds after turning on, then I have redone all solderings ( full solder removal and put fresh new ) in the hot areas, still no luck.
As I have issues in both +12V and +5V I would say that the issue before the transformer, may be it is worth replacing the Mosfet IRFEC40 again ? ( I'm pretty sure I have already replaced it )
Thank you

 

[amedeo_68k]

Hi everyone, I finally solved this, it has been tricky. The issue was a bad CNY17-3, even If it was already been replaced 2, the key is to get a really good one. The Philips one works perfectly