68kMLA Classic Interface

This is a version of the 68kMLA forums for viewing on your favorite old mac. Visitors on modern platforms may prefer the main site.

Click here to select a new forum.

Diagnosing SE Analog board (bad flyback?)

Posted by: dramirez on 2022-09-12 17:28:10 Fan doesn't even turn, does faint noises, like it's trying to rotate. Voltages: 0.038 volts instead of 12, 0 instead of 5, bad power supply? I don't think so! R19 and L3 were very hot (over 50C, 122F). CR3 was removed, tested bad. With CR3 removed the fan works and normal voltages are read. Replaced CR3, fan doesn't turn again. I'm planning on replacing CR2 and Q2, but I can't find a replacement for BU406 in stock on DigiKey (BU406G) Could it be that the flyback is bad too? How can I test it? Thanks for your help, guys!

Posted by: joshc on 2022-09-12 17:47:07 Doubt it's a bad flyback. Check for weak solder joints on the analog board as well. Reflow all the common areas, all joints for the connectors, especially the yoke connector but also all the power connectors.

Posted by: cheesestraws on 2022-09-13 03:42:01 The schematics I've got don't have lots of the designators on, annoyingly. Which resistor is R19 and which inductor is L3? That sounds like something has shorted somewhere that shouldn't have shorted...

Posted by: dramirez on 2022-09-21 00:03:51 Thanks for any help guys!

Posted by: dramirez on 2022-12-17 12:58:52 Fixed, CR2 was shorted!

1