Fixing a Macintosh Classic Rev. 2 Analog board (240V) that shows no signs of life

[Ferrix97]

This is the third time that I found a board with this kind of issue, the same fix also works with board that produce low voltages at startup and ramp up as they heat up.

The Board in question is a 630-0560-240V. This is a 240V international board, but the same fix should work on the US ones.

The symptom is complete absence of voltage on the secondary and no (or very little) drive to the switching MOSFET, The problem is usually a bad TDA4605 IC (or optocoupler and its zener diode).

I've seen this issue only on newer boards (where the chip just says Germany, not W Germany like the older ones). The design of the newer revision board is also a little different from the older one (they have at least another diode and capacitor).

If you have low voltages (like a checkerboard pattern with both boards recapped), try heating the chip and the area around it briefly, it should come back up immediately.

I also made a quick schematic around that chip, once I tidy it up I'll upload it.

 

[Elfen]

Any pictures? I like pictures... (LOL!)

So,  with these parts, it is best to place them? I'm sure that I have the older Classic II analog board as the logic board is a "Prototype-4-EPROM Chip" board.

 

[bibilit]

I have at least two AB board with no sign of life, i ordered a couple of those IC's, will let you know.

 

[marcelv]

Can confirm this, have a AB with no voltage on the secondary side, primary side seems ok, every component checks out, Replaced the capacitors on the secondary side did not fix it.
 
When I replaced the TDA4605 the board started working!!
 
This is the component I replaced:

 
This is another one with a W on the ic. This board is from a classic II
According to the first post these do not fail as much as the one without the W