DrJosh9000
Member
I'm in the middle of fixing a Plus. The main issue I'm fixing is a dead flyback transformer + BU406 transistor (both of the parts are clearly faulty), which I feel happy doing. I want to check my thinking about the rest of it.
The board has part number 630-0108 and is marked "INTERNATIONAL" on the top edge. Good for me, because I'm in Australia. I started with Larry Pina's Macintosh Repar & Upgrade Secrets which has the parts lists for both U.S. and Int'l versions of the boards, and has been useful so far.
The bad flyback is marked 157-0042C, which (going on the book) suggests someone did repairs in the past. I got a 157-026C to replace it ($$$
). All the shiny, higher-voltage capacitors suggest someone in the past did a thorough recap too.
Proceeding with Chapters 5 and 6, after swapping the flyback and testing the BU406 (a three-way short, lol) I went to check R48 though R55. All fine, except... Larry lists R55 as 20kΩ. But somebody put 33kΩ here. My question is: why someone would do this (did they just run out of 20k-ish resistors?) and whether it is better to replace with a 20k or leave it. On the one hand, the Mac used to work like this, and to my mind the higher value might mean it has to dissipate less power - but on the other hand, maybe it led to the other components failing somehow?
The corresponding part in the US model (R52) is a 10kΩ, so I suspect that the different resistor value is related to the higher supply voltage. Flipping over to Thomas Lee's Classic Mac Repair Notes, since it has schematics - page 21, figure 14: switching inverter core. But I'm not sure how well I understand Thomas' notes, or how well it translates to the international board.
The board has part number 630-0108 and is marked "INTERNATIONAL" on the top edge. Good for me, because I'm in Australia. I started with Larry Pina's Macintosh Repar & Upgrade Secrets which has the parts lists for both U.S. and Int'l versions of the boards, and has been useful so far.
The bad flyback is marked 157-0042C, which (going on the book) suggests someone did repairs in the past. I got a 157-026C to replace it ($$$
). All the shiny, higher-voltage capacitors suggest someone in the past did a thorough recap too.Proceeding with Chapters 5 and 6, after swapping the flyback and testing the BU406 (a three-way short, lol) I went to check R48 though R55. All fine, except... Larry lists R55 as 20kΩ. But somebody put 33kΩ here. My question is: why someone would do this (did they just run out of 20k-ish resistors?) and whether it is better to replace with a 20k or leave it. On the one hand, the Mac used to work like this, and to my mind the higher value might mean it has to dissipate less power - but on the other hand, maybe it led to the other components failing somehow?
The corresponding part in the US model (R52) is a 10kΩ, so I suspect that the different resistor value is related to the higher supply voltage. Flipping over to Thomas Lee's Classic Mac Repair Notes, since it has schematics - page 21, figure 14: switching inverter core. But I'm not sure how well I understand Thomas' notes, or how well it translates to the international board.
