68kMLA Classic Interface
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| Mac Plus analog board questions | |
| Posted by: DrJosh9000 on 2022-03-19 19:23:17 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 ($$$ ðŸ˜). 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. | |
Posted by: cheesestraws on 2022-03-20 02:51:46how well it translates to the international board. I drew out—and @michimartini checked and corrected and made readable (and added component values!)—schematics for the international ~230V PSU here: 128/512/Plus 240 V PSU SchematicsI know a number of folks have been here wrangling 240V Pluses/512s/128s recently, myself included. Schematics for the 120 V version of the power supply are reasonably available, notably in Thomas H. Lee’s commentary on the design. However, the 240V version is laid out differently enough to be...
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Their schematics show 33k for that resistor, same as you have. In general, the PSUs are basically similar but not the same, and you shouldn't expect the US PSU to tell you much about component values or component numbers on the International boards. However, Thomas Lee's notes about what each bit of circuitry is doing still holds as far as I can see. | |
| Posted by: DrJosh9000 on 2022-03-20 04:01:00 Oh, awesome! Thanks! | |
| Posted by: DrJosh9000 on 2022-03-27 23:02:48 In case anyone cares, I'm pleased to report that the Plus is working again. | |
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