MacPlus analog board with resistor on the back

[pfinn]

Hi, I've recently acquired two mac plusses which I'd like to restore to proper working condition. I've found a resistor on the back of one of the analog boards which looks very odd to me. Somebody has definitely been inside the machines before me, so I was wondering if this factory default for this revision or somebody has been hacking on it. As can be seen on the protective paper cover it has been running hot too. It seems to be rated 410k ohm, but measures 470k. Anybody familiar with this?

 

[RetroTheryboy]

That looks like an 220V-240V international analog board, and looks like someone has been hacking it tbh.

There is not supposed to be a resistor like that across the transformer/inductor like that on the 240V version.
That resistor goes across the Live and Neutral through the fuse.

There is such a 470K R61 resistor arranged in that fashion on the 120V version but thats got a very different design. Maybe they were trying to replicate the EMI filtering of the US analog board.

International Analog board schematics

128/512/Plus 240 V PSU Schematics

I 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...

68kmla.org

 

[pfinn]

Thanks for the quick reply! Q10 also seems off to me. Weird soldering job has been done on that board. And the battery compartment is red. Haven't seen that before either, but maybe that's more common.

 

[RetroTheryboy]

what kind of issue do you see for Q10? red battery holders are certainly rarer than the black, I do have both

 

[pfinn]

It had weird solder blobs that stuck out from the rest. Solder wasn't shiny and evenly distributed and its pins were bent in a way to stick to the board without solder (middle pin opposite direction). I've removed the rifa caps and gave the voltage pot meter a little nudge. Measured the resistance and it was infinite. After turning back and forth a bit, it started working again (110 ohms now). Aaaand: computer works again! Display is a bit jittery vertically, will have to look into that, but floppy mouse and keyboard all seem to work. I think this is the Mac Plus that kept rebooting (low voltage?). I'll measure the 5 and 12 voltages later, see if it needs adjusting.