How to easily probe Mac Classic board?

[zachlapidus]

Hey, working to troubleshoot my simasimac Classic after doing a recap of the logic and analog boards and trying to repair damaged traces. It's coming to the point where I think I need to start measuring voltages on the board and trying to hook up a scope - it's become a nightmare of lifted pads and corroded vias.

First option -- how can I get my hands on an extension cable for the 14 pin mini-fit jr cable? I don't have a crimper, but would get one if it's truly the only way.

Second option - I have a Siglent SPD 3303-C triple output bench supply that I use for building synthesizers, but I've only ever used two outputs in a simple +/-12V configuration. Can I use this bench supply to power the logic board? It has a third output that is statically set to 5V. Assuming I set output 1 and output 2 to 12V each and bridge output 1's positive and output 2's negative terminal, would I just connect a jumper from output 1's positive output to the negative jumper of channel 3 to get the voltages I need? I bought a J12 breakout from Joe's Computer Museum thinking that perhaps I might need to do this at some point.

 

[ymk]

First option -- how can I get my hands on an extension cable for the 14 pin mini-fit jr cable? I don't have a crimper, but would get one if it's truly the only way.

You can use part of an ATX extension cable for this.

 

[jasonfortheworld]

I can't help with probing, but when it comes to an extension cable I'm pretty sure the classic boards use the same kind as the SE and SE/30 which means you can use a 24pin ATX connector like what is used on modern motherboards. You just want to match the keying of the pins and let the extras either hang off the side, or you can cut them like my cable in this photo.

 

[djc6]

I use a 20 pin ATX extension cable on my SE/30 - should also work on a Mac Classic since it uses same J12 connector. Here is the one I bought:

https://a.co/d/2Vw66J7

Here is a good video:

These could also be useful - but only for probing the signals on the J12 connector, but if you see simasimac signal then your video and power are good so you won’t gain much insight probing J12 probably:

J12 Breakout for Classic and Classic II – Joe's Computer Museum

jcm-1.com

J12 Passthrough for Classic and Classic II - Joe's Computer Museum

Want an insanely easy way to gain access to the video signals on the J12 connector on your Macintosh Classic or Classic II logic board? Look no further. The J12 Passthrough for Classic and Classic II by Ron's Computer Videos has you covered! Just plug it into your logic board, and then plug...

jcm-1.com

 

[cheesestraws]

Yes, it's a Molex Mini-Fit Jr. Same connector family as the ATX PSU, the SE AB-LB connector, and thousands of other things. Hopefully needless to say it doesn't have the same pinout as any of the others though - even the SE - which is just inviting trouble.

As people have alluded to upthread, these have the useful property that you can use a bigger connector and only use a subset of the pins, which is why the ATX extension cable trick works. Which is quite handy. But do watch out for wire gauge and quality, some cheap ATX extenders are seriously crap.