Mac Classic - will it ever run again!!

[rubber_jonnie]

Looking at the 3rd party schematics for the Classic, it doesn't look like the -12V and -5V are used on the board and the +12V is only for the floppy connector and serial ports, so you should be able to power the board from +5V only externally and then take the v-sync, h-sync and video from pins 4, 5 and 6 of the power connector. (+5V on pins 2 & 3, Ground on pins 10, 11, 12, 13 and 14)

I do dislike the way that the 3rd party schematics are done. The +5V, +12V, -12V and -5V symbols are extremely confusing and able to be misinterpreted.

Yeah, the 3rd party stuff is really a nightmare, I presume you mean the Bomark ones? They are the only ones I have found so far, and having fun and games because where I stored them on OneDrive I had some issues and some I am having to fix as the Mac doesn't like them.

[stephen_usher]

Yeah, the Bomark ones. They're really bad.

[Tomswork]

@rubber_jonnie

Reach out to Kai Robinson

@Kai Robinson

Tom

[rubber_jonnie]

Looking at this, personally I'd solder wires to the ground and +5V points on the underside of the board and power it from a bench power supply initially. You can then get the two syncs and video and try to see if it's outputting anything.

There is actually an edge connector on the logic board, and most of the needed power connections are available there, it makes it a bit easier. I believe it was a burn in test connector.

[rubber_jonnie]

@rubber_jonnie

Reach out to Kai Robinson

@Kai Robinson

Tom

Cheers Tom.

I'll get the analog board checks done first as I'm confident that is where the main problem is right now.

[rubber_jonnie]

Yeah, the Bomark ones. They're really bad.

Yeah, they're better than none, but some very odd stuff in there. Can be difficult to read.

[Tomswork]

The analog boards are a pain on the classic the psu is integrated into a lot of the basic functions but if you can get sync to it the flyback may hum. You can check the speaker for the beep but the analog board needs to work a bit for the amp to work

[rubber_jonnie]

The analog boards are a pain on the classic the psu is integrated into a lot of the basic functions but if you can get sync to it the flyback may hum. You can check the speaker for the beep but the analog board needs to work a bit for the amp to work

No beeps at all, just the fan running. The speaker was working before all this so I have no reason to think that's failed, but I can feed it a signal from my function generator to test it. There's no flyback hum, no tube heater, which did all worked previously so at some point something there has died I'm sure.

[stephen_usher]

I'm pretty sure that the video circutiry requires the v-sync/h-sync signals before it will power up.

[rubber_jonnie]

I'm pretty sure that the video circutiry requires the v-sync/h-sync signals before it will power up.

Pretty sure you're right. I pulled the analog board and reflowed all the wiring harnesses and had a good poke around on the analog board and found this:

IMG_20211214_221650.jpg

Yes, there is a gap between that copper section in the middle and the pad one up and to the right!!! And that pad is attached to QV1, which according to the schematics drives the composite video out to the neck board of the CRT.

So bodge wire in place and now we get this:

IMG_20211214_223632.jpg

For me, going from nothing at all at the beginning of the evening, I'm taking this as good progress. So, CRT works, back to working out what's up with the logic board! I'll start by double checking the voltages, make sure they are up to spec.

Gotta love these old machines, but damn they drive you mad!