exxos's Atari, Amiga & retro forum

It looks like there are voltage test points, like 2.5 above the trimmer pot. So assume 2.5V is a nominal voltage so sure if you check that already or not ?

There are apparently a large number of mistakes in those schematics.

exxos wrote: 21 Dec 2021 14:24 It looks like there are voltage test points, like 2.5 above the trimmer pot. So assume 2.5V is a nominal voltage so sure if you check that already or not ?

I saw those 'Test' points too, so will try and figure something out. They don't however equate to real test points on the board.

However it's pretty tricky measuring as you have to assemble the machine and getting into stuff is done by going under the CRT, and through a nest of cables, so I'm going to see if I can figure out what's what on the PCB side and mark them all up so I can test from the bottom, avoiding the flyback area!!

rubber_jonnie wrote: 21 Dec 2021 15:14 I saw those 'Test' points too, so will try and figure something out. They don't however equate to real test points on the board.

However it's pretty tricky measuring as you have to assemble the machine and getting into stuff is done by going under the CRT, and through a nest of cables, so I'm going to see if I can figure out what's what on the PCB side and mark them all up so I can test from the bottom, avoiding the flyback area!!

For fear of sounding dumb here, Could you not just power it up with some dummy load resistors like on the Atari power supply ? Feedback is only normally done on the 5V and/or 12V rails so as long as they under load it should regulate and start up fine.

exxos wrote: 21 Dec 2021 15:22

rubber_jonnie wrote: 21 Dec 2021 15:14 I saw those 'Test' points too, so will try and figure something out. They don't however equate to real test points on the board.

However it's pretty tricky measuring as you have to assemble the machine and getting into stuff is done by going under the CRT, and through a nest of cables, so I'm going to see if I can figure out what's what on the PCB side and mark them all up so I can test from the bottom, avoiding the flyback area!!

For fear of sounding dumb here, Could you not just power it up with some dummy load resistors like on the Atari power supply ? Feedback is only normally done on the 5V and/or 12V rails so as long as they under load it should regulate and start up fine.

I wish it were that simple but the flyback and video output side is on the same board as the PSU (The analog board) in the Mac Classic, so if I did just do that, the flyback would still be powered on. I also don't know how the video section would react to being run disconnected, as there is also the focus/deflection ring connection to consider. Having both on the same board is great for making it compact, but not so great for working on. Apple also made all the leads stupidly short, which makes things awkward to get at as you have to assemble it. Sadly I don't have any suitable connectors to hand to make an extension for the power lead.

I have a dead mainboard (2 actually) that I can load it with, but the problem is the same, all the high voltage parts will power up at the same time. Maybe I'm just being nervous, (I'm allergic to high voltages) but I can more easily identify the test points on the back of the board and test more safely that way. You are correct though, I should check them.

It is odd, it's like a capacitor is failing to charge fast enough, then when it does, it all starts just fine. I'll look at those test points later tonight hopefully.

I keep thinking about getting a Man Classic but then I read this topic and think NAAAAH. :lol:

PhilC wrote: 21 Dec 2021 16:18 I keep thinking about getting a Man Classic but then I read this topic and think NAAAAH. :lol:

What's a Man Classic? :lol:

I can see why this topic would put you off for sure!

rubber_jonnie wrote: 21 Dec 2021 16:29 What's a Man Classic? :lol:

It's a Mac from an alternate timeline where Steve Jobs got his way. :P

derkom wrote: 21 Dec 2021 17:22

rubber_jonnie wrote: 21 Dec 2021 16:29 What's a Man Classic? :lol:

It's a Mac from an alternate timeline where Steve Jobs got his way. :P

I thought it was some kind of male grooming item, but I like your idea better 😂

Could some of the diodes or resistors be breaking down ?

Tom