Mac Classic - will it ever run again!!

[JezC]

Yeah, I've seen a lot of that on Adrian's Digital Basement.

Annoyingly this morning I powered it on and got a checkerboard pattern, so it seems I'm not done with it yet, or it's not done with me!

Starting to sound like another Mega saga...

[rubber_jonnie]

Yeah, I've seen a lot of that on Adrian's Digital Basement.

Annoyingly this morning I powered it on and got a checkerboard pattern, so it seems I'm not done with it yet, or it's not done with me!

Starting to sound like another Mega saga...

You're not wrong, but on a positive note, I know it does work and I know the CRT is good, which was a worry.

I was getting worried I was running out of machines to fix, so not overly worried its playing silly buggers again.

[mrbombermillzy]

Starting to sound like another Mega saga...

You're not wrong, but on a positive note, I know it does work and I know the CRT is good, which was a worry.

I was getting worried I was running out of machines to fix, so not overly worried its playing silly buggers again.

Sorry Jon. Looks like early congrats jinxed it a bit. :(

[rubber_jonnie]

You're not wrong, but on a positive note, I know it does work and I know the CRT is good, which was a worry.

I was getting worried I was running out of machines to fix, so not overly worried its playing silly buggers again.

Sorry Jon. Looks like early congrats jinxed it a bit. :(

:lol: I blame you entirely!!

Interestingly, I left it powered on for a while just now, and out of the blue it binged and powered up, and then started up but wouldn't recognise the extra RAM.

I'm wondering if some of the other caps are unhappy, it's almost like it needed to charge up.

[rubber_jonnie]

Ok, so definitely PSU instability going on here, need to look at the caps I haven't yet replaced I think.

[Tomswork]

The macintosh just wants to slow down your list

Tom

[rubber_jonnie]

The macintosh just wants to slow down your list

Tom

I think you may be right!!

[Tomswork]

@rubber_jonnie

The analog board does have a good mix of capacitors and x and y rifa caps to if I remember

Tom

[rubber_jonnie]

@rubber_jonnie

The analog board does have a good mix of capacitors and x and y rifa caps to if I remember

Tom

I have replaced the recommended electrolytics, plus the RIFAs, but there are still some left, I'll look at the schematic and figure it out.

[rubber_jonnie]

Ok folks, I'm after some suggestions from the PSU experts amongst you regarding what what additional caps/components you 'might' think would be good to replace in the PSU side of my analog board.

I am continuing to investigate myself, so I may end up answering this anyway, but I know some of you good chaps are a whizz with PSUs, which are a bit of a black hole for me.

Basically the PSU does the following:

1. Power on the machine, no bong followed by a checkerboard.
2. Leave the machine for 5 minutes and it starts to bong and reset.
3. After a few more minutes the bong/reset loop stops and the machine will try and boot, but is not reliable and won't recognise the RAM card.
4. After about 10 minutes, it can be rebooted and will work reliably

To be clear, the CRT is working fine, so I don't believe there to be a power or signal issue to that part of the analog board, at least not once the PSU finally staggers into life!
This is the schematic, which I am struggling with a bit as they aren't great and I am still building up my knowledge of PSUs.

Screenshot 2021-12-17 at 19.31.42.png

I believe it is probably a switching PSU, and the starting capacitor is failing and taking time to build up enough charge to reliably start the machine when there is more than just a HDD attached.

In terms of what I have already replaced so far this is it:

CP8 & CP12 in the 12V output circuit
CP6, CP7 & CP36 in the 5V circuit

In addition I have replaced CP2, CP9 & CP10 as they're in the same general area, and as it happens every last one of these were identified as leaking with brown goo at the bottom of each one.

In addition, these are recommended to be replaced in the Mac Classic re-cap guide I found. These are all for the power circuit, hence the CP name.

I'm going to change the Schottky rectifier diode at DP6 which feeds the 5v supply (The 5v supply seems unstable when measured, plus these components are pretty cheap), and am intending to replace CP3 (This 'might' be the starter cap, I am not 100% sure due to lack of PSU knowledge) and CP11 and CP37 as they seem to be part of that circuit too.

In addition I am considering changing CP1, which hangs off the +tive end of the rectifier, but given they are about £14, I thought I'd ask here first you chaps think changing it might help?

I'm not 100% sure, but it looks like it's a smoothing cap to me, so I'm not convinced changing it will do very much at this point.

Any advice or pointers you can pass on at this point would be most welcome.