exxos's Atari, Amiga & retro forum

So, I decided that it was high time I did a little maintenance work on my Mac Performa 6200. The CD eject button hasn't been right for a while so I decided to fix it.

It was a purely mechanical problem, so some fettling later I fit the front panel back on the machine and power on to check my work.

The machine is about to achieve desktop, when I hear a loud fzzzt and it powers off. My mind instantly goes to RIFA, but no acrid smoke and the machine has powered off, not usual RIFA activity.

So I pulled the PSU out, the machine is quite modular so not an awful task, and find this.


Looks a some other component or components have gone foom.

Let's hope there's a schematic for this PSU.

:fire: :fire: :fire:

Looks like that TO220 needs a bigger heatsink ? Assume that's why the PCB is burning.

exxos wrote: 27 Dec 2022 13:57 Looks like that TO220 needs a bigger heatsink ? Assume that's why the PCB is burning.

Yeah, something isn't losing heat as quickly as it needs to, will have to pull the transistors and see if they've died and how bad the PCB is, it's scorched underneath too.

exxos wrote: 27 Dec 2022 13:57 Looks like that TO220 needs a bigger heatsink ? Assume that's why the PCB is burning.

Or it's failed in some other way and gone short.

To do that level of scorching something would have had to have been glowing red hot, or the PCB got cooked enough that the fibreglass resin has turned to carbon and become conductive. I've seen that happen. The PCB then shorts everything and starts to burn.

stephen_usher wrote: 27 Dec 2022 14:55

exxos wrote: 27 Dec 2022 13:57 Looks like that TO220 needs a bigger heatsink ? Assume that's why the PCB is burning.

Or it's failed in some other way and gone short.

To do that level of scorching something would have had to have been glowing red hot, or the PCB got cooked enough that the fibreglass resin has turned to carbon and become conductive. I've seen that happen. The PCB then shorts everything and starts to burn.

You could well be right.

I think this capacitor failed for a start:


And that would be consistent with the fzzzt or fssst sound I heard. It's likely been cooked by what seems to be a fairly poor PSU design.

There is a dead 7805 next door to the main mosfet. I have a 7805, so have replaced that, but I don't have an IRF820 (Mosfet) to hand so will need to order one.

It also looks like this PSU is know for overheating and burning the board as I quickly found out when searching for the PSU schematic. This is not my board BTW:


There is a small resistor too, R14, that's been cooked and needs replacing. Since this machine won't see a huge amount of use, I will try to repair temporarily for testing, then look later for a replacement.

It seems that the Performa CPU is not particularly well designed or built based on my searches so far.

The weird thing that I've found is there are some areas where the board is baked, and the solder has been cooked and turned into a grainy mess that is really hard to clean up. I've managed, but I can't help but think that could have been causing arcing as well, leading to more heating, as in some locations where the solder was grainy, there were loose components. R14 was one of them.

It's really quite the mess.

Parts ordered, as usual my big box o' capacitors doesn't have a 22uF 35v Radial capacitor. So many capacitors, never the ones I need...

EDIT: Searching for the PSU schematic is much the same as searching for the Archimedes PSU, so if anybody can point me at one, that'd be great. TIA.

rubber_jonnie wrote: 27 Dec 2022 16:27 The weird thing that I've found is there are some areas where the board is baked, and the solder has been cooked and turned into a grainy mess that is really hard to clean up.

These boards I used to work on (Bellfruit scorpion 1 MPU) used to be a bugger for that. Saw this image on the net. You can see the row of resistors which have been replaced. Generally 2 watt .Even though they were mounted away from the PCB, they used go burn the board over the years. No amount of solder or flux would get the resistors out of the board. Even when we did manage to get them out, no way would they solder back again.

This board must have been a easy one. But it was a common thing on lots of MPUs. Pretty amazing so much heat can travel down resistor legs enfusing them into the board like spot welding. Don't get me started on fuse holders.. :lol:

exxos wrote: 27 Dec 2022 17:26
These boards I used to work on (Bellfruit scorpion 1 MPU) used to be a bugger for that. Saw this image on the net. You can see the row of resistors which have been replaced. Generally 2 watt .Even though they were mounted away from the PCB, they used go burn the board over the years. No amount of solder or flux would get the resistors out of the board. Even when we did manage to get them out, no way would they solder back again.

This board must have been a easy one. But it was a common thing on lots of MPUs. Pretty amazing so much heat can travel down resistor legs enfusing them into the board like spot welding. Don't get me started on fuse holders.. :lol:


$_86.jpeg

Wow, they're big old resistors!! Bet the smaller ones just turned to dust due to overheating over time.

Wow, have got a BT problem, no engineer available until 4th Jan and I'm back to work on the 3rd...

Luckily I have BT Halo, but it's 4G backup and the signal here is bloody awful, just one bar and maybe 4Mbps if I'm lucky, a far cry from the 450Mbps I was getting over wireless and nearly 1Gbps over a wired connection.

I'm really hoping it pings back into life, or at least the 4G signal improves following a relocation of that router.

It is great it worked just fine and took over seamlessly, but some sort of notification would have helped, rather then me noticing it was slow and checking.

Hope it's fast enough for work on Tuesday!!!

Well BT are here poking around, seems the light levels are low in my fibre to the house, so he's off poking around at the next connection point outside. Luckily I have 4G backup that automatically kicks in when the fibre is down, so up and running at least and since I moved the 4G backup, my signal is much improved, as has the speed.

As for my Performa PSU, well it looks like it's dead Jim. There is clearly something else wrong with it, and due to the general burniness of it, I really don't want to spend a lot of time resurrecting it. It's no phoenix, but the parts may be useful in the non scorched areas.

First things first though, I just need to verify the machine works on bench power.