Mac Classic - will it ever run again!!

[rubber_jonnie]

@Tomswork That's really helpful, thanks so much.

Hopefully I may get to do that tonight. What with work and other stuff getting on to it has been tricky, and I definitely won't have much time next weekend. I've got details of where to check on the FDD port so that should be nice and easy to do.

I have been over every chip with my microscope at 1000x and I don't see any corrosion, or cracks/tears, but I think it would be worth revisiting just to be sure, especially in the area around the capacitors that I replaced. What did also occur to me is that given the proximity to the capacitors, I should perhaps remove the power connector and make sure there are no shorts under there.

I'll also have a look around the larger external connectors, though I would be surprised if there is a problem there since other than the ADB port they never get used.

I've also started a thread over at https://68kmla.org/forums/index.php?app ... ent-646601, so hopefully a 2 pronged attack will bear fruit

[rubber_jonnie]

@Tomswork I measured the 5v and 12v rails at the floppy port and it was fine. I also removed and refitted the power socket and found a lot of cap gunge underneath, so that was cleaned up.

What was disturbing though is that after refitting the socket (Oriented correctly BTW) when I powered on the CPU was getting very hot quite quickly, so I switched off in a hurry. The socket is off again and having picked through the schematics I now have the pinout so I need to check over the power rails for potential problems like shorts. Other chips were cool.

It's a bloody frustrating little machine, and for everything to be SMT bar the ROM, not going to be easy to troubleshoot.

I need to try and run it under a load and double check the PSU outputs, but I'm really concerned that it's dead properly now.

I think it'll be a miracle if this runs again.

[Tomswork]

@rubber_jonnie

The cpu may have gone with the rom. The cpu is soldered to the board. But hot air solder paste some reflow to clean the pads solder wick and ipa will get you back to install a new cpu but it will be on CPR for awhile I think


Tom

[rubber_jonnie]

@rubber_jonnie

The cpu may have gone with the rom. The cpu is soldered to the board. But hot air solder paste some reflow to clean the pads solder wick and ipa will get you back to install a new cpu but it will be on CPR for awhile I think


Tom

That's my worry, as the voltages seemed good when I tested. I have a spare CPU removed from an STE which will likely work OK, and I have programmed up a 27C400 EPROM with an image of a Classic ROM. I really need to go check the outputs of the PSU, which I'm off to do now since I've worked out the connector pinouts from the schematic now. There seem to be 2 5v connections plus a 12v and -12v (Unless the schematics are lying).

I suppose I could try giving it a nice clean 5v from my bench PSU and see how it behaves too.

[rubber_jonnie]

Ok, so I've pulled the CPU and with power on, nothing else get's hot, so maybe CPU and ROM are dead. Checking the voltages on the power connector (Now re-soldered), I get what I expect, 5v, 12v and -12v.

Wondering if the CPU and ROM had some sort of pact to die together!! I certainly can't read the ROM for sure,even with my new 27C400 adaptor for the TL866. I can certainly write 27C400 EPROMS and read them back, though the original ROM is a mask rom from what I can tell, so that may be part of the problem.

Voltages across the RAM seem good too and I can't find any shorts at the power plug, so that all looks fine. Ordered an SMT PLCC socket so I can pull the chip again easily if I need to.

I also checked the GND and power pins in the CPU area and they are all fine with no shorts, and the CPU, had it been fitted, would have been getting 5v at both VCC locations. Nothing else around the connection array seems to have a short either.

I guess we'll see what's what when I can get the CPU back in.

[rubber_jonnie]

On a positive note, I've confirmed that the 27C400 is a match pin for pin for the TC534200 mask ROM indicated on the schematic, and I have 2 27C400 EPROMS and ad they write/red fine in my TL866 with adapter.

[exxos]

Sometimes mask ROMs have a inverted enable pin.. So I'd be careful of that...

[Tomswork]

The socket will make things easier. Maybe check online with other boards. Yours looks like it maybe a few capacitors short around the power socket

Tom

[rubber_jonnie]

The socket will make things easier. Maybe check online with other boards. Yours looks like it maybe a few capacitors short around the power socket

Tom

Yes, there are 2 caps missing there, but that was only whilst I was doing some testing as I thought I had a short. They have been replaced already.

[rubber_jonnie]

Sometimes mask ROMs have a inverted enable pin.. So I'd be careful of that...

I've checked the datasheets and everything seems the same:

ROM_Compare.JPG (80.02 KiB) Viewed 3706 times

The TC53420 is the item specced on the Mac schematic, and the datasheet says it's a mask rom. I can't really do more than compare, especially since the machine is dead. Just gotta get a CPU back in it next.