Mac Classic - will it ever run again!!

[stephen_usher]

Have you monitored the power rails during this time? I'm really wondering if it is a PSU problem and not a fault on the motherboard, with a component being temperature sensitive and working once it's warmed up, after all this was what was happening to the RAM chip I was having issues with on the Apricot FP.

Before you go down the PSU rabbit hole power the machine up and let it get stable and then squirt freeze spray on different parts of the motherboard and see if things break. (You could also potentially do this on the analogue board too.)

[rubber_jonnie]

Have you monitored the power rails during this time? I'm really wondering if it is a PSU problem and not a fault on the motherboard, with a component being temperature sensitive and working once it's warmed up, after all this was what was happening to the RAM chip I was having issues with on the Apricot FP.

Before you go down the PSU rabbit hole power the machine up and let it get stable and then squirt freeze spray on different parts of the motherboard and see if things break. (You could also potentially do this on the analogue board too.)

I can't monitor right now as I've removed a couple of components from the analog board so I can get their details, however I would say that all the docs I've found thus far say it's the analog board, and in particular the PSU.

Also, the motherboard is a known working one that Tomswork sent me. The Analog board is original hence my focus there, especially given the state of the original caps that I've already replaced, 7 of which were leaking, which might point at other failures.

I may well reassemble over the weekend and monitor the rails as you've suggested if I get time, as we have quite a bit on. Also, I don't have any freeze spray right now :(

[rubber_jonnie]

Hurrah, my MBR1045s arrived today, so that's one thing I can get done :)

[stephen_usher]

:cheer: :cheer: :cheer:

[rubber_jonnie]

Ok, so the new diode is in along with all but two of the caps apparently related to power (They are all marked CP, CP3 and CP11 are yet to be replaced), which I've had to order.

Power seems better, in that the time to power on is much shorter, with less of a bong loop, and certainly one of the replaced caps was completely dead and fails when tested.

I spent some time monitoring the power rails too, and there is a clear issue. Initial power on gives 12.36v and 4.8v and I get a corrupted screen. Once I get a bong, now after a couple of minutes rather than 5, power jumps to 5.14v and it boots. The 12v rail is quite consistent.

I also seem to have a problem with the RAM expansion in that it sometimes works and sometimes doesn't.

It can stay out until I can get a stable power on though, as I can boot to the ROM (Thanks for the key combo @stephen_usher, more digging into the power side needed though.

What's annoying is that it takes a long time for the PSU to get into the state where it won't start up right away, which is a bit limiting.

And boy do I hate discharging the CRT whenever I remove the analog board. It hasn't actively cracked yet and may have a built in discharge resistor, but it does rather worry me!

[stephen_usher]

It does have a discharge resistor, so they're relatively safe.

[rubber_jonnie]

It does have a discharge resistor, so they're relatively safe.

Nice to know!

Still gives me the heebie jeebies though!

[exxos]

Sounds like there is some weird feedback issue regulating voltage.. Maybe check out the capacitor at the bottom left CP24.. There is a preset trimmer there as well PP1, Maybe make a note of its location and give it a squirt with contact grease lubricant or something and give it a rigorous lefty righty turn. Also check out the electrolytics above / around the TDA IC, looks like CP4, CP20..

[rubber_jonnie]

Sounds like there is some weird feedback issue regulating voltage.. Maybe check out the capacitor at the bottom left CP24.. There is a preset trimmer there as well PP1, Maybe make a note of its location and give it a squirt with contact grease lubricant or something and give it a rigorous lefty righty turn. Also check out the electrolytics above / around the TDA IC, looks like CP4, CP20..

The trimmer pot, PP1, I can say is fine, it's already had a drop of contact cleaner and works nicely.

CP4 I replaced yesterday, CP20 is a small ceramic, which I probably have in stock so can change, and will do so. CP11 and CP3 will also be replaced once the parts come.

CP24 is a mylar cap that I don't have a replacement for, so I'll look into that too.

Finding stuff is fun on these analog boards with all the integrated video circuitry too, where does one part end and another begin!

[rubber_jonnie]

"Hmm, CP20 is marked as .1 on the diagram, and all other caps are rated in uF on the schematic, so by my reckoning, it should be .1uF, which I would expect to be marked 104, but it's marked 102, and according to the guide I have, that should be .001uF.

I can only assume a mistake on the schematic since they aren't Apple originals, so I'll just have to go with the original value of 102 I think.

Not confusing at all and it doesn't look like a rework either."

EDIT: Sometimes I write guff and have to check my work (I cross check A LOT!!). I was mixing up CP20 and CP24. They are relatively close on the physical PCB, so I mentally transposed them.

Off to order the correct caps, good job I checked beforehand.