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Weird issue with recapped Macintosh Classic analog board
Posted by: latum69 on 2021-12-16 02:25:29
I have a Macintosh Classic (analog board 630-0560 220V), I have already replaced all the electrolytic capacitors that were leaky, and now I get perfect voltages on both 5V and 12V rails. I've recapped also the logic board. The Mac works just fine, but only with a power cord without the ground pin. If I use a regular 3 pins power cord, the machine shows the checkerboard pattern on screen and after a minute or so it boots (with a "shaky" image) and after another minute the image stabilises and it works just fine.
I suspect that it's somehow related to a capacitor, and I'm thinking about the 2 RIFA security capacitors (RIFA PME290MB Y 10n). I'm not an expert in electronics, so maybe I'm totally wrong here..
Posted by: Big Ben on 2021-12-16 02:40:01
Hello

First you might want to avoid using the computer without grounding, something is not working properly at all.

You should replace the security capacitors, did you a have a picture of them?
If they show some cracks you must replace them without question.

I use new Rifa but it seems better parts exits for those, never took the time to do some research but, maybe someone here have more experience about this?
Posted by: latum69 on 2021-12-16 02:57:32
I think that the 2 RIFA can be the culprit, and yes, those have a crack on top. Nevertheless, I can't find an exact match for a replacement, these are PME290MB 10n Y and I can find only these ones: KEMET, PME295, 4.7nF, 480V ca. Could those work?
Thanks!
1639651956927.png
Posted by: Big Ben on 2021-12-16 03:02:09
Kemet is the new owner of Rifa IIRC.
They are definitely damaged. White one usually cracks from top. Yellow ones on the side.
I’ll look for the replacement part.
Posted by: latum69 on 2021-12-16 03:03:16
Kemet is the new owner of Rifa IIRC.
They are definitely damaged. White one usually cracks from top. Yellow ones on the side.
I’ll look for the replacement part.
Thanks!
Posted by: cheesestraws on 2021-12-16 03:16:36
I use new Rifa but it seems better parts exits for those, never took the time to do some research but, maybe someone here have more experience about this?

I tend to replace with new RIFAs, but that's mostly because I have enough already to do all my compacts. Other line filtering caps will do just as well, but you need to make sure to get the X and Y designations right. In fact, some compacts come from the factory with other kinds of line filter.

I mostly went with RIFAs because I don't trust myself with mains electricity, so getting the exact replacement part with the same part number felt safest, because I'm neurotic.

KEMET

KEMET now own RIFA, so that's expected. I suspect reducing from 10nF to 4.7nF won't cause any issue: in theory the machine will actually run without it at all, it's just rude to anyone else using the radio spectrum around you (that is, everyone). The important thing is the X/Y rating: the PME295 looks like a Y1 cap which is, as I understand it, a more stringent safety rating than Y2, so it's probably OK. I'd say that PME295 looks reasonable, but a second opinion would be a good move.

And just as I finish typing this:

I’ll look for the replacement part.

A second opinion approaches. Excellent :-D
Posted by: latum69 on 2021-12-16 03:27:57
Actually the replacement I can source locally is a Y1cap, it's a KEMET, PME295, 4.7nF, 480V ca 1639654072142.png
Posted by: cheesestraws on 2021-12-16 03:58:27
Actually the replacement I can source locally is a Y1cap

sorry, that's what I meant: the original PME290 is a Y2 cap, the PME295 is a Y1. It is OK to replace a Y2 with a Y1: Y1s have higher safety requirements than Y2s.
Posted by: latum69 on 2021-12-24 09:15:15
I replaced the 2 cracked RIFA security capacitors with the 2 KEMET, PME295, 4.7nF, 480V ca, but nothing changed, I mean the machine boots only with a power cord without the ground pin. So most likely there is something else going wrong on this board. :-(
Posted by: cheesestraws on 2021-12-24 10:10:45
That sounds to me like something is shorting out on the chassis perhaps?
Posted by: ymk on 2021-12-24 11:07:41
Any chance the hot and neutral on that outlet are reversed?
Posted by: latum69 on 2021-12-25 05:56:09
Any chance the hot and neutral on that outlet are reversed?
I checked that and they are not reversed
Posted by: OsvaL on 2023-11-05 06:42:49
I have the exactly same problem with my Macintosh Classic. The only way it will boot the OS is plugging it bypassing the ground pin with an adapter. Other symptom I have is that when I connect the ADB keyboard only some keys actually work. Don’t have the knowledge to fix this issue.
Posted by: bromaster on 2024-03-10 03:17:18
Optocoupler...
1