Weird issue with recapped Macintosh Classic analog board

[latum69]

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..

 

[Big Ben]

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?

 

[latum69]

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!

 

[Big Ben]

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.

 

[latum69]

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!

 

[cheesestraws]

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

 

[latum69]

Actually the replacement I can source locally is a Y1cap, it's a KEMET, PME295, 4.7nF, 480V ca

 

[cheesestraws]

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.

 

[latum69]

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. :-(

 

[cheesestraws]

That sounds to me like something is shorting out on the chassis perhaps?

 

[ymk]

Any chance the hot and neutral on that outlet are reversed?

 

[latum69]

Any chance the hot and neutral on that outlet are reversed?

I checked that and they are not reversed

 

[OsvaL]

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.

 

[bromaster]

Optocoupler...