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Mac Plus 240v Analog board CR21 diode
Posted by: pfinn on 2023-04-23 09:40:07
Hi folks,

I've got a Mac Plus international analog board here with a blackened CR21 diode. When measured it has about 40 ohms resistance regardless of polarity. This is a good indication this diode has blown right? Should I get a replacement and try again or could there be something else at fault which needs correcting first?

I'll post updates here in my search for a replacement part.
Posted by: Nixontheknight on 2023-04-23 09:53:22
Hi folks,

I've got a Mac Plus international analog board here with a blackened CR21 diode. When measured it has about 40 ohms resistance regardless of polarity. This is a good indication this diode has blown right? Should I get a replacement and try again or could there be something else at fault which needs correcting first?

I'll post updates here in my search for a replacement part.
It might be an indication. If your multimeter has a continuity or diode check mode, it should beep or show 0.7V in one direction and do nothing in the other, if it beeps in both directions or shows little to no voltage drop, then it is blown
Posted by: pfinn on 2023-04-23 10:53:16
It beeps in both directions. It shall forever be remembered as the little diode that could.
Posted by: pfinn on 2023-04-24 03:13:04
I don't think CR21 is the problem. According to the Dead Mac Scrolls:

"With the power off, check the in-circuit resistance across
CR21 (IR31 D Q, R-Schottky Barrier, 40V, 6A). If the reading
indicates 00/00 (not 52/53), replace CR21 with a heavy-duty
MBR360 rectifier. If the in-circuit reading is normal (52Q/53 0 ), see
the next entry."

My readings are 38/37, so maybe not right on the spec, but CR20 isn't either. My multimeter continuity tester reads 40 ohms as continuity I think.
Posted by: pfinn on 2023-04-25 07:33:12
Is this something that's a normal find or do I have a couple of Frankenstein Mac Plusses here? This is a different machine from the one with the resistor on the back of the analog board. Notice the transistor with the resistor doing a little dance together. It does not look factory default to me. 😳

IMG_20230425_162707498_rotate.jpg
Posted by: RetroTheryboy on 2023-04-25 10:36:53
I have that too. 20230425_183327.jpg
Posted by: pfinn on 2023-04-25 12:00:03
Hey wow, I didn't expect that! It's not in the schematics. Is yours a revision C too? My 1070-A doesn't have that, while the C does...

Thanks for checking!

I wonder what engineer suggested this change and got approval. 😀
Posted by: RetroTheryboy on 2023-04-25 12:39:30
It's also a rev C , my rev A does not have it but the rev A has the Red battery holder.
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