68kMLA Classic Interface

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Diagnosing a Plus that won't turn on

Posted by: EvieSigma on 2019-03-02 12:40:33 I have a Plus (that was converted from a 128k) that doesn't power on at all. Where can I probe with my multimeter to see if the power supply is putting out any voltage?

Posted by: IlikeTech on 2019-03-02 16:16:47 The connector that connects to the logic board on the analog board side. Attached is the pinout of that connector. IlikeTech

Posted by: EvieSigma on 2019-03-02 17:12:54 Thank you, that is exactly what I was looking for! I'm thinking something in the power circuitry is toast as literally nothing happens when you flip the power switch. The CRT heater doesn't even come on.

Posted by: EvieSigma on 2019-03-22 11:48:20 Took almost a month to get back to this but sure enough, I have basically no voltage on the +5, -12, and +12 lines, literally less than a volt. Bad power supply?

Posted by: dochilli on 2019-03-25 00:55:25 Perhaps this may help: http://www.ccadams.org/se/classicmac2.pdf Search for Larry Pina: Macintosh Repair & Upgrades Secrets. There you will find a lot of information about the Macintosh Plus. I think, you need a recap of the analog board.

Posted by: Ildefonso on 2019-03-28 23:52:46 I solved a similar problem in my mac plus changing the R46 resistor (look at Larry Pinas “The Dead Mac Scrolls”, page 23)

Posted by: EvieSigma on 2019-04-14 00:07:36 Where can I find schematics of the 128k analog board?

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