JCE_Microlith
New member
I'm trying to troubleshoot a bad Mac Plus analog board. I was given the computer in non-working condition probably 20 years ago, and I've never really made much of an effort to repair it. I just fixed another Plus analog board by re-soldering connector pins, and that has me excited to try and fix this one. I plugged its logic board into the working Plus chassis, and confirmed that it is good. When powered on, the speaker on the analog board makes a high pitched, chirping, sounding like it's being modulated by an envelope generator controlled by an LFO. With the logic board connected, there is no startup chime and no picture on the screen. With the logic board disconnected, I measure 13v _AC_ on pin 6 of the logic board connector, which should be 5v DC. The same on the 12v line. I measured some resistances across diodes that, according to "Dead Mac Scrolls" would indicate that the FB transformer is bad, if they read 0 ohms, but they were pretty close to the values the book claimed they should be. Measuring across CR20 as per p. 24 I got a resistance value that kept increasing, as if it was charging a capacitor somewhere, but it was definitely not 0 ohms. On p. 25 of DMS, it suggests replacing a number of capacitors, which I may do. They aren't visibly bulged or leaking electrolyte, but I do understand that they may fail without any visible sign.
But the fact that I'm getting an AC voltage on what should be DC power supply lines to the logic board, and the chirping speaker, make me think that some kind of rectifier is gone. Could my odd measurement across CR20 indicate that it is not shorted but rather open? Other thoughts before I go blindly replacing caps?
JCE
But the fact that I'm getting an AC voltage on what should be DC power supply lines to the logic board, and the chirping speaker, make me think that some kind of rectifier is gone. Could my odd measurement across CR20 indicate that it is not shorted but rather open? Other thoughts before I go blindly replacing caps?
JCE
