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Low Voltage on Macintosh Classic ii Analog Board

DomDom

Member
My mac classic ii I recently got when I turned it on it would have a wavy screen pattern and after that, I recapped all Capacitors analog and logic. I tested logic on the working mac analog and the logic board works fine. I have also replaced all ic chips on the analog board after all this and still get the wavy pattern and my 5-volt and 12-volt rails are 4.3v and 11.03v with hard drive and floppy disconnected. I know diodes, transistors, etc can go bad and are there certain ones that can go bad?
 

Nixontheknight

Well-known member
My mac classic ii I recently got when I turned it on it would have a wavy screen pattern and after that, I recapped all Capacitors analog and logic. I tested logic on the working mac analog and the logic board works fine. I have also replaced all ic chips on the analog board after all this and still get the wavy pattern and my 5-volt and 12-volt rails are 4.3v and 11.03v with hard drive and floppy disconnected. I know diodes, transistors, etc can go bad and are there certain ones that can go bad?
there is a pot you can adjust on the board that will adjust these voltages, just make sure the other voltages aren't where they should be either, if there are other voltages
 

bibilit

Well-known member
DP3 and DP4 are both diodes you should replace in that case, inexpensive and known to go bad with age.

4,85 should be the minimum required.
 

bibilit

Well-known member
What voltages are you getting right now ?

The 5 volt line should be around 4.85 the 12 volt around 11.90.

if you are not close, you can adjust with PP1 to get as close as possible with those, if the 12 volt line goes too high while the 5 volt line stays low... don't go further !!

If your capacitors have leaked badly, start cleaning the board again, and reflow here and there (improving voltages) if you are not getting better, probably a leaky component somewhere.

depending on what Analog Board you have (two types where available at some point) probably a diode or the capacitors close to the Optoisolator;
 

joshc

Well-known member
A TC1 multi-function component tester could be useful, then you don't have to guess as much. You can remove a diode and check it. You can test if a resistor is bad. And so on. It will also tell you the capacitance of a capacitor, among other things.
 

DomDom

Member
I've already fully recap them analog born and the voltage I'm getting is just barely 11 volts with the pot turned all the way up and 4.3 volts. I've already replaced all IC's on the board in thoroughly clean the board with 99% IPA and lots of scrubbing. I've already replaced two diodes and nothing I have a REV B board. I have the tool to check the components but I'm probably just going to replace all the transistors and I've already checked all the diodes they are good. I'll check the resistors today but I'm wondering if anyone has the part numbers for the transistors on a REV B board?
 

bibilit

Well-known member
Keep in mind that if a diode is not shorted and just leaky, you won't be able to spot it.

at first i tested Dp3 and Dp4 and were found Ok.
 

alexGS

Active member
I've already fully recap them analog born and the voltage I'm getting is just barely 11 volts with the pot turned all the way up and 4.3 volts. I've already replaced all IC's on the board in thoroughly clean the board with 99% IPA and lots of scrubbing. I've already replaced two diodes and nothing I have a REV B board. I have the tool to check the components but I'm probably just going to replace all the transistors and I've already checked all the diodes they are good. I'll check the resistors today but I'm wondering if anyone has the part numbers for the transistors on a REV B board?
How did you go with this one in the end?
I had identically-low 4.3V from two different analog boards after replacing all the parts that you did. One of the boards is fixed after replacing the two mains filter caps. The other, still no change and I don’t know what’s keeping the voltage low.
 
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