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| Low Voltage on Macintosh Classic ii Analog Board |
Posted by: DomDom on 2022-08-10 18:13:08
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? |
Posted by: Nixontheknight on 2022-08-10 18:14:14
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 |
Posted by: bibilit on 2022-08-10 23:42:14
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. |
Posted by: DomDom on 2022-08-11 03:58:47
Okay thanks I found them and do you know the specs for those diodes or the item number for them? |
Posted by: bibilit on 2022-08-11 04:25:43
1N4148
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Posted by: DomDom on 2022-08-15 09:47:09
1N4148
Ok After changing those diodes it really is trying to boot because I see the standard boot screen but I'm still getting low voltage and it still has a wavy screen and won't do anything else. I probably just going to replace all the transistors and I was wondering about those transistors part numbers or if the wavy screen pattern has a different solution to it. |
Posted by: bibilit on 2022-08-15 10:15:18
First replace the Optoisolator CNY17 and chip TDA4605. Those are the usual culprits. |
Posted by: DomDom on 2022-08-15 14:37:27
First replace the Optoisolator CNY17 and chip TDA4605. Those are the usual culprits. Already done |
Posted by: bibilit on 2022-08-16 00:47:44
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; |
Posted by: joshc on 2022-08-16 00:55:12
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. |
Posted by: DomDom on 2022-08-16 07:05:43
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? |
Posted by: bibilit on 2022-08-16 08:14:54
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. |
Posted by: alexGS on 2022-11-20 10:54:21
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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