68kMLA Classic Interface
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| Click here to select a new forum. | | Mac Plus Analog Board repair help | Posted by: opcode on 2025-01-28 15:45:01
I have a Macintosh Plus that I bought and restored a year ago, and it has worked beautifully since then. At the time, I fully recapped both the analog and digital boards.
Unfortunately, a few week ago, I accidentally plugged in what turned out to be a defective cable into the SCSI port. This seems to have caused a short circuit—the cable became hot, and the screen went black. I quickly turned the unit off, but the damage was already done.
I can see the power supply took a hit because, ever since, it has been making a clicking sound at approximately 4.2 Hz. A visual inspection didn’t reveal any damaged components. With no logic board connected, I checked the +12V and +5V lines using an oscilloscope and observed the voltage trying to establish itself before cutting off and restarting in a loop, -12V is relatively stable. Checking the power lines' load resistance didn’t indicate any obvious short circuit either—though I couldn’t find the expected load values.
So far I tried to replace CR20, CR21, C24, C25, C29, C31, Q8, CR18, U3, R51, R52 - didn't change anything. Setting R56 to the minimum lowers the frequency of the flupping sound but that's it. All testing has been done with the logic board unplugged.
Do you have any recommendations on how to move further to diagnose this?
Thanks, everyone! | Posted by: Arbee on 2025-01-29 09:55:34
If there's no shorts on any of the rails, it's likely something is drawing excessive current, probably due to a near-short. Check the components connected to the SCSI port (the 5380 main SCSI chip being a likely suspect) and look for signs of the magic smoke having escaped. | Posted by: wskjinfen on 2025-02-28 02:10:09
Please eliminate the short circuit of the motherboard first
check whether the connection wires between the logic board and the main board and the fuses on the connection wires are blown.
“clicking sound at approximately 4.2 Hz”means the switching power supply is unloaded | Posted by: opcode on 2025-03-02 16:22:25
by unloaded do you mean that the power supply will just not be stable an keep clicking until the mother board is actually plugged? | Posted by: opcode on 2025-03-02 16:38:11
I might not have been clear in my initial post but all the testing I made was with the motherboard unplugged, it doesn't look like the motherboard has any issues, so I've been focusing on the power supply alone for now. | Posted by: wskjinfen on 2025-03-04 08:39:01
I might not have been clear in my initial post but all the testing I made was with the motherboard unplugged, it doesn't look like the motherboard has any issues, so I've been focusing on the power supply alone for now. The Macintosh power supply cannot be unloaded and requires a minimum resistance, otherwise the overvoltage will trigger the crowbar circuit and cause flip-flip. When repairing, you can temporarily connect a 10 ohm cement resistor between +5V and GND and a 20 ohm cement resistor between +12V and GND. | Posted by: giolicious on 2025-03-04 22:47:59
The Macintosh power supply cannot be unloaded and requires a minimum resistance, otherwise the overvoltage will trigger the crowbar circuit and cause flip-flip. When repairing, you can temporarily connect a 10 ohm cement resistor between +5V and GND and a 20 ohm cement resistor between +12V and GND. Yes you need a proper load for the analog board.
@opcode have you replaced the RIFA caps? Mine has 3 at C33, C37, and C38. | | 1 |
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