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Macintosh II startup circuit is always on

Aektann

Well-known member
After replacing the broken power switch my Macintosh II motherboard begins to startup immediately after power cord is connected. I tried to de-solder and remove power switch to restore the previous working state with no luck. My board turns up every time when plugged to power. It seems like the power circuit is always on. What should I pay attention first?
 

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Verault

Well-known member
Its just a basic Mac II? IF I recall (been a while and I cant find my pdfs) wasnt D5 or D4 involved in the startup voltage to turn on the psu? It could be stuck. Are you sure its not the psu?
 

Aektann

Well-known member
Its just a basic Mac II? IF I recall (been a while and I cant find my pdfs) wasnt D5 or D4 involved in the startup voltage to turn on the psu? It could be stuck. Are you sure its not the psu?
Its just a basic Mac II? IF I recall (been a while and I cant find my pdfs) wasnt D5 or D4 involved in the startup voltage to turn on the psu? It could be stuck. Are you sure its not the psu?

yep, it’s a genuine Mac II board, early revision. The problem is I don’t understand what was broken after the power switch was installed on board. It was the only modification I performed.
 

Verault

Well-known member
do a continuity check on your power switch. IS it always on or does it only chime when you push it?
Also are those original or new batteries?
 
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LaPorta

Well-known member
Are you sure the power switch itself is not broken? As in, always closed and not open when not depressed?
 

Aektann

Well-known member
Are you sure the power switch itself is not broken? As in, always closed and not open when not depressed?
I de-soldered power switch and checked the board without it. It continues to power on each time I plug a power cord. Before repairing it behaved correctly. I don’t know what could be broken since that time
 

joshc

Well-known member
Thankfully, this is a topic that's well documented/explored here:


Your Mac II is starting, so you know the battery part of the circuit is OK - it wouldn't start without that being OK. I think you need to poke around the bits in the 'Power On' section of the circuit:

1627617856577.png

First off:
The 7V rail also goes to the emitter of transistor Q3, which is a PNP transistor. We’ll see in a minute, but when the power button is not being pressed, the transistor is turned off, so it shouldn’t be drawing any current.
The interesting bit here is Q3 - if that transistor is off, the power supply is off. And that transistor is supposed to be off until the power switch is pressed.

According to that page:
Since 7V is going into the left side of R14, there is no current between the emitter and base of the transistor. The transistor is turned off.
Because the transistor is turned off, the power supply control pin (J18, pin 15) is left floating (remember, the +5V output is not supplying 5V yet, since the power supply is off).
Thus, the power supply stays off.
Hopefully that will give you some ideas of other bits to check.

Before repairing it behaved correctly.
You mentioned replacing a broken power switch before, what made you think it was originally broken? What other work did you do? (I can see the batteries have been replaced), was there anything else?

A photo of the back of the board may help myself or others identify a potential problem area.

I hope that helps. It's a lot to get your head around, but approach it methodically from the power switch through to each step.
 

Aektann

Well-known member
Are you sure the power switch itself is not broken? As in, always closed and not open when not depressed?
Yes, I’m pretty sure. It was de-soldered again to check if the board could power on normally. No way, it continues to fire up immediately even without the switch
 

Verault

Well-known member
Again, you should test the contiunity of the switch. You can also desolder the switch and see if the system still turns on. IF it does you have additional problems. Which Is what I would lead to as I havent seen one of those power switches on any of the MACS which had them go bad (other than being snapped off)
 

Aektann

Well-known member
Again, you should test the contiunity of the switch. You can also desolder the switch and see if the system still turns on. IF it does you have additional problems. Which Is what I would lead to as I havent seen one of those power switches on any of the MACS which had them go bad (other than being snapped off)
Yes, I’ve already done this check and got the motherboard staring each time when I’m powering my setup. As I have an explanation of schematic I’ll do the check of NAND gates and transistors down the circuit.
 

Aektann

Well-known member
We found a cause, one of the axial capacitors started to leak and made a shortage, making the power controlling transistor always on.
 

Verault

Well-known member
Before going further it would be a good idea to change all the capacitors. I know the Axials are far less prone to leak than SMD, but they are 35 years old, and it would be one less variable to consider,.
 
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