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Macintosh II Will Not Power On.

mg.man

Well-known member
I'm pretty sure the II needs a steady 5V on the (white?) pin coming off the logic board - hence having 2 x 3V batteries. I'm doing this from memory, but the way it works is that the soft power circuit (when pressing keyboard or rear of case power) will cause 5V to be present on this pin, kicking the PSU into life. Once the PSU comes up, 5V is passed through the soft power circuit keeping that pin (POWER GOOD?) active.

Most of the issues I've encountered have been with the long (cross 'board) trace on the top of the 'board that connects the ADB / power switch area of the 'board (extreme right-rear corner when looking from the front) to the soft-power circuit (at the rear of the NUBUS slots). Leaking caps have often eaten away at this trace. To rectify this, I run a jumper wire (I use wire-wrap wire) from the appropriate places along the bottom of the 'board. This results in an invisible repair once the 'board is back in place.

I have also seen one 'board where there was an issue between the soft power circuit and the 5V pin on the logic board header the PSU connects to. In this later case, although the II would jump-start, it would immediately shut down as soon as power was removed from the pin.

For a source of 5V, I use one of those cheap USB power banks with a USB cable with the appropriate wires bared and connected to that (white?) pin on the header and GND. This can be left powered on for those situations where the PSU 5V is not getting passed back. Since you've jumpstarted once, hopefully you know the pins to use. If you used a 3.6V "PRAM" battery, that may be enough to kick the PSU on, but it might not be enough to keep the II going - so you might need to try the power bank method.

From your initial post - you got death chimes - it sounds like the "feedback" (5V good) side is working. If you haven't already, I'd pull the RAM out and check the contacts are clean. If you have all 8 slots full, you may want to work with BANK A only and see if you can get it going. If all SIMMs are the same capacity, you have 8 to play with.

If the SIMM edge connectors are grotty (technical term 😉) then I recommend you get one of these - https://www.amazon.com/Fiberglass-Pen-Scratch-Brush-Single/dp/B0892V7QV5 (just an example, there are sets as well) - and gently clean the edge connector pads.

Let us know how you get on.
 
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