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Help to make IIsi PSU work for LC

9166188

Well-known member
Is it doable to use a IIsi PSU for a LC? I hv a working IIsi PSU and a LC motherboard that I want to test out. Can anyone share the pinout of a IIsi PSU so that I can wire them to my LC motherboard? Also, is it possible to turn on the IIsi PSU (which doesn't have a power-on switch) ... like the hack of using a paper clip on an ATX PSU molex connector? Otherwise, I can only see a -5V on pin 10.
 

MacKilRoy

Well-known member
I’m not fully knowledgeable in such things but I note the IIsi has soft power and the LC does not. I do not know if that would impact the ability to use it or not.
 

macintosh2002

Well-known member
The IIsi's power supply connections are internal, the LC models have a lead-out cable and different connections. So it has to be rebuilt. The size doesn't fit either. Would be easier to repair a broken one that came from the LC series.
 

9166188

Well-known member
My goal is to test out whether my LC board is good rather than using the IIsi PSU for it. If this needs modification, I will pass. I'm looking for low hanging fruit and have a quick test. I also have an ATX PSU. With only 5V, 12V, Ground wired to my LC, would this simple hack enough to boot up my LC (without -5V)? I just need to know if the LC board display something on screen and make a sound.
 

MacKilRoy

Well-known member
My goal is to test out whether my LC board is good rather than using the IIsi PSU for it. If this needs modification, I will pass. I'm looking for low hanging fruit and have a quick test. I also have an ATX PSU. With only 5V, 12V, Ground wired to my LC, would this simple hack enough to boot up my LC (without -5V)? I just need to know if the LC board display something on screen and make a sound.

I could be wrong but I’ve understood the LC requiring -5v only for serial ports. Without it, you should still be able to boot and have sound. I’m sure someone will correct/confirm.
 

cheesestraws

Well-known member
I could be wrong but I’ve understood the LC requiring -5v only for serial ports. Without it, you should still be able to boot and have sound. I’m sure someone will correct/confirm.

Pretty sure you're right: I can't remember for the original LC, but certainly on the LC II -5 V is only used for differential serial. You will have an almost completely working machine without it.
 

max1zzz

Well-known member
The LC dose not require -5v to work, as @cheesestraws says it is only used for serial
The LC will actually often work with just +5v connected, however you have to be a little carefull testing like this as the EGRET derives it's supply from the 12v rail so stability is not garanteed like this (5v dose get backfed onto the EGRET's supply line through something if +12v isn't there, but this is not ideal)

TL;DR - Only +5v and +12v are required for a LC to boot
 

9166188

Well-known member
Thanks ALL! It's good news that -5V is non-essential. Bad news is: my LC makes no chime and no display after powering on. I've started another thread about this.
 
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