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My experience buying about 25 different LC/LC2/LC3/LC475 machines is 75% of the time the board needs a recap to work properly. Most also need the PSU recapped or replaced. 
 

I’ve had a variety of issues resolved by recapping including SCSI, sound, ADB, and video not working, as well as machines that just don’t boot. 
 

With machines getting close to 30 years old, it’s shortsighted to avoid recapping. Especially since time causes more damage and 1 year makes a big difference. 

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8 hours ago, Challenger 1983 said:

Sorry I really should have mentioned that I completely recapped the logic board a few weeks ago, which brought sound back to the machine, it was just yesterday I went to power it up and got nothing 

If the logic board was working, it probably is still; in my experience, at least, those power supplies don't tend to take the board with them if they don't fail dramatically.

 

8 hours ago, Challenger 1983 said:

Like with a PC power supply for now? 

The LC boards only need 5v, 12v and ground.  There's also a -5v, but for testing purposes you can leave that out.  The pinout is here:

 

https://old.pinouts.ru/Power/mac_lc_power_pinout.shtml

 

The connector is compatible with the Molex KK396 (if I remember correctly); when I built a replacement "PSU" I used cheap KK396 clones from eBay.

 

With an ATX PSU you'd have to manually fiddle with the soft power of course.

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8 hours ago, Challenger 1983 said:

Thanks, My only question is why are there two ground wires? 

I assume to make sure that there's enough current carrying capacity to take all the current back to ground?  There's no point having big chunky voltage wires if the ground connections aren't up to the job :-).

 

8 hours ago, Challenger 1983 said:

I apologize for asking so many questions 

No apologies needed, you are in the right place to ask these questions and a lot of them are questions I asked when I was trying to build my PSU-thing. :-)

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8 hours ago, cheesestraws said:

I assume to make sure that there's enough current carrying capacity to take all the current back to ground?  There's no point having big chunky voltage wires if the ground connections aren't up to the job :-).

 

No apologies needed, you are in the right place to ask these questions and a lot of them are questions I asked when I was trying to build my PSU-thing. :-)

Thank you so much, today I’ll hack an ATX power supply to confirm that everything works and I’ll look at getting/building a proper power supply for it 

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8 hours ago, Challenger 1983 said:

and I’ll look at getting/building a proper power supply for it  

Repairing your existing one may just be a "replace-the-capacitors" job.  That often seems to fix them.

 

Otherwise if you do need a replacement, I'd look at the macATX LC (I think it's @Compgeke's project), which is a pre-made converter for ATX PSUs especially designed for picoPSU type arrangements.  I don't use one, but that's only because I built mine before theirs came along.  Given how nice theirs looks, if it had been around at the time I probably wouldn't have bothered building one from scratch.

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8 hours ago, cheesestraws said:

Repairing your existing one may just be a "replace-the-capacitors" job.  That often seems to fix them.

 

Otherwise if you do need a replacement, I'd look at the macATX LC (I think it's @Compgeke's project), which is a pre-made converter for ATX PSUs especially designed for picoPSU type arrangements.  I don't use one, but that's only because I built mine before theirs came along.  Given how nice theirs looks, if it had been around at the time I probably wouldn't have bothered building one from scratch.

I think I’ll trying replacing caps first, I’ve also heard that sometimes solder joints crack so I’ll reflow everything first 

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8 hours ago, Challenger 1983 said:

Update: after leaving it plugged in and turned on for about 15 minutes it suddenly chimed and booted, however the audio is very faint, I’m assuming these are symptoms of bad caps in the PSU?

 

I had an issue similar to this on my IIsi that I recapped. I simply removed and resoldered down the caps and it worked after. Perhaps try that on the caps near the sound?

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9 hours ago, Challenger 1983 said:

I’ll try that, did that also solve the booting issue?

I had several issues. Video, scsi etc. I removed and soldered again. I was told that even though there’s a solder connection if it’s a cold solder it won’t electrically connect. So attaching caps but with a cold solder joint results in no conductivity. That is possibly one of your problems. Very easy to rule out in a few seconds (heat, remove, clean, resolder). 

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8 hours ago, MrFahrenheit said:

I had several issues. Video, scsi etc. I removed and soldered again. I was told that even though there’s a solder connection if it’s a cold solder it won’t electrically connect. So attaching caps but with a cold solder joint results in no conductivity. That is possibly one of your problems. Very easy to rule out in a few seconds (heat, remove, clean, resolder). 

Alright I’ll try that, thanks 

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Does anyone know how to adapt an ATX power supply to this, I’m going to get the MAcTX power supply but i just want to verify that i didn’t damage the board while reflowing the caps

edit i just bought a picoATX 160, i couldn’t find the 80, is that okay?

Edited by Challenger 1983
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Of course. You won’t be using more than 40W so 160 is more than enough!

 

9 hours ago, Challenger 1983 said:

i just want to verify that i didn’t damage the board while reflowing the caps

Take your multimeter and check on the logic board’s PSU connector if there are any direct shorts to ground (and to other rails). Don’t use continuity mode. Anything above 3-5ohms is fine but your multimeter might beep for values under 35. FYI, on my Quadra 950, there’s about 30 ohms between 5v and ground, my multimeter beeps yet there are no shorts.
 

If you used a standard iron (ie not a hot air station), nothing apart from the cap pads could have been damaged. If you really want to test everything, you can try to trace them back to a via or another pad but some traces may be hidden. 
 

To be honest, your board is most certainly fine, the fact your PSU is clicking without the hard/floppy drives means it can’t maintain the proper voltages without a  load. Yet another sign of failing output caps inside the PSU. In other words, the PSU is getting weaker and weaker.

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