1.2a & 1.8a LISA Power Supply, - Common Issues?

[uniserver]

Hello,

I have 2 LISA power supplies here.

One thing I can say, for both power supplies

They get very hot.

You can even see black marks on the bottom of the PCB's,

and the ½ - 1 watt resistors on the 1.2 amp version are Crunchy/Crackly looking.

All though, the crackly resistors seem to VOM fine, not to say they do not change once the PSU kicks on.

My god would it have killed them to throw a fan in there?

Anyways,

Here are my symptoms:

1.2a Will Run for a while , and then shut off, Assuming from a heat induced issue.

- was going to just change the caps, but i want to make sure there is not any other common issue

I can take care of while its open.

1.8a Has a dead short some where, was working, and that all of the sudden it stopped working. The 3amp 250v fuse was nuked,

replace the fuse, plug it in and it nukes again

I can include pictures as well - thank you for sharing any experienced thoughts you may have.

 

[maceffects]

The 1.8a Lisa PS is junk, I don't recall the exact problem with those but they went out all the time, I use to have some parts for them, but I am sure they are lost or trashed by now. Now the good news is the 1.2a is a good power supply, and everyone I've used had got really hot in that area, adding a fan would be a wonderful idea... Not sure why $10,000 didn't get you a PS fan... But hey

 

[James1095]

Apple was very adamant about not having fans, it wasn't until the Mac II and SE that any Apple computers had a fan. I think it was a Steve Jobs obsession.

Blown fuse is almost always a shorted chopper transistor. Nearly all switchmode power supplies have the same basic layout. Input->fuse->rectifier->bulk filter capacitor->chopper->transformer->rectifier->output filter. Look for a beefy 3 pin device on a heatsink near the big capacitor.

Shutting down after it's been on for a while is frequently a cracked solder joint or component drift causing either the output to drift up and trip the overvolt shutdown, or the shutdown circuitry to drift and falsely trip.