IIfx ADB power problem

[Macsimus]

Hi, folks!

I'm having a problem with a IIfx. It's unable to start from ADB power. I recapped it and replaced the PRAM and startup circuit batteries. I've tried:

• Both ADB ports. No power, but typing on the keyboard works fine.
• Different keyboards. Those same keyboards work fine to power-on my other ADB Macs.

Power-on from the switch on the back of the machine works fine, and general operation of the machine is perfect.

My electronics knowledge is rudimentary, but I reviewed the schematic for the IIfx's power circuit and did a bit of checking with a multimeter. There is no continuity between the ADB ports and R4. Should there be? Or does UB16 interfere with that? There is continuity from R4 to elsewhere, like R23, C27 and UJ18, and R4 registers a correct 100 ohms.

I don't see any obvious trace breaks, though I only have a magnifying glass, not a microscope. Overall the board is pretty clean with no obvious cap corrosion.

Any ideas of what else I can look for?

 

[retr01]

Hi @Macsimus!

I have a question:

• Have you tried removing the power key and the ALPS switch and cleaning it? (Need to rule out mechanical)

 

[Macsimus]

The keyboard isn't the issue. The power button works fine when I use it with my other Macs.

 

[Kai Robinson]

Hi, folks!

I'm having a problem with a IIfx. It's unable to start from ADB power. I recapped it and replaced the PRAM and startup circuit batteries. I've tried:

• Both ADB ports. No power, but typing on the keyboard works fine.
• Different keyboards. Those same keyboards work fine to power-on my other ADB Macs.

Power-on from the switch on the back of the machine works fine, and general operation of the machine is perfect.

My electronics knowledge is rudimentary, but I reviewed the schematic for the IIfx's power circuit and did a bit of checking with a multimeter. There is no continuity between the ADB ports and R4. Should there be? Or does UB16 interfere with that? There is continuity from R4 to elsewhere, like R23, C27 and UJ18, and R4 registers a correct 100 ohms.

I don't see any obvious trace breaks, though I only have a magnifying glass, not a microscope. Overall the board is pretty clean with no obvious cap corrosion.

Any ideas of what else I can look for?

Here's the schematic of the IIfx startup circuit:

​

Well, my money would be on R3, Q1, C61, C62 or a trace somewhere. Worth checking, anyway

 

[Astal]

I'd definitely look at the traces, possibly something is damaged under one of the caps.

 

[retr01]

It's easy to overlook. Yeah, you have to magnify your view!

 

[Astal]

This area looks suspect to me.

I think Kai is on to something with those parts.

 

[retr01]

And look at the bottom of the logic board in that area.

 

[Macsimus]

Thanks, all! I'll do some more digging and report back if I find anything.

 

[jajan547]

Check these areas look suspicious to me.

 

[Astal]

Check continuity with a multimeter