IIcx Power Intermittent on-off Issue

[chrisrueckert]

This IIcx loops from on to off as soon as a known working (and recapped) PSU is connected to mains. The loop cycle goes a brief moment in "on" mode, then to off, then on again, etc.. No peripherals were connected to the board while powered on.

What I have done so far:

- Recapped logic board, checked continuity of all caps
- Replaced UM2, UK2, and UL2 with 74HC74D and 74HC132D (they seem to work in a IIci - are these also ok for a IIcx?). Continuity of all pads of the chips is ok. The IIcx didn't do anything prior to replacing these.
- Fixed two broken traces (one next to C10/C11) and one from on UK2 from pin 2 to pin 8. Continuity also ok.

Any thoughts on what else I could check? Thanks!


View attachment IMG_1768.jpg

 

[robin-fo]

There are a few suspect looking traces near F3 and to the right of C8. Did you check these? However that sounds like an improbable cause for your issues..

 

[chrisrueckert]

Yeah, these had minor corrosion, thanks for pointing them out. But they were not yet broken.

Any other ideas how to resolve the power issue?

 

[Addicted]

I am reworking a IIcx right now that has green corrosion (zombie dust..) on the solder pads of all eight 74F258, U[KLM]2, C8 - C11, C15 - C16, and on my board those diodes look suspect, too.

The symptom is the same, the PSU won't remain on after I press the button. It doesn't stay 'on' long enough to chime. IIRC from the schematic, there's a series of gates that keep the PSU on once it's up? Ties in with the ADB keyboard-on-off key control. Will review.

I think it's just humidity or a poor solder lot to blame - the battery does not seem to have ruptured. The battery holder is the original and clean, and the logic board in general looks great. Just pads are green. No orange crust anywhere.

I will be recapping the logic board and replacing the ICs I mentioned (if I can source them all). If that's not enough, I will open the PSU and see what I can see.

I'm waiting for parts to arrive and will follow this topic, adding my outcome once I have done the rework.

 

[Addicted]

PS: after more inspection, I think the battery on the IIcx logic board I am repairing did rupture at some time in the past. Whoever cleaned the board did a fine job, but I noticed.. all the electrolytic capacitors' markings are washed off. Somebody used a ton of cleaner if they got it on top of the cans! I believe once I lift any of the chips in that corner of the board, I'll find leftover battery crud. I just hope the solder pads for those PSU logic gates are still useable.

 

[Addicted]

Follow-up: I didn't fix my IIcx, and now think the problem is not the logic board.

I recapped my IIcx, then replaced UK2, UL2, UM2, D5 - D7, Q2, Q3, as well as the RAM bus multiplexors H1/2/3 - J1/2/3. These chips all showed signs of corrosion on their leads and I was able to inspect the pads once I lifted the old parts. The traces and pads all seem fine. There was some green dust under many of these ICs.

I lost only one pad in the process - on the RAM multiplexors, not in the PSU control circuit - and placing a good solid bodge wire to fix that was easy.

But.. I still have a PSU that does not stay on. It comes on and shuts right back down, so fast that it pulses twice before I can get my finger off of the power button. The logic board LED (DS1) lights momentarily and I hear the PSU relay click once each cycle. This is the same behavior as before I replaced all those ICs. I have enough confidence in my rework that I now suspect the PSU itself. I will look into recapping it.

 

[alexGS]

View attachment 74707
View attachment 74709

The capacitors C15 and C16 seem to be two different values on yours, Chris? I thought they were both 10uF.

 

[chrisrueckert]

They both are 10uF/16V, just different sizes. Thanks for checking the picture nonetheless. Here's another picture of them:

 

[alexGS]

They both are 10uF/16V, just different sizes. Thanks for checking the picture nonetheless. Here's another picture of them:

Cheers Chris, I got confused with the numbers on them
I finally managed yesterday to get a IIcx board to power on; in my case it was traces visible near the + silkscreen marks for C8 and C9. Those traces are very thin and if they look bad, they sometimes are.

I think you could possibly invest some time in cleaning up UK2, UL2, UM2 as it looks like there’s been a fight there against oxidation I find the flux in a syringe and solder braid really helps!

Although, having spent an hour doing exactly that to another IIcx, it turns on now but I think it has a ROM fault (immediate death chimes)… so sometimes the best-looking boards still don’t work anyway. I’ll try it with a ROM SIMM before I do anything else.

Best of luck!