Macintosh IIcx boot problem

[cesare]

I've got a IIcx here which arrived to me with a dodgy power supply, and some corrosion from leaking caps near the battery, but it was pretty solid, and with a little bit of work i got it up and running - all good!

However, it has an intermittent boot problem - it would sometimes fail to boot, the speaker would 'click' at power on, but nothing further would happen. Anyhow, this went from being an occasional problem to a continuous problem, as of now, it fails to start at all.

With the bare motherboard on my desk being driven by a bench supply, I get some interesting results which might shed some light on what is going on. With just 5V supplied, I often see sensible looking activity on a scope looking at one of the ROMs. I see activity on CE/OE lines, and data on the data bus, so it looks to me that the processor is running and trying to do useful stuff.

However, if I apply the 12v before the 5v, I never see any bus activity, and if I apply the 12v line after 5v, all activity on the bus immediately stops. So, it feels to me that the 12v is either leaking somewhere it shouldn't within the board, say, keeping a line high that shouldn't be, or some device which is powered by the 12v is breaking the bus when powered up.

So, what is driven by the 12v line? Anything which is addressed and on the bus? Audio related maybe?

Any advice appreciated, it's a lovely looking board, and it was working great for a while so it feels this isn't totally terminal (fingers crossed).

 

[Fred1212]

Recapped?

 

[cesare]

Hi, yes, recapped board - replaced the SMDs, checked the electrolytics and they measure fine, so have left those.

I've traced the 12v to the two sound chips, UB15 and JA15, and removing these, the machine runs as before with the 5v and the 12v applied. Replacing UB15 and I get the dubious results from before. I don't understand how the sound is organised, with UD14 also being involved - is UD14 doing the digital domain sound playback and feeding UB15/UA15 for the analog domain side of things, volume control, that sort of thing?

It's odd since these chips aren't on the bus at all, so this would suggest something is getting messed up either with an output they are driving or something like that. I'll try restoring UA15 and see if this gets things moving along. I'm guessing there is something else broken on the board and i'm poking around the wrong area trying to deduce what the real issue is!