LCIII - Recapped and alive! 12 hours later... DOA

[skyhawk3355]

Hi All,
I'm either cursed or the cats are effing with me.
I recently acquired an LCIII, tested it, and got no video or chimes. Once I recapped the PSU and the logic board I was getting chimes and the question mark floppy. So i was pretty satisfied for the evening. Today, I go to power it on, no video and no chime. WTH!
Now, there is some medium levels of corrosion around U4 and U5. I've pulled up the Bomarc schematics and can see that U5 is part of the audio system, but not sure about U4.
I've got good voltages coming out of the PSU and have an oscilloscope at the ready. Would anyone have any recommendations as to where to probe next?
See attached a pic of U4/U5
Thanks!

 

[finkmac]

did you actually clean that area? it looks pretty grody.

 

[skyhawk3355]

Yeah, the best I could with IPA and Q-Tips. Should I attempt to de-solder and clean it up? I was afraid of breaking the legs on the ICs

 

[skyhawk3355]

UGH! cleaned and re-soldered U4 with no change. While poking around I found a cap that I installed yesterday loose. re-soldered the cap and it fired right up.
Guess the moral is to triple check your own work

 

[djhaloeight]

For what it’s worth, my LCIII+ died a few years after being fully recapped. The traces on the RAM address bus had corroded internally, due to electrolytic cap juice pooling there before it was fixed.

 

[robin-fo]

Isn‘t the LCIII the one with the reversed capacitor? Did you solder in that one properly?

 

[djhaloeight]

I forgot to mention that max1zzz (who is a serious Mac mad scientist!) very kindly diagnosed the open traces on my LCIII+ and transferred everything from my board to one of his new production boards. That's how I found out it was rotted traces. I'm unaware of any reversed capacitors on these machines. I've also yet to tackle a recap job myself, but I've got a Yihua combo hot air rework/soldering station that I need to set up. I've got a dead LC board that probably has bad traces but I'll give it a go on that before I try the 475 and my working LCII, both of which need caps.