Mac LC-II: sound only without PRAM after power on

[Kubik]

Folks, I wonder if you have any idea WTH is going on.

I have recapped an LC-II board. It works fine but has weird sound problems.

Without PRAM battery, when the Mac was off for at least a few minutes and I turn it on, I got a nice, loud startup chime. Also, after booting to OS, the sound works fine.
When I turn it off and on again, I get no chime, or just a faint, distorted chime, and no sound in OS.
Turning it off, letting it sit for a few minutes and turning it on again, I get loud chime and sound works in OS.

With PRAM battery, I only get chime and OS sound right after inserting the battery. Then, no matter how long was it off, there's no chime and no sound.

It feels like there's some sort of configuration written into PRAM that causes the sound to fail. Without battery, after some time for any caps to bleed off, the defaults are loaded into PRAM and sound works. As soon as PRAM gets initialized, as long as it keeps the configuration, sound does not work.

I did full recap of all SMD electrolytes, but I did not touch any tantalum caps. There's one tantalum next to the sound chip I might try to replace when my SMD soldering tweezers arrive.

I am not an expert on Macs - I just kind of like them...

BTW: this board has ROM chips in sockets - is that normal? All other LC-II's I've seen recently have them soldered down...

Thanks!

 

[GiGaBiTe]

You may have to take the logic board back out and give it a good wash in the sink and scrub around the chips with a tooth brush and soap. Rinse and dry the board, then follow up with Lectra Clean electrical parts cleaner and hose the entire board down. and let it drip dry.

I suspect there's some leftover electrolyte on the board causing a short somewhere. If a vigorous cleaning doesn't work, you'll need to start pulling chips off the board and cleaning under them, as well as testing passive components near where the capacitors were to see if any were damaged.

BTW: this board has ROM chips in sockets - is that normal? All other LC-II's I've seen recently have them soldered down...

Most LC machines have ROMs in sockets.

 

[Kubik]

OK, thanks. I have already tried the dishwasher and it did not help. I'll try replacing few passive components in the analog part of the audio (it sounds like some sort of analog distortion or overdrive), but pulling chips off is probably not worth the time.

Interesting - I have two boards with soldered ROM and just one socketed board