LC III - No sound, no video

[charkes]

I bought an LC III logic board cheaply, and the seller didn't know if it ran. The fan spins and the three lights on the keyboard flash but there's no chime or video. I've recapped the board and there was very little corrosion. I have soldered in the C22 cap reverse from what the silkscreen says (learned that when recapping another LC III).

I can't see any obvious signs of corrosion except for the pins on the IC at U4 -- corroded to nothing. I have an LC III donor logic board (it works, except for sound, but the traces around U1 are in shambles). I removed the U4 from the donor board and soldered it into place on the new board. The pads are horrible but I've checked continuity on the other parts of the board and the new U4 chip is connected.

I'm getting the same nothing: fan spins, three lights flash, no chime or video. I plugged headphones into the speaker port and get that deep "click" sound of the power turning on but there are no other sounds. I've tried putting the ROMs from the donor onto the new board and nothing changes.

I've measured pin 4 on U22 and I get 5V (reference: https://68kmla.org/bb/index.php?threads/lc-iii-no-video-no-sound-and-weird-schematic.39651/page-3 )

Are there any other voltages or continuity I should check? Thanks!

 

[Garrett]

Have a good look on the backside of the board at the solder joints for the ROM sockets. If you have no video or chime, my bet is that it's poor contact on the ROMs.

 

[charkes]

Have a good look on the backside of the board at the solder joints for the ROM sockets. If you have no video or chime, my bet is that it's poor contact on the ROMs.

Thanks for the suggestion. I reflowed the solder joints on the ROM sockets and added a bit of solder to each. I also reflowed the solder on every pin on the EGRET chip. Unfortunately, no change.

 

[charkes]

Pin 15 of the EGRET (U7) is at 5V so that chip is working. (reference: https://68kmla.org/bb/index.php?threads/lciii-is-not-working-no-picture-no-sound.37477/post-405734)

I'll take the advice in that thread and clean the heck out of the board with vinegar and then alcohol.

 

[charkes]

I've cleaned the board with vinegar, a toothbrush, distilled water, then alcohol. I tested the continuity of the EGRET chip and Pin 3 did *not* connect to the Y1 crystal, despite it being right there and the trace looking perfect. I added a bodge wire.

It chimes! (and reboots with command-control-power)

But there's no video displayed.