2021-07-09 Since I'm in need of additional projects (actually not at all) and practice with the new oscilloscope is definitely needed, I decided it was time to purchase a dead Quadra 605 motherboard. Seller listed as "Macintosh LC 475/Quadra 605 Logic Board Main Board Motherboard AS IS. Recapped but not working. Bad trace somewhere. Or my PSU is bad. Something I don’t feel like diagnosing." Further messaging back and forth didn't gain much additional info. So here are some pictures of the board after it arrived:
Top of Board (sorry on the glare - i don't have the knack for the polarization filter yet)
Apple 820-0364-A (C) 1993
Original 68LC040 @ 25MHz
Top of Board - RAM Side
Two Apple 630-0044 256K VRAM Modules
Top of Board - SCSI / Floppy/Power Side
Top of Board - PDS Side
Top of Board - I/O Port Side
Bottom of Board (Sorry for glare again)
Bottom of Board - PDS Side
Bottom of Board - I/O Port Side
Bottom of Board - RAM Side
Bottom of Board - SCSI / Floppy/ Power Side
So here we go... Swapped it in for a known good LCIII motherboard. Powered it on quickly... No smoke yet... Powered on again to check voltage on input side - normal, but my 12V rail is a little high. No Chimes, no video output. Fan Runs. HDD spins up. FDD does it's power up twitch. Nothing else. Significantly, no magic smoke... Course it could already have escaped. So I guess it wasn't his Power Supply. So now we probe the capacitors... huh...This is weird - no good voltage at C137, C136, C149 or C150. Wish I had a wiring schematic for this board. Time to unsolder and look at the board under the capacitors....
I think I found the problem around C137 and C136... Looks like the leaky electrolytic capacitors have struck again. (Red boxes highlight possible damage) There's at least 3 vias that need repaired and one broken trace on the bottom side of the board... I've never attempted repairs with such tiny vias before...
2021-09: I attempted 2 via repairs on the board... Pretty pleased with them although they look ugly when looked at under detail. Found some nice circuit board drill bits on eBay and managed to drill out two defective vias and pass some tiny wire through the now open holes. But still no boot yet... But I'm now stuck with a puzzling problem...the next coulple vias all connect to internal circuit board plane(s)... Is there multiple layers or just one? I can't seem to find any way of figuring this out... And no luck on wiring diagrams yet.... Any ideas?
Found a datasheet for the U32 DA7052A 1W Audio amplifier (PDF, 94KB). It appears we're missing connections to pins 1 and 8. Pin 1 is Vp and pin 8 is audio out (neg). But there is no copper between the via right below pin 1... Do we make a wild guess and assume its right? If only there was published repair information on this type of equipment!