Quadra 605/LC 475

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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)

Original 68LC040 @ 25MHz

Top of Board - VRAM/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!~~

2024-03-25: I've acquired a little more equipment since the last time I looked at this board over 2.5 years ago... An oscilloscope and a hot air rework station. I'm now able to confirm some things I couldn't before...

The first convenient tool is the hot air rework station... It is a cheapo, but it does let me lift the DA7052A IC off the board to check the via and the trace to the pin. This via is definitely bad and goes to pin 1 as guessed earlier. (there is no voltage on Pin 1 - should be +12VDC)

Flipping over to the backside of the board shows the trace and a test point right near by. We have +12VDC at the test point. Should be a fairly easy via to repair!

After repairing three vias (blue boxes in the two images above), I have a small amount of success! I finally have sad mac chimes. Unfortunately, I don't have a monitor setup to test video output and/or check and see if there are any codes. I still have at least a couple more vias to investigate and repair (red boxes in the above images). I'm feeling successful and don't want to push my luck too far!
Pin 22 of U30 connects to internal circuit board plane - beeps to C153 via closest to C193.
C137 + pad connects to internal circuit board plane - beeps to C178 pad closest to R105

2024-04-05: I got stalled by some questions about vias and decided it was a good idea to finally create an account at 68KMLA after being a lurker for way too long. You can read the post there if you want. Lets just say I got the exact schematics I needed to get the repair done! (Make sure you stop by the post and thank codevonlux after grabbing a copy of the schematics!) Board is now working! So how bad does it look? These are the last two pictures I'll probably take; board has been coated to hopefully prevent future damage to the repairs...

And this is where this particular page will probably end... I'll probably do a couple upgrades to it (more ram, maybe vram and I'd love to put a full 68040 in it.) I've spent a lot of time repairing this board and learned a nice bit. I'd love to learn more about the development of this board, especially about all the custom chips on it! Hopefully, this can assist others in getting their boards running again!

2024-04-22: Update: Shameless plug for MacMemories; Found a 64MB 72-Pin SIMM; bought and installed and now have 68MB of RAM available on this little machine. You can get one for yourself HERE!