Quadra 840av chimes and powers back off

[error1]

I recently came across a Quadra 840av that was in a pretty sorry state after 20 years in a damp garage. I asked the seller to test if it was working and he said it more or less exploded when he plugged it in.

I still decided to buy it for $20 in case it was repairable, and the logic board actually doesn't look too horrible after cleaning it. No battery leakage!

The power supply was completely fried and full of old wet paper dust so I've gutted it and mounted a SFX pc psu inside it and made an adapter cable to still have soft power. So far so good.

The problem now is that the machine rapidly powers itself off if I insert any RAM, if I leave all the ram out of it I get a boot chime and then nothing.

I would like to know if any of you have seen similar behaviour from your Quadras and what you did to fix it. I am worried about the old PSU maybe having destroyed components if high voltage jumped to the low voltage side but it is still showing signs of life after all.

The board has been through the dishwasher twice and scrubbed with a ton of isopropyl to remove two decades of corrosion. The caps had not leaked visibly, but I've gone through and replaced all of them with new electrolytics. I'm at the point where I'm thinking about trying to desolder individual chips to check for corrosion and bridging under them (the VRAM chips especially have pins that are bent around the underside of the chip and hard to clean with the chip in place)

I've also ordered a set of tantalum capacitors to try another replacement in case the cheap electrolytics I had on hand aren't good enough.

If i hit cmd-power to interrupt it really early after powering up I can make it play the neat crash sound and stay powered up, I suspect the reason it powers back off is because it detects a short while trying to initialize graphics or scsi or something... It is frustrating because it feels like I am so close to getting it working

I've been told that getting the boot chime at least means my ROM and CPU is OK since the CPU has to be involved to play any sound at all.

https://www.youtube.com/watch?v=MD6iN1Umyd8

 

[techknight]

The fact that you rigged in a 3rd party power supply has me suspicious. 

For S&G, I would hardwire the soft power and see if it stays up and actually boots up. Or if it shuts down again (not enough power)

 

[error1]

The fact that you rigged in a 3rd party power supply has me suspicious. 

For S&G, I would hardwire the soft power and see if it stays up and actually boots up. Or if it shuts down again (not enough power)

It turns out you were spot on with this suspicion

I came across another Quadra 840av for sale locally that I bought, this one had battery acid damage but the rest of the machine looked in much better condition than my existing one. I tried swapping in my recapped logic board and lo and behold it boots just fine with an original PSU!

Now I have to try to repair the new logic board, maybe I can end up with two working Quadras! I'll use a beefier ATX supply for the next one, I think the small SFX supply I used maybe can't deliver enough amps on the 5v rail, or the thin wires in my adapter causes the voltage to drop too much.

Thanks for the suggestion!

 

[error1]

I'm really surprised by all the differences between my two logic boards, the working one has the ROM chips soldered and much less capacitor damage while the non-working one has a SIMM ROM and copper traces that look like they are about to detach from the substrate.

According to the wikipedia article for the 840av the soldered ROM is a later revision of the board. It has a much smaller CPU heatsink too.

Most worryingly, several tiny capacitors on the back of the board seem to have fallen off in the dishwasher, with one of them dangling by a single leg and another getting stuck to my hand when I removed the freshly washed board to dry it.

The dishwasher should not cause this, I suspect either someone has been tinkering with the board before (overclocking?) or there were several bad/cold solder joints.

I think those tiny caps are mostly there for signal filtering on the data and address bus and maybe not critical, but I'm having a hard time finding them on the old schematics I got. Anyone seen anything similar on other macs from the early 90s?

I measured the dangling one to 100nf, I'm tempted to just install new 100nf caps on the pads where it looks like the originals might have broken off.

I'm really happy the first logic board sprung to life, this second one is starting to look kinda rotten. No boot chime at all either