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Quadra 700 powers on, but no chime or video

Yodd

Member
Hola folks!

I recently received a new to me Quadra 700. It was sold as not booting.

While it does power on (power led is on), there is no chime (I do hear a click/pop from the speaker) and no video. While the scsi drive spins up, it doesn't boot from it as far as I can tell.
Motherboard is squeaky clean with barely even a trace of dust. For a 30 year old machine, it doesn't look old at all. It almost looks like new old stock. Zero battery leakage as the previous owner pulled the battery out a decade ago before it ever leaked.

So far I have:
Removed all vram and dram memory simms. No change.
Installed a new PRAM battery. No change.
Disconnected both SCSI hard drive and Floppy drive. No change.
Tested the PSU outside the machine and the 5v/12v and -12v rails all look good. So PSU doesn't seem to be suspect.
To bypass the PSU, I built an ATX adapter cable and attempted to boot the machine from an ATX psu. Exact same thing as before. So I think I can rule out the PSU as being suspect.

I started probing around the board and found a whole bunch of the tantalum capacitors that appear to be shorted to ground. The ones in the middle section of the board front to back. Not the ones under the PSU and not the ones behind the expansion slots, but the ones in the middle of the board. Both sides of these caps appear to be grounded, so something, somewhere is shorting out and causing this. I doubt all these tantalum caps are bad, so my guess is some component in that region is causing it and that's what is preventing the machine from booting.




Anyone with ideas?
 

Yodd

Member
Looks like the 5v rail is shorted to ground somewhere. I can buzz out continuity from the motherboard power connector between ground any of the 3 5v pins. So I'm guessing a component somewhere on the board connected to 5v is toast. Couple be a cap or an IC of some kind.

Has anyone seen this? I'm not sure if there is a specific component that I should be looking at first.
 

Daniël

Well-known member
Has anyone seen this? I'm not sure if there is a specific component that I should be looking at first.

My Quadra 700 has a short somewhere as well, on the 5V line too I believe. I've not yet been able to debug mine, I've tried taking off tantalums on the 5V rail and that didn't fix the short. A short on the 12V line was due to a tantalum cap, that went away when I took that out.

The way to diagnose it would be to get a lab PSU and feed a slight bit of current onto the 5V line, and see what gets warm. Given I don't have a lab PSU, my Q700 has been lingering on a shelve ever since.
 

Yodd

Member
My Quadra 700 has a short somewhere as well, on the 5V line too I believe. I've not yet been able to debug mine, I've tried taking off tantalums on the 5V rail and that didn't fix the short. A short on the 12V line was due to a tantalum cap, that went away when I took that out.

The way to diagnose it would be to get a lab PSU and feed a slight bit of current onto the 5V line, and see what gets warm. Given I don't have a lab PSU, my Q700 has been lingering on a shelve ever since.

I wish I had access to a thermal camera like a Flir. That would make identifying the hot/shorted component a lot easier.
 

trag

Well-known member
Sticking your thumb on each chip to see what's warm can work too. Although, if you don't know how warm it would normally be...

Years ago, a friend sent me an S900 board to look at. Turns out there was short between the 5V and 3.3V supply inside the PCI-PCI bridge chip that Umax used for the four lower PCI slots. That chip was getting very hot.
 
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