Dell Optiplex 780 Schematic Diagram

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Lorin Mandaloniz

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Aug 3, 2024, 5:31:13 PM8/3/24

to acimcrewach

I disconnected all the IDE/Floppy ribbons and removed the ram, but still no video.
It does not even beep and error codes.
I see the keyboard lights work on power up.
but I don't think it is posting as I don't hear any beeps.

Mine is also dead sadly but I have a rough idea what is wrong with mine. Looks like a resistor on the power chain to the RTC IC (5v up to that resistor but only getting 2v out, the RTC IC needs 5v so I will have to swap that resistor out (assuming I'm right, just playing it by ear as I don't have a board view or schematic)).

Quick question, is the CD on the same IDE chain as the HDD or is it plugged into an expansion card of some kind. Mine only came with a floppy and as mine is dead I haven't been able to experiment with it yet.

Yes, Thats how I messed mine up. I was trying to add a CD-ROM drive on the same single IDE channel.
The best way to add a CDROM is with a dedicated controller, either on a sound card or ISA controller.
The bios does not support CDROM. So it wont pick it up.
Do you have any documentation for this computer ?

These should be running at 5V I think (based on rtc ic data sheet that indicates that AVDD is a 5V pin) but all I am getting is 2.2V. No idea where it goes from R135 as this is as far as I could trace it with a continuity meter (you may have better luck).

Not to do the typical "It's old! CHANGE THE CAPACITORS!" thing, but I do notice the solder joints around the SMD caps on my board are really dull looking, and yours appear to be the same... Are they leaking do you think?

The machine was working (obviously, dell didn't ship a dead unit (I hope)) but power up was intermittent. It would turn on and boot once, if you had to power cycle for some reason the machine was just completely catatonic when you tried for a second power up, no beeps no nothing.

I did manage to get into a boot up diagnostic (no idea how) which indicated the clocks weren't syncing (or words to that effect) which is why I started checking the clock chip for issues and found the incorrect voltages.

Also, FYI - I'm running a standard ATAPI CD-Rom and IDE hard drive on the same cable to the boards lone IDE port. The BIOS on these old machines will never detect a CD-ROM drive - the driver and MSCDEX takes care of getting it working.

Set HDD to Master, CD-Rom to slave. HDD on connector on cable that's closest to motherboard connector, CD-ROM on the other end.
Drive 0 in BioS set to your HDD settings then leave Drive 1 set to "Not Installed"

Strange, esp as they are the same model. I'll get a photo of my board tomorrow and post it then we can see just how much they differ. I assume they changed the design at some point in the models life and we have different versions.

Two notes on mine, I have the VRAM and L2 cache upgrades which explains the daughter boards above and to the right of the processor respectively. Both of which were removed during troubleshooting so we can rule them out as the culprits.

Also found the photo I took of the on board diagnostic results. Still have no clue how I got this running (maybe automatic, if so why hasn't it shown up again?) which shows something is very wrong on the timing front.

As for the PSU, it appears to be a run of the mill AT unit. Pin out is the same of every other PSU I've come across. And I have two for this model of machine both check out ok, voltages within normal range and work fine on other AT boards.

Caps are probably the culprit but figuring out the exact power chain is going to be problematic without a schematic or boardview to go by. Probing around testing for continuity only got me so far. I don't see any cracked joints but I'll give the board a more thorough going over when I can to rule that out.

Not sure if this will be much help but I found a list of all the part numbers of the machine. At least it yields the part numbers for both OEM and IBM parts (not sure why, did IBM use the same board somewhere?) which might pin down a viable schematic or boardview.

Thanks for the info and update. Please keep us informed as to your progress if you wish to continue.
Some day when I have time I might get around to checking all the Caps, resistors and diodes on my motherboard too.

Sorry for microwaving such an old thread, but I have 433 LV. It works, but once or twice it died on me (no display, keyboard blinks, fan spins). Not sure why ?
PSU seems to have different markings (it is p4 and p5), and I'm not sure if I can plug regular P8 and p9 connector into it.
Question, can you upgrade its CPU with just sliding 486DX2 or DX4?
Also, does anybody know how to get to bios in it?

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