Dell does not support Win 10 (no drivers) for this older model.  It may work but how well is questionable.

However, that is probably not the problem.  Which version of the 390? There is a Desktop, Mini-Tower and Small form factor model.

Do you get any beeps or diagnostic lights at power on?  Does the fan(s) work? Do you see a light on the motherboard?

There is no "Win 10 short cut key".  It either works or doesn't. From your description there is no video at all even at startup (correct?).

the word video card means PCI express GPU video card, and is in the x16 PCI-slot

to test the PC the card is rumoved.

remove dvd cables

and hdd cables

all USB things, as all things in all PCs can short out and we remove all those

last is all ram DDR but one, in bank0 channel A,  (test all ram 1 by 1 in that one slot) if the PC is dead or worse dead silent now, the PSU is dead. (99% ) most times the mobo is not bad, but sure can be.

spare PSU on EBAY.

spare mobo on ebay is  a whooping $11.  

remember there are 2 answer,  dells,  and what really works.

As you now know your dell.com support page shows no W10 support, end story just 2 clicks.

I do no not understand any  of this.

  Everything works fine on the other computer and television until I hook the monitor up to the Optiplex computer

not sure at all what you did there,?

so lets do DEAD PC 101: dell has dead PC page to but lets just do that.

1: you push the power button, what does it do turn blue/white or yellow or blink,?

2: the PC makes sounds (noise) , fans, does it? y/n it must. (and beeps got those?)

3: the PC then allows the DVD tray to eject, y/n does it, this is sign of 5v and 12v power good.)

4: did you remove the video card first then test the onboard video jack first?  oops. ?

5: the VGA monitor will show NO SIGNAL if the VGA port is dead, (called iGPU inside your CPU)

6: if all this fails do the strip down test. or answer above first, first things  first.

Just so you know, my 380 and 790  and up PC all run windows 10, even 64bit with the right processor plugged .

H61 chip set and gen 2 CORE i3 up. its not trash PC at all.  with this.

I have G41 running w10 32 and 64bit and all the way back to D945G chipsset, yah with P4 HT 651 cpu.

amazing no?  the sound can fail but who cares, plug in any sound card you want or usb for sound and w10.

I can prove this. Check out BOTH here>

or post a photo of your PC with the left lid removed, so we can see what inside, or tell it,?

A Desktop can have many things inside, what does yours have.

also Microsoft and intel do support most of what you have there,  not just dell.

unplug the HDD then all PCI-cards first.

then test HDMI (onboard) and VGA onboard. first. no GPU Cards first.

if silent and dead, and stripped down the PSU is bad if the power button cables not fallen off.

the x16 video card , slot has lock there,  do not force the Card out.

the lock lever is on the inboard side of the x16 slot you tickle it upward and the lock releases so the card x16 comes out.  this is how all X16 slots work every one.

also even my oldest dell the intel HD drivers work. from MS plug and ply, even up to 1600x900 resolution.

on some PC old the applications for security like fingerprints and all that are dead for W10 , so the CIA only cares right?  most folks only need at the max youtube to work ? that you? and firefox. and email.

The device manager in my oldest PC from dell or intel 2004 have  clean slate. in Device manager.

all from MS dot com, PNP

your core gen2 processer is not chopped liver. well not yet.

savvy2

Thanks for your reply.  I updated my post to answer your questions and included pictures. Since I am new to hardware diagnostics, I am not sure what to do next.

fireberd

Thanks for your reply.  I updated my post to answer your questions and included pictures.

Diagnostic lights 1 & 3 with blue power button light indicate a RAM error.

First, anytime working on an MB (motherboard), make sure computer is unplugged.

Remove RAM.  Reinsert one stick at a time.  Plug computer back in and start.  Does error repeat?  If not, continue procedure.  This is to rule out any bad RAM sticks.  Careful not to wiggle sticks up & down when inserting as to not break them.

It's possible simple reinsertion of the RAM could take care of the problem.  I've been there.  But when the Hynix RAM my refurb came with failed again, I replaced it.

What brand RAM do you have?  If the RAM isn't Dell, Crucial comes highly recommended by our Rockstars as it's guaranteed to work in Dell's and has a lifetime warranty.

To save money on an older machine, I use A-Tech RAM, which also has a lifetime warranty.

Kingston Value RAM (KVR) rarely works in Dell's.

In the 390, all RAM sticks should be the same brand, capacity, and speed.

Thanks for posting the photos.

A copy & paste from Dell Diagnostic Indicators:

Also, buying a used computer makes it a good time to replace the CMOS battery (on motherboard) - size 2032 Lithium.  A low battery can cause all sorts of bizarre problems.

Optiplex 390 Tech Guide