We have mercury 845 gvm motherboard dead what can we do to repair

Electrolytic Capacitor replacement is about as far as you can go, unless you have a reflow machine, and access to purchase the various chipsets on the motherboard.

[Most of the time when it is stated that a motherboard is bad, I have found it was a bad Power Supply instead. LED lights light, fans may spin, but the Processor is not getting enough power to turn on. Try a KNOWN to be good, compatible Power Supply for a test unit. Perhaps there is an unused computer that you can borrow one out of for a test ]

Visual signs of capacitor failure. (Problem is they don't always show signs of failure. The Electrolytic Paste inside the capacitor can dry up ) {Motherboards and Power Supply's most generally stick with Radial Aluminum Electrolytic Capacitors}

http://www.capacitorlab.com/visible-failures/

A capacitor's values are rated in MicroFarads, and Voltage. A lot of Electrolytic Capacitors, (Radial Aluminum) are marked with a manufacturer code, and do not state uf or Volts. (The symbol for Microfarads looks like uf. This is actually the symbol, ?F. It is also abbreviated as MFD)

The main manufacturers are Japan and China, (Taiwan)
Info to help you decipher the manufacturer code markings,

http://www.hardwaresecrets.com/printpage/How-to-Identify-Japanese-Electrolytic-Capacitors/595

Radial Aluminum Electrolytic Capacitors are Cheap.
(I borrowed two 330uf 10 volt capacitors off of a new/old Pentium II motherboard, and installed them in a modem that needed it.

http://www.mouser.com/Passive-Components/Capacitors/Aluminum-Electrolytic-Capacitors/Aluminum-Electrolytic-Capacitors-Leaded/_/N-75hqw?Keyword=radial+aluminum+electrolytic+capacitor&FS=True

Soldering/De-soldering leaded capacitors:
http://www.youtube.com/watch?v=YCSNWi3UHf4&feature=related

(You're not going to see any 200 volt 'caps' on a motherboard. Power Supply? Yes)

For further questions concerning your problem, please state in a Comment.