Cooling fan motor grinds and very noisey would like to replace cooling fan,where ca i obtain oem part for this mg250dfx

I rather suspect IC1 TDA7293 is the cause of your problem. If this is the case you will need to check the speaker/s

Well without testing a bunch of parts. I would say some component in the clean preamp has changed. It may have happened over time and now you are noticeing it. without checking all the parts it would be hard to tell. Get it to a service center. Or you may send it in to Pro Line Music
215-736-8055

Usually an aluminum heat sink is connected to the fan or near it and an amplifier chip/stick is on that. If that is what smoked, you need to take it in for replacement. Continued powering of this could damage the speaker.

DO NOT FACE THE GUITAR TOWARDS YOUR AMP. IT WILL MAKE IT SQUEAL.

Since you just purchased this, verify that your guitar and cables are good on another amp and if they are good, then exercise your warranty on the Marshall.

Connectors and cables for guitars are chronic problems so verify they are good to avoid looking foolish.

If you have no experience with amp repairs you are better off having it serviced. This problem seems to be related to bad solder joints in the FDD circuit exacerbated by the cooling fan aging and wearing out. Namely R107 and R96.

Try carefully cleaning your cooling fan with qtips for the blades and a vacuumn. See if that helps the fan. If not it will need to be replaced. If you are experienced with amp repairs have a look for loose parts on the circuit board. If you can safely disassemble it, look at the soldered side for bad solder joints. From another poster on here with the same problem I have learned that resoldering R107 and R96 helped. Make sure they aren't burned out or have changed in value.They are in the FDD circuit. Have a look at your owners manual and it will make sense. The schematic is available here:http://www.ampix.org/displayimage.php?album=search&cat=0&pos=0 If it doesn't come up use the search function on that site. Do not attempt to service this yourself unless you are absolutely sure you won't hurt yourself or the amp.

I have read that the problem is in the FX send/return jacks which oxidize if not used much. All you have to do, apparently, is ram a jack plug in and out a bunch of times. This breaks up the oxidation, and allows the amp to work properly again.

There may be thermal sensor that protects the power amp that is sensing over temperature and turning off the power amplifier stage.

Try cooling with a fan and see if it lasts longer. If you are obstructing the cooling in any way, try to avoid that.

As a test, leave on for an hour and THEN try driving sound. If it works for a while, then driving is what is overheating it.

VERIFY that you are NOT running too low impedance speakers!!!

Most amps will drive down to 4 ohms. That is a single 4 ohm speaker or two 8 ohms in parallel MAX.