Still Cant add memory

Tried three sticks of ram now and still no joy. firstly i purchased:
256mb PC133 SDRAM SoDimm Laptop Memory / RAM from a seller from hong kong who is a top rated seller on ebay, this didnt work.

Then i tried:
Kingston 256mb PC133 SDRAM SoDimm Laptop Memory / RAM which i no others have successfully used but no joy.

Then i wondered if i had purchased a recent akai whereas you may only be able to use there brand, so my last ram stick was a 128 akai ram stick specifically for the job and still nothing.

When installed it just says mpc 500 startup and stays on this screen. When i remove the ram, the same comes up but then the mpc loads with the 16mb listed.

I purchased the mpc off ebay for close to £200 then spent another small fortune on ram, now am skint and refuse to jump on the machine until setup properly with the extended ram.

I think this can only point to there being a problem with the mpc, i havent noticed any other faults though and theres no return from the seller, ive even been to a computer troubleshooter just to make sure i slotted the ram correctly.

Does anybody have any ideas to what could be wrong? could it need resetting ifso how? should i keep trying ram? or does it sound faulty?

go to ur local computer store and let them kno about the specs
let them test it for u and wat ever fit purchase
b4 some one behind u find it for cheap

After fiddling with the machine again today it turns out that it wouldn't even start up without the memory in which made me even more distraught.
Then i got to thinking that if it wont start maybe its due to power, so i have just returned with 6 AA battery's and its not only started but its loaded with memory to which is a major relief, so it turns out after all that its a faulty power supply cable, so its just down to finding one now.

got 1 for 15 dollars
give me ur email
and ill send u a plug thru paypal

You certain that you were seating the memory properly? The AA/power explanation doesn't sound right to me.