How to fix 990 erro code without changing a drum ?

An SC990 is usually an error indicating maintenance is overdue. The only way to verify that is by printing out the SMC report and seeing the subcodes. In the case of this model when is the last time the drum and developer was replaced?

sc 990 relates to the nv ram chip on the main board also check the values of the lazer unit are correct and not changed

Error code 990 indicates software problems yuo can check in SP7-403 for information.
Error code 540 will proberly indicate some heating trouble in the fix, you have to reset the machine with service code SP5-810.
Code 350 will disappear if you restart the copier.

Copy cartridge (drum unit) must be replaced
Waste toner full (replace drum cartridge)
Copy cartridge has run limit off copies

No. The CRUM is integral to the Drum Cartridge. The only way would be to swap CRUM from a cartridge that failed for a reason other that it reached end of life.

Although SC990 relates to a software issue, it is most often actually caused by an aging drum - These machines can be strange that way. I would start by replacing the drum, and recalibrating it - Make sure to also replace the PCR.

Please let me know if it works :)

if you are getting the sc990 code, you will need to have someone come out and do preventative maintenance (PM) on the machine. the 990 code has to do with the drum quality, and the only way to fix it is to, at least, have the drum unit rebulit.

There is a chip on the back of the drum unit that keeps count. It cannot be reset. You may try to find a replacement chip on the internet. I would recomment the rechargerstore.com They have good products. I do not know how long your drum will last, you may end up having to replace it for copy quality problems at some point.

ERROE E000602-0003

Error 990 can be cuased by many things. Most of the time it is related to the Hard Drive. you could try replacing it.