Uni-T UT804, FI2804MT calibration procedure

[Lajt]

Hi to everyone,

Some time ago I bought a bench multimeter: Française d'Instrumentation FI2804MT
Seems to be re branded Uni-T UT804 inside and out.

I bought it as defective. DC voltage was constantly 1500V and all else worked with no issue.
I was hoping for it to be some simple malfunction but everything was pointing towards ES51966A.
And yes after replacing the ES51966A it works as it should except that it is out of cal in all the ranges by a small amount.

I searched all the internet but could not find calibration instructions.
Sent e-mails to: Française d'Instrumentation and Uni-T but they never replied.
I even tried to cal it as UT71E since inside it is almost same as UT71D and Volcraft VC940 but it didn't work.

Does anybody here knows what is the cal procedure for this instrument?

Thank you