[Test-Equipment] HP 410B help

[Joseph Orgnero]

Hello all,
I'm trying to bring back to life one of these old workhorses but I cannot
get a proper reading. It zero sets OK but in trying to get  a reading, ex. 1
VDC, the needle drifts very slowly but never quite gets to the value and
when I disconnect the source it drifts again slowly towards zero without
quite reaching it. This happens in all settings (DC,AC, ohms). I changed
tubes and checked voltages and it all seems OK.
Any ideas as to what could be causing this problem?
Thanks
Joe Orgnero  VE7LBI