@Shafi
.... i was going to test it against my 400Mhz HP54502A scope, but when i turned on my old reliable, she is no longer reliable. It popped up with Calibration Memory checksum error. So i spent quite a few hours trying to recalibrate it and failing, it seems that the old HP is dead now
Off Topic. if need continue, then other thread.
My opinion is that your HP54502A need only new NVRAM and then need do perfect recalibration just following exactly service manual.
(54502A is very sensitive in calibration prosedure and it fails very easy. Sometimes it need some small adjustment before selfcal go ok but inside scope adjustments need special knowledge and instrumets - other way it only go more bad. Also you need good perfect set of calibration cables. )
After NVRAM change it first need factory default and then do ALL steps exactly as service manual explain for calibrating - The manual does not contain any unnecessary words or sentences.
If selfcal fails in any step --> stop and start totally beginning with full factory defaults (reset all selfcal data).
If it fails all times in same step then need do service manual adjustments exactly with required instruments.
If still fails there is HW problem. (mostly littlebit bad input attenuator module.)
Selfcal cables need be perfect quality, many coaxial cables with cheap noname BNC connectors may cause problem becouse specially shield connectors are loose. Also old machine BNC connectors may be dirty. (use pure isopropyl alcohol or "dry" video cleaner. Front BNC' do not use whatever "contact spray", they may damage input attenuator module behind BNC)
BTW 54502A is good scope specially for timing measurements. It is 6 bit adc but it is not problem if use this scope for purposes for what it have designed. (extremely nice is that it have real 50 ohm inputs for rf work. This can not solve using 50 ohm terminator in normal high imbedance input scopes)