“Apparently some transponders need to warm up briefly in standby (STBY) before they will operate in any other mode.”
The time depends on the age of the transponder. Because of the mode S uncertainty in the UK I fitted a second hand transponder and was given an old but serviceable encoder. The encoder requires 15 min to stabilise completely. I turn it from stby to alt as I line up as by then the errors will be quite small. The system is still working well after 5 years but may one day be upgraded to S.
Modern solid state units do not require warm up but banging an old style unit from off to Alt could spill some coffee as the errors will be 1000’s of feet.
Rod1