Re: Dropping mains voltage to mains transformer approx 20V
248V is in spec for UK mains (230V +10%, -6%; 216-253V).
Without knowing the model number, I'd be surprised if this high quality HP device would be unhappy with this - if that was the case, the designers would have included taps on the transformer to adjust for the expected range of AC voltages. Remember - it used to be 240V +/-6% (226-254V) in the UK.
Following that thought-process through, that suggests to me that there is a thermal fault with the instrument. It could be that the series pass transistors could do with their thermal grease replacing and the screw connections re-torquing. It's also possible that one of them is dying - I've certainly had old power transistors play up when hot (current gain falls away). Could even be capacitors...
The voltages prior to the regulators will be nominal at best, and the designers would have taking into account the full range of expected voltages.
Give us the model number, and perhaps David (Radio Wrangler) might be able to comment further. But certainly, that's where I would go first.
Mark
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