It is a discontinued product and Fluke says the newer ones are better and don't give any service manuals directly.
I have two 1520 meters with the same issue.
The newer ones are not 'better',
smaller and lighter perhaps, big price tag, full of inferior spec multimeter features you may not need, but not 'better', that's just sales BS
The 1520 is in a working class of its own
I would check the selector dial area for dirt, dry tracks and bad contact if you haven't already.
Check the input sockets are not dirty or turned black and sooty.
See if the glass fuse is ok, and for any small board cracks
You might get lucky otherwise it may be the HV capacitor and diode array thingies
It's unlikely BOTH would have a tiny transformer problem
They are worth fixing but not at Fluke prices,
or sell them as 'parts or not working' on Ebay and take a look at the Fluke 1507 which you may like too.
Not as 'nice' a meter, no bargraph, no low ohms, but you get a 50v to 1000v range, and option for auto timed insulation tests
1507 is easy to use (no menu BS) and works great if you can't fix or get another 1520