since the radio is used, have you done a through cleaning of the
switches and pots, took a peek inside, and lubricated the fan bearings?
Often these radios do not like to sit idle, once cleaned up, they work great.
Then you can start to study the schematics and use the test points to
verify what is going on inside the radio.
Otherwise you can end up looking for a problem that is not there and
replacing good parts for no reason.
can you tell us what the reading is on the meter for your HV high voltage
and what the idle current set to?
all the manuals for operation and service are in pdfhttp://www.n6wk.com/kenwood/
please join the ts-520 yahoo group if you have not done so already.