"author" pondered the futility of human existence in
an uncaring universe and yet found the courage to write:

The DX66 takes 4 type C batteries so the supply voltage must be 6V.
I should point out that there is a hum without the headphones but it is
less notisable when using the speaker. I became aware of the hum
after using headphones.

The schematic shows each end of transformer secondary connected to
a diode ...a filter cap across it ...and a transistor with a zener
diode on the base at the output of the power supply.

Can I modify this with something to get rid of the hum?

Rather than internally modifying the unit, it would be simpler to buy
or build a 6V regulated supply and connect it to the battery
terminals.

I don't know how experienced you are with a soldering iron but with
modern integrated-circuit regulators it is not at all hard to build a
decent supply. An easy approach is to start with a 12V wall wart (new
or salvaged from some other appliance) and use a LM317 regulator. You
need the higher voltage wall wart because the LM317 drops about 3V.
Google for "LM317" to get manufacturer's data sheets which contain
example circuits, or email me and I'll send them to you.


--
hambu n hambu hodo