I have an HP204 audio generator, with shot batteries,
has anyone modified one of these to run strictly off
the 120v mains?

Paul-

I don't have any information about the older 204A or 204B. I purchased an HP
204C from someone on E-Bay, that turned out to have badly leaking batteries. I
could tell from the photograph that there had been leakage, so I bid
accordingly! The corrosion was so bad that it would have been difficult to
renovate the power supply. I also took the approach of changing it to an
AC-only power supply.

I was able to find a military manual on the web, TM 11-6625-1589-15-1, for the
HP 204C. I also found a manual for the HP 204C/204D, but it was incomplete.
Together they have about anything you need. I think you can find them by doing
a Google search.

It seems that the batteries are divided into four groups of five NiCd cells
each. They are wired in two pairs, for a plus 13 volts and minus 13 volts
supply. They normally operate in a trickle-charge mode, so each cell is about
1.3 volts. The AC battery charger provides sufficient current to operate the
oscillator and also charge the batteries.

The solution is simple. I used two twelve volt zener diodes, with each wired
in place of a pair of battery packs. The battery charger supplies sufficient
current so voltage doesn't drop below the zener knee of the diodes. Although
twelve is less than the usual thirteen volts, I found a reference in the
manuals that the power supply voltage was rated to be a minimum of plus and
minus eleven volts. Certainly the voltage would have been twelve volts nominal
when operating from batteries alone.

I can't find my notes, but believe the diodes used were 1N4742A, 12 Volt 5% 1
Watt zener diodes. Two are supplied in a pack from Radio Shack, stock number
276-563.

The hardest part was cleaning up the mess from the batteries!

73, Fred, K4DII