Replacing a missing/obsolete AC power supply with DC power supply
From the circuit it seems AC input is used to provide +/-5V rails, and capacitors on those inputs are rated for 16V. Also note that the ground is part of AC supply too. It is not single 20VAC power, it is 2 x 10VAC supplies.

I'd suggest either using center-tapped 2 x 9-10V transformer from some old device, if you can find one, or two 10-12V DC adapters.

The DC input is regulated to +12V and it has capacitor rated for 35V. Third DC supply of about 15-20V should be OK for this.

So, 3 DC adapters in total. Or you can find transformer with 3 secondary coils, rectify one in your adapter and connect them together for common ground.

UPDATE

After some digging on the web found this. It confirms my guess above how the whole thing was wired, although I did not expect two transformers in there. Good to know exact expected voltages too.