Okay, strip the radio to the transformer and keep the secondary clean. First, get a good 12‑V supply if you can avoid the old AC. If you must keep the transformer, wire the primary with a gauge that matches the current—no 24‑AWG for a 150‑W radio. Use ferrite beads on each wire to cut noise, then run the secondary through a 0.1 µF ceramic and a 10 µF electrolytic to filter out ripple. Keep the wires parallel and at least a centimeter apart from the transformer windings to avoid stray coupling. Twist the input and output leads tightly and finish with heat‑shrink tubing; no messy braid. Ground the transformer case and the chassis separately—no shared ground, no hum. And for a nostalgic look, solder everything on a small perf board, then mount it in a metal box with a clear lid. If you think the radio is the problem, replace the transformer with a modern isolation transformer. Done.

Just make sure you double‑check the voltage ratings on the new transformer. If you see that hiss again, you’re probably missing a short or a bad connection. Keep the wires short, keep the heat‑shrink tight, and you’ll have a clean loop without the noise. Happy tinkering.