Hi Rob,
I tried mine with the car battery as well. Didn't see any difference. I believe a 10A power supply is good enough to test the unit. Mine is a 20A as well. I hope to find some information online so I can fix it myself. I know Konigs Klassik has a good reputation and I bought some parts for my Becker radios before and was happy with my purchase but sending the unit for repair is not cost effective for me. Apart from the repair cost, shipping solely is a killer. If you send yours for repair please let me know about the time and costs.
Regards,
Taha