I am new at this and have the same problem with 3 ac/dc transistor radios. I am not following exactly what contacts are being shorted out. Is it the + and - contacts on the AC? I marked up photo with arrows would take the guesswork out for me.

 

I would say its the dc jack socket thats faulty, most mains/ battery radios switch off the battery positive lead via the mains lead jack socket.


Post pictures please

Yes. Try shorting the switch pins on the socket.

 

Repairing portable radio, AC works, batt doesn't. I have 2 radios from the 1960s-70s. Both are VISCOUNT 1860.
https://www.radiomuseum.org/r/viscount_18_transistor_acdc_afc_6_band_1860_japan_304.html

It has a direct AC plug-in with a two-prong connection. The AC works fine but the batteries do not work at all. It has an AC/DC switch, but switching it does not help. Both radios have the exact same issue.

 

If it has an AC/DC switch then that is where to start (and is most probably the problem part).
Contacts in switches get corroded over time. There are three possible solutions.
1. Vigorously move the switch back and forward many times. This makes the contacts rub over each other and may be sufficient to fix the problem.
2. Use some switch cleaner and get it inside the switch.
3. Replace the switch but it may be difficult to get an identical replacement switch.

 

Looking at that radio, it has a built in transformer, so probably no jack socket maybe an AC/DC switch, if neither then it will be a diode in series with the batteries.

 

Repairing portable radio, AC works, batt doesn't. I have 2 radios from the 1960s-70s. Both are VISCOUNT 1860.
https://www.radiomuseum.org/r/viscount_18_transistor_acdc_afc_6_band_1860_japan_304.html

It has a direct AC plug-in with a two-prong connection. The AC works fine but the batteries do not work at all. It has an AC/DC switch, but switching it does not help. Both radios have the exact same issue.View attachment 190113

Some AC plug in's have an auxiliary contact that disconnects the battery. Have you tried unplugging the AC connector?