[Vlad95]

Hi Arran.
I continue to disassemble and I see that the interstage transformer is completely beyond repair. It is placed in an iron box and completely filled with bitumen. There is no way to clean and rewind it. But I plan to use for the first step the circuit upgrade we discussed above with a symmetrical choke instead of a phase inverter transformer. As a choke I plan to use a small 120-240V converter transformer, I have a couple of them, it should be popular for this purpose. In the future, I may find a ready-made transformer with a combination of about 120V + 120V + 120V or something like that. I can apply a similar method to the output transformer. I have changed similar parts several times to fix the problem of a broken output transformer in several of my portable tube radios. In this case, rewinding is extremely difficult, as the audio transformer in this radio is very small and has a huge number of turns of very thin wire. For this case, I used with good results a 120v x 6-12v power transformer from an old power supply from small devices such as a phone charger. The winding data for this transformer matched the original requirements quite closely.

[Arran]

Vlad;
It's possible that the tar/bitumen using to pot the box may have lead to the failure of the transformer, either that or the paper insulation, one or both may have been acidic, or hydroscopic, and corroded the wire. One nice feature of the metal box is that a more modern replacement transformer, or choke could be hidden inside. I think I melted the tar out of the box upon the suggestion that it could be repaired, I even went a step further and attempted to peel the paper back to find the break, I did not get very far. Somewhere I have a plastic container full of the laminations from this transformer, which I had the idea of maybe finding a substitute transformer bobbin that would fit the iron. The choke substitute is an interesting idea, though you do loose some element of amplification, I think I worked on a Canadian Westinghouse set that had a similar arrangement to your family battery radio from Ukraine, I never thought that the Soviet equivalent of a 1N5G could be used for audio output tubes.
Regards
Arran