Zenith Transoceanic

[boora2]

Picked up one of these when I went to have a look at the PA at a dogtrack,rats,as usual,the guy said it just stopped,fitted a new battery,nothing,a CRO at the speaker terminal indicates plenty of life,nothing,replacement electros,nothing,however it will play fine if an external power supply is hooked up to the terminals,connect the battery,dies,supply foldback operates,current plus 40a before crowbar trips,any ideas,weird hf oscillation?