I had one of those years ago - was an old Navy Shipboard Studio Monitor. It had been modified for shipboard use by installing some isolation transformers. I fixed it twice - ate the SCS (SG613) one time and I had to troubleshoot the B+. From what I wrote on the Sams folder, I gather it was a 4.7uF 160V cap that opened up.
Second one was a bad solder joint - those early Trinitrons were famous for bad solder joints on the coils near the Horizontal (H?) board - also check the pincushion coils and transformers.
BTW, if you do blow the SG613, Prelco ( no affiliation! ) has them for 28 bucks.
But try to find an SG629........
Cheers