[Eric Adams]

Every time I think I know what I'm doing, I get hit with something like this. (Although, once I solve the problem I sure feel great)
OK, 38-12. The motorboating is a putt-putt sound, about 2 per second. In other words not real fast. Doesn't change with volume or station position. It was there before I did anything.
What I've done so far: All tubes tested and replaced 78 and 6A7. All caps replaced. All micas tested, within specs. All resistors tested and almost all replaced. All coils and transformers tested ok. I can put a signal to the center terminal on the volume control and it comes in loud and clear, along with the putt-putt.
All voltages check ok except one. The screen (S) voltage on the 78 tube should be 90v and I have 27v.
So now I'm stuck. Any ideas?
http://www.nostalgiaair.org/PagesByModel...013236.pdf
Thanks.