Hi mate,
If you had a similar issue to mine it was due to the -150V rail being off by about 2v. The adjustment for that rail is on the righthand side (viewed from the front) and down the bottom between two valves from memory. I no longer have the scope and have sold it to a bloke who uses it daily.
I would also check the valve and switch which connects whichever plugin module, as that commonly stops the ability to adjust the waveform from that module. I believe that was mentioned in this thread too.
The usual suspects aswell check that all the valves are glowing, and that the timed relay is clicking over its around 20seconds or so.
Good luck, I'd like to hear more.
Cheers!