Are we talking about a HP 1740A scope? if so I had a similar problem on a HP 1727A scope
, if not then disregard everything I am going to say. On mine the 120V was low, about 20V, I had to replace a couple of transistors on in the power supply including Q3. You should be able to replace Q3 with a PN2222A, that is what I put in my scope, and it has held up fine so far.
I should warn you that the rectifier bridges in these scopes are prone to failing, I have seen several instances of weird intermittent issues caused by the diodes, including my scope, although I have not replaced them yet I will soon.
You should get one of those cheap component testers on amazon or ebay, they are about 20 bucks and can come in their own acrylic case, worth their weight in gold in my opinion.