HP 120b - Vertical vernier affects position

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I just picked this up on FBM, and after some contact cleaner, it's working okay - clean/bright/sharp trace, controls all work, etc. Enough to tell me that it's worth fully fixing up.

It's got one real functional issue. Adjusting the vertical amplitude via the vernier expands the trace as expected, but also moves the trace's position.

It's a pretty complex three-stage differential setup, with direct coupling and cross-coupled plate-grid feedback. The service manual just says to investigate the vertical amplifier ... Uh, thanks I guess? I know my common-cathode stuff, but not a lot about differential amplifiers. The 12AU7s test a little weak, but still serviceable (and I don't have any spares on hand anyway).

Before I dive into thoroughly analyzing the circuit, any quick ideas I should try?

(I'll also mention that vertical sensitivity isn't great, and LF response isn't that hot either. The calibration trace is awfully slopey. I suspect these are all related issues.)

[TimFox]

Have you tried adjusting the balance trim/control R16?  The usual method on older oscilloscopes is to adjust it until there is no change in the trace position when changing vertical gain.

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Have you tried adjusting the balance trim/control R16?  The usual method on older oscilloscopes is to adjust it until there is no change in the trace position when changing vertical gain.

I haven't, but I will. The description of R16 isn't explicit that it impacts trace position, so I missed that. Thanks, I'll try that tonight!

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Just following up on this - yep, tweaking R16 fixed it right up! Of course, I didn't realize that R16 is the "BAL" control on the front panel until I had pulled it from my shelf and opened it up 

I discovered that the "bad vertical sensitivity" issue was because someone had taken off the vert and horiz knobs and put them back on incorrectly, so that the vert was off by one position (i.e., it was on 10V/div when it looked like 1V/div) and the horiz was off by four positions. THAT got fixed for sure.

Ran it through a basic calibration and it looks good to go. Not even seeing any power supply ripple, which is a relief - that's a LOT of filter caps to replace!