1971 HMV Trent problem.
 

My 1971 HMV Trent television works, aside from one weird video problem. It seems that the vertical or something is screwed up, as it won't center when I use a digital signal like a converter box. Only on analogue signals it will stay centered.

It's TRYIN' to tell you something, but you AREN'T listening... (Grin)

Lol. I am turning 18 this year and hopefully I am going to rock this restoration, whether it is a bad tube or not.

If it a monochrome or color set? Mono sets typically have centering rings on the yoke, color sets typically have a control.

Is synch at black or white video level in your country? Here in the USA synch is black, and sets with poor synch detectors and vertical stages tend to loose centering or roll on letter boxed VS. native 4X3 content. Could be synch separator, vertical (more often drifted resistors/bad caps than tubes), or the AGC could be misadjusted.

Monochrome. We did not get color until 1975.

Do you still have true over-the-air analog signals in your country? What country are you in? The output of the "converter box" is an analog signal itself, but it could still be different enough that your TV set has trouble with it.

Is you system 625 lines 50 frames/sec. Before color became popular, your system was known as the CCIR standard.
Some refer to it a PAL, which only pertains to the color system, Phase Alternating Lines.
In the US, we had color since 1954, but it didn't start getting popular until about 1963/64. That's when more color programing became available.

The set is a VHF only set, which sucks...

And a VHF only tuner in a 1971 set?

Would I be right if I guessed you're in Australia?

Okay, I need to ask if your picture is merely off-center (but steady), or rolling (moving up or down the screen and reappearing on the other edge of it).

If it is steady, try the settings on your converter box (usually accessed by the remote control of the converter box) and try different "aspect ratio", "zoom", "display type","shape", "PAL", "NTSC", "50Hz" or "60Hz" settings.

If it is rolling, the sync signal is not as strong as the sync signal on analogue television broadcasts had been. Perhaps try a separate RF modulator (which could be a VCR) instead of the converter box's own.
If your television has valves, try replacing the valve that controls vertical sync.