V-Hold constantly out and seemingly unadjustable (Matsui 1091)?

The V-hold is running a distortion from bottom to top of screen every 1 minute or so. No knob to adjust on exterioir of set or on light inspection of interior adjustment knobs.
Picture perfect apart from this annoying bit. Any ideas where v-hold can be tweaked on the PCB or other area, or indeed, if this isn't symptomatic of a non v-hold problem. Cheers, in adavnce.