TORQUE AND TENSION CHECKS
Torque is a turning force, and is applicable, in videorecorder service, to the tape spool turntables. It is measured in gram/centimetres (g-cm) the second term referring to the radius of the tape reel. The most important such reading is that of take-up torque, the ‘twisting power’ of the right-hand reel during record and playback modes. It must be sufficient to reliably pull in the tape issuing from the capstan, but not so great that the tape is stretched – a typical figure is 80 g-cm. To measure it a special torque gauge is available. It fits over the take-up turntable and has a spring-loaded dial/pointer scale on top, directly calibrated in g-cm; in use it is gently held at the rim, permitting the upper section to slowly rotate while the reading is being taken. An out-of-spec torque normally calls for cleaning, adjustment or replacement of the slipping-clutch in the take-up drive – generally a worn clutch will increase torque.
In fast-forward and rewind modes the torque will typically be more than 400 g-cm, and the absence of any form of driver clutch means that any shortcoming is likely to be due to slipping belts or idlers, or a defect in the drive motor or its electrical drive circuit. The type of torque gauge mentioned above can also be used for this check – an alternative in either case is a special cassette containing spring- loaded spools with dials and pointers instead of tape spools.
The most important tape-tension check is that of running back- tension, the degree of braking on the left-hand (supply) tape spool during record and playback. On this depends the video-head tip penetration into the tape; the chroma-luma image registration (via the ‘stretch-factor’ imparted to the tape); the skew-error (sideways tilt from head switch-point) in the picture; the intimacy of tape contact with the sound and control-track heads; correct interchange with other tapes and machines; and the risk of permanently damaging the tape. The correct figure varies between manufacturers and formats, depending on the number and type of guides between feed spool and video head: a typical figure is 30 g-cm for VHS.
There are several ways of measuring back-tension. The cheapest is the use of a spring-loaded straight- or sector-type tension gauge, which is in effect a weighing machine calibrated in grams. It is attached to the end of the tape of a full reel recovered from a discarded cassette, then (with the videorecorder in ‘play’ mode) the tape is passed over the back-tension pole and pulled to the right at approximately the correct (format) speed. The reading is taken from the scale as the gauge is moved. This is not easy or accurate, and a simpler (but much more expensive) method is to use a directly calibrated tension gauge (e.g. Tentelometer) which has two fixed arms and one central ‘deflectable’ one; the tape is threaded between these to give a direct readout of tension. In some machines, however, it is difficult to find room to fit this instrument’s probes into the tape path, and like the reel-and-pull system just described, account must be taken of the diameter of the tape reel on the feed spool in use. For these reasons, a third method is favourite: a specially produced cassette containing two spools linked by a few minutes’ worth of ordinary tape, which is played in the normal way while a calibrated dial on the feed side indicates back-tension directly in g-cm. They are available from videorecorder manufacturers and component distributors. Also available are alignment tapes with on-screen indication of back-tension and head-switch point.