TAPE GUIDES AND THEIR ALIGNMENT
The most demanding and critical adjustment procedures on the tape deck have to do with the tape guides. The number of guides on the deck varies with formats and threading arrangements, and is least with moving-arm M-wrap systems and most with loading-ring systems. All guides define the shape of the tape path; the more important ones determine the angle and running level of the tape across the heads.
Guide adjustment is normally only required when the guides
themselves have been replaced or disturbed by necessary replacement of associated parts. In all other cases of mistracking or tape- running error, every other possibility should be explored before breaking the factory paint seals and attempting to reset guides. Their adjustment is carried out with reference to (a) an oscilloscope trace of the envelope pattern of the output signal of the heads during replay; and (b) careful observation of the ‘lie’ of the tape ribbon on the guide and head surfaces it passes over.
Fig. 18.8(a) shows the sequence of components on a typical VHS deck. The two crucial guides are the entry and exit guide rollers at each end of the tape’s head wrap. Their height is initially set up by use of a jig which rests on the deck’s surface. Using an alignment tape in replay the height of the entry guide is now set for a flat shape at the beginning (LHS) of the envelope waveform, avoiding the ‘bot- tleneck’ effects shown in the waveforms of Fig. 18.9(a). When it is correct there should be no wrinkling or curling of the tape at the guide; if there is, trouble upstream (typically a bent tension pole, or incorrect supply-reel height) is indicated. The exit guide height is now trimmed for a flat waveform at the end of the head sweep. Correct setting is when the bottleneck effects at RHS of waveform (Fig. 18.9(b)) are eliminated.
A representation of the tape path of a V8 format videorecorder is given in Fig. 18.8(b). The first guide, no. 1, moves vertically and sets the running angle of the tape at the entrance side. It is set to poise the tape marginally above the guide rabbet at the entry side. Guide no. 2’s flanges do not contact the tape at all, and it has two adjust- ment points: height and tilt, the latter biasing the tape downwards onto the head rabbet, as per VHS practice. It is set for a flat entrance waveform. Guide 3 has little effect on tracking or tape running – it absorbs tape tension fluctuation, like the impedance roller described earlier for VHS.
At the exit side, guide 4 comes first, and has the same function as no. 3: it absorbs fluctuations in tape running. Guide 5 is the main exit governor, and like no. 2 is adjustable in respects of height and tilt. It is set for a flat exit waveform from the test tape. Alignment of guide no. 6, beyond the capstan, is concerned with tape running dur- ing reverse mode. Succeeding guides 7 and 8 are non-adjustable, being merely tape-steering rollers on the threading ring; guides 9 and 11 are deck-mounted pillars which define the tape’s routeing on its way into the take-up spool; they work in conjunction with guide no. 10, which in fact is a pull-out pole used for initial tape threading. Its action was illustrated in Fig. 18.4, where it forms one pole A.