A VCR is very much a mechanical entity and as such is prone to wear, especially when it is being made to operate for twenty-four hours per day, seven days per week. Anyone who has worked with timelapse VCRs will be well aware of their need for periodic servicing and when this requirement is not met, the user soon discovers that their replayed images are either non existent or too broken up and jittery to distinguish anything of value.
In this article we shall be looking at the standard maintenance requirements for VHS VCRs (remembering that the mechanical features in S-VHS machines are identical to those of VHS – it is the signal processing that differs) as well as giving some tips on common problems.
Before we can begin to discuss fault symptoms and servicing, it is important that the reader understands the basic functions of the various items in a VHS deck. Thus we shall begin with a brief overview of the VHS deck.The deck can be divided into two parts; the tape path and the tape transport. The tape path components are responsible for ensuring that the video heads scan the tape at precisely the correct angle in order to produce correct tracking, and that the control (CTL) and audio tracks are laid down in a linear fashion along the bottom and top edges of the tape. The tape transport components are responsible for moving the tape through the machine at a variety of speeds; 23.39mm/sec for ‘Standard Play’ mode, and a range of other speeds for visual search, fast wind and timelapse.
Fig 1 shows a typical tape path with the primary components numbered, while Figs 2 and 3 show a more detailed view of some components.
Let us take a look at the basic function of each of those in the above illustration.
2: Back Tension Arm:This maintains constant head/tape tension. In a simple machine, this is done by adjustment of a brake band on the supply spool. However, more elaborate models employ a motor beneath the supply spool, which is given a slight reverse current. Where a felt brake band is employed, periodic replacement is required, but adjustment can only be done with a torque gauge or a special test recording. Incorrect setting can give noise bars and may mean rapid head wear.
2, 13: Guide Poles: These static poles set the height of the tape at the full erase and audio heads. These never require adjustment – unless tampered with!
3: Full Erase Head:This erases all previously recorded video, CTL and audio information prior to re-recording.
4, 11: Impedance Rollers: These prevent tape flutter at entry and exit points. Note that on many machines one or both of these may have been omitted.
5, 10: Guide Rollers: These rollers provide a smooth, jitter-free movement at the two points where the tape has to make a 180 deg turn. They also set the precise point at which the video heads enter and exit the tape by altering the height of the tape. These are adjustable, and require re-adjustment whenever the video head drum is replaced. However there is no margin for error in this adjustment – especially in the case of a timelapse VCR – and these should not be tampered with unless you have been taught the correct alignment procedure.
6, 9: Slant Poles: These tilt the tape such that it lies flat against the video head drum. Their angle ensures correct tracking. They are non-adjustable.
7: Video Head Drum: This is officially termed the upper drum assembly. In a timelapse VCR it contains four video heads, although because two heads are often cut into one ferrite tip, to the naked eye the drum often appears to have only two video heads.
8: Lower Drum Assembly: This contains the drum motor which is responsible for rotating the head drum at 1500 RPM (25 rev/second) precisely. The machined edge – known as the ruler edge – sets the precise height of the tape at the drum assembly.
12: Audio/Control Head (A/CTL): This contains two separate heads; the top head is set to record (and replay) the audio signal along the top edge of the tape. The bottom head records (and replays) the 25 Hz CTL pulses along the bottom edge of the tape. This head will be found to have a number of alignment screws surrounding it, however correct alignment can only be achieved using an alignment tape and an oscilloscope.
14: Pinch Roller and 15 Capstan: The pinch – or pressure – roller presses the tape against the capstan. The capstan is rotating at a precise speed and the combined action with the pinch roller causes the tape to be pulled through the machine.
The tape transport components are much less prescriptive and a comparison of different decks would reveal a variety of designs. However the machine must contain some belt driven pulley & gear arrangement to drive the take-up and supply reel tables at various speeds. Clutch assemblies comprising of dual nylon gears or pulleys with felt pads in between are common to allow the take-up reel table speed to vary; bearing in mind that the tape is emerging from the capstan/pinch roller at a constant rate, but the diameter of the take up reel alters constantly as it fills up. Thus it is necessary for the reel rotation speed to reduce, and this is usually achieved using a slipping clutch.
During record, the 50Hz field sync is divided by two and recorded as the CTL track, hence the CTL frequency is 25Hz. When viewed on an oscillo-scope, the CTL signal appears as shown in Fig 4. The duty cycle (the ratio of period A to B) is 60 – 40 per cent. The CTL pulse is a very important signal in a VCR, as it performs a number of functions. The most important function is its use as a reference for the capstan servo during replay, without which the servo will constantly alter its speed (an effect known as ‘hunting’). This results in a replayed picture where the tracking is constantly moving in and out, producing a bar of noise which comes and goes every few seconds. In some cases the machine is designed to put out a mute signal such as a blue screen in the absence of a CTL track signal which will hide this effect. This is not particularly helpful to the service engineer! As well as maintaining correct tracking, the CTL pulse can also be used for other things such as the real time tape counter and event search (when an alarm event occurs, some machines momentarily alter the duty cycle of the CTL pulse to mark the point on the tape).
When the classic tracking fault described above is evident, then the first port of call for the service engineer must be the CTL signal.
Why four heads?
A VCR requires two video heads to record TV images. The tape is wrapped around the drum assembly by 186 deg and the heads scan the tape in turn. Rotating at a rate of 25 rev/second means that one head will record or replay the odd TV fields, and the other the even fields. Thus one revolution of the video head drum produces one TV frame.
The heads follow a diagonal path up the tape (Fig 5) where each track (TV field) is 49mm wide and is set at an angle of 5.3 deg (note that you rarely see it drawn at this angle). For correct replay tracking it is essential that the heads follow the tracks precisely. However the angle of 5.3 deg is determined, among other things, by the tape transport speed of 23.39mm/sec and if this alters, which it does during search and still frame, the tracking will be incorrect. To overcome this problem, timelapse machines employ two additional heads on the drum. During normal record and replay these are not used, however as soon as any other mode is entered these heads become active – sometimes in conjunction with the two standard heads and sometimes instead of these. In other words, four heads are required to obtain stable still images and clear visual search.
You may encounter some interesting effects when one of the heads becomes clogged with ferrite particles; i.e. ‘dirty’. If head 3 or 4 is clogged, the machine will continue to record and replay in 3Hr mode satisfactorily, however when ‘Still’ is selected the picture will break up. Similarly, if heads 1 or 2 become clogged, the machine will not record or replay, but it may produce clear still images from recordings made on other machines.
Perhaps the most common problem encountered with any VCR is a worn pinch roller. Because the roller is constantly pressed against the tape it can either become permanently distorted or hard and shiny (Fig 6). When this happens, the pressure on the tape becomes uneven, causing it to ride up or down the capstan. This in turn will cause the tape to curl on the exit tape guide pole (see Fig 1) and thus move away from either the CTL or audio head. In other words, the most common cause of loss of CTL pulse (and hence the cycling bands noise) is a worn pinch roller.
Pinch roller replacement is generally a simple process involving the removal of a retaining clip or screw. No re-alignment of the mechanism is necessary, but be aware that if the pinch roller is worn, other items in the deck must also be reaching the end of their serviceable life.
Other causes of loss of CTL pulse are a dirty (oxide build-up) CTL head, a dirty capstan, or a worn or mis-aligned A/CTL head. Cleaning will be dealt with in a moment, however alignment is more involved and is out of the scope of this particular article. The CTL pulse will also go astray if the bottom edge of the video tape is damaged. You may have come across a symptom where inspection of the tape inside the cassette reveals the bottom edge to be serrated. This can be caused by severe curling at the exit guide pole, or by other curling caused by incorrect back tension. Whatever the reason for the serrating, once a tape is in this condition it must not be used because it will not be possible to record a CTL track. Furthermore, any information on the tape will be unrecoverable because the machine will not be able to read the existing CTL track.
Grainy images or general poor tracking is most often due to dirty or worn video heads. The basic diagnostic procedure for this is simple; first clean the heads, and if the picture is little different, the heads must be worn. Although the actual head replacement is relatively simple, because a perfect still image is essential for a timelapse machine, the guide rollers (Fig 1 items 5/10), must be re-aligned using the correct procedure, which is not so simple and should only be attempted if you have had the correct instruction.
A machine that ejects the tape while leaving a loop hanging out of the cassette front is suffering from some form of tape transport problem. When ‘Stop’ is selected, the loading arms which carry the guide rollers and slant poles retract to a position in front of the cassette case. However the tape would be left in a loop around the head drum were it not for the action of the supply reel which performs a slow rewind action for a few seconds. If this function fails, a loop is left outside of the cassette.This may be caused by the failure of a clutch or idler, and the only way to diagnose this is to look at the operation of the machine while it is dismantled and observe the unloading operation a few times. However, failure of the unload action can often be caused by nothing more simple than a foreign body inside the machine; cassette labels are the most common. Thus, a quick visual inspection is always worthwhile.
Cleaning
Different areas of the machine require different cleaning techniques. The video heads may be cleaned using one of the many head cleaning cassettes on the market (the ones that use a ‘wet’ cleaning method generally work the best). However where a video head is severely clogged there is no better method of cleaning than the traditional manual method. This is as follows…
Use an aerosol isopropyl head cleaning solution and an approved cleaning swab. Do NOT spray the head directly, the sudden cooling action of the propellant has been known to cause the ferrite head tips to crack. Spray the solution onto the swab and rub each head tip SIDEWAYS about ten times. One vertical rub on the swab will break the head tip! How hard should you press? Look at your finger nail and when the blood just leaves the tip of your finger the pressure is about correct.
Use the swab to remove any other oxide from the surfaces of the upper and lower drums, paying particular attention to the corner of the ruler edge. Make sure that the last thing to touch the drum surface is the swab and not your fingers. Do not run the machine until all of the cleaning fluid has evaporated. If in doubt, wait about two minutes.
The A/CTL head can be cleaned using isopropyl fluid, but in this case feel free to rub as hard as you like. You will not damage this head by excess pressure, and the oxide build-up is often difficult to remove.
Oxide on the capstan is common and can be difficult to remove using isopropyl fluid. I have found that the best means of cleaning this (and any other stainless steel finished components) is metal polish. This will strip away the oxide very quickly, however make certain that you then remove all of the metal polish, otherwise it will do the same to the video tapes as they pass through! The pinch roller is always best replaced and not cleaned, however other rubber items such as idlers and belts can be cleaned using either methanol (which can be difficult to get a hold of) or mild detergent and water.
Finally, what should you expect to see replaced during an annual service? At the very least the rubber components must be replaced i.e. the pinch roller, belts and any idlers in the tape transport. The upper drum assembly should be replaced, even though these can often produce acceptable images beyond 12 months, because it is unlikely that it will function for two years.The back tension band (if incorporated) should be replaced and the mode switch (no room to discuss this in this article) cleaned or replaced.There is a school of thought that says the lower drum assembly must also be replaced annually. From my experience this is rarely necessary.
