In the September issue of CCTV Solutions we identified the function of the primary deck components in a VHS VCR and looked at the basic servicing procedures relating to cleaning. We also identified some common fault symptoms. In this article we shall move on to discuss more involved servicing procedures including video head replacement and identify the precautions which must be taken when dismantling a VCR.
Let’s begin with a recap – and a little self test! Before moving on, look at these questions (box below) and see if you can answer them. All of this was covered in the September article …
Where do we begin?
If you had a go at the Q & A how did you do? The important thing is that we have once again identified some key fault symptoms.
Whatever the symptom and reason for removing the VCR covers, it is always best to begin by cleaning the entire mechanism, including all heads, tape guides and other tape path components, and tape transport components.
However, where should we go if fault symptoms of the nature described in the four questions/answers persist? Well, one of the most unreliable components in a VCR is the pinch roller, and thus a visual inspection of this is highly recommended. Remember that it generally becomes concave in shape, however in some cases the rubber becomes hard and shiney. The general rule is, if in doubt take it out (and replace it). The cost of this item is not high, and fitting a new one ensures without doubt that this part of the mechanism is functioning correctly.
Having established that the tape path exit point is correct, it is a good idea to do the same for the entry point. Thus a check of the back tension is called for.
Back tension brake band
In the majority of machines the back tension is controlled by a mechanical servo employing a felt brake band around the supply reel table and a tension post close to the full erase head (Fig 1). The felt pad is prone to wear, or sometimes it may detach from the brass or nylon brake band. The only way to detect wear is to check the back tension using some form of test cassette.
There are two types available; the torque cassette which has an in-built gauge giving a direct reading in gram-centimetres (gcm) when the PLAY mode is selected, or a test cassette with a specially recorded test card which indicates the amount of back tension by the degree of horizontal picture jitter. The torque gauge is available from SEME, (order code TEST603), while the test tape is available from SEME (order code MBSWISS4) and CPC (order code IN01115).
The torque gauge is by far the more effective tool in the long run as it lasts for many years. However it costs in the order of GB pound 150 and you cannot really use it effectively without knowing the specified reading for every machine you work with. On the other hand, costing around GB pound 25, the test cassette is far less expensive and there is no need to know the back tension specifications. However the tape has a limited number of uses.
Using the tension gauge, for the majority of machines the specified torque lies between 30 – 40gcm. Some however, may require a higher tension and you must consult the service manual or contact the manufacturer’s technical support to ascertain the specified torque for any machine. When using the test tape, the tension band is adjusted for zero jitter on the replayed test card.
If the back tension is incorrect it is best practice to replace the tension brake band, not forgetting to adjust it to the correct tension.
I should mention at this point that some machines – in particular a number of Mitsubishi models – do not employ a back tension brake band. Back tension is applied electronically using a motor, and from my experience this rarely requires adjustment. Basically, if there is no brake band in evidence you may assume that an electronic method is employed and a check of the torque will usually confirm that the tension is correct.
Once you have ascertained that the back tension is correct, you can now proceed to look at other parts of the machine in the confidence that the tape entry and exit points are correct. Note that it is not possible to perform the other adjustments that we shall be considering if the pinch roller is causing tape transport problems, or the back tension is incorrect.
Video head replacement
Replacing the upper drum assembly is usually a fairly straightforward mechanical procedure. However before the machine will function reliably it is necessary to adjust the entry and exit tape guide rollers (see Fig 2 overpage).
The four heads on the drum all differ slightly, and thus it is essential that the new drum is fitted the correct way around otherwise the machine will not function correctly. There is usually some form of indication such as arrows or different coloured wiring, but make sure that you have identified this before removing the old drum.
Drum replacement usually involves de-soldering the wires or terminals on the top of the drum (make sure that you don’t de-solder the actual heads. Look at the new head drum to identify these as they will already be soldered). Remove the two retaining screws and gently pull the head drum off. If it does not want to come, DO NOT under any circumstances attempt to lever it off; the most minute damage to the lip on the lower drum will result in permanent lines across the picture.
If the drum is stuck, try heating it with a hair dryer – that usually works. You can also purchase a head puller from CPC to make the job easier.
Fitting the new drum is simply a reversal of the removal procedure. When handling the drum you should be wearing cotton gloves as the oils in your skin can result in corrosion of the nickel plating in the months following replacement. This will lead to tracking problems. If you choose not to wear gloves, make sure that you clean the head drum following replacement.
Tape guide alignment
The entry/exit tape guide rollers (Fig 2) ensure that the tape is at the correct height at the video head drum. I cannot stress enough that this adjustment is critical and the slightest error will result in a degraded picture. A slight misalignment often results in an acceptable replayed picture in the 3Hr mode. However, timelapse or still mode performance is poor with tracking errors and/or vertical picture bounce. Alternatively, tapes recorded on the machine may replay correctly, but when replayed on any other machine the tracking is poor.Alignment can only be performed using an oscilloscope.
The procedure is as follows:
The guides are aligned by observing the FM video signal coming off the tape in the replay mode and adjusting them until a correct waveshape is achieved. It is important that a good quality recording is used as the playback test tape. You can make your own using a known good machine (preferably brand new) or you can purchase an alignment tape from a number of VCR spares suppliers, such as CPC or SEME.
Audio/control head
We looked earlier at the problems which can result in a missing CTL pulse, however one component that we did not consider was the Audio/Control head stack (A/CTL) itself (shown in our September article as Fig 3). As we saw in the September article, cycling noise bars resulting from loss of CTL can be caused by a dirty CTL head, however this head can also wear.
A worn CTL head is difficult to see. But this is not all that important because, if cleaning of the CTL head fails to resolve the problem (assuming that you have also checked/replaced the pinch roller), then it is almost certain that the head will require replacement. A/CTL head stack alignment is more involved than for the tape guides, yet it is just as critical and therefore the machine is best returned to a trained VCR service engineer.
Other considerations
For the service procedures we have considered in this article it should not be necessary to remove anything other than the top cover and perhaps raise a hinged PCB. Beware the temptation to remove other mechanical items without having been given the proper instruction. For example, simple and obvious faults in the tape transport area may be tempting but you would need to remove the cassette lift assembly. In some machines this is a straightforward operation, but in others the lift is meshed into a chain of tensioned gears and cams. As soon as it is removed all of these spin out of alignment and the entire mechanism may require realignment!
Thus, if you are the more adventurous type, consult the manufacturer first of all to determine whether or not the carriage can be removed without the need for re-alignment.
Spare parts for VCRs are generally available direct from manufacturer. Some manufacturers, however, are reluctant to release spares if you are not one of their appointed dealers.
Alternative sources of supply lie with trade suppliers, but make sure that you specify that you want manufacturer’s original spares. There are many pattern spares (direct equivalents) in circulation and while these are generally much less expensive, I have discovered that many of these fail long before one would like. These components are sometimes satisfactory in domestic VCRs, however for 24 hour continuous operation where the end product is video evidence, original spares should be used.
