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Notes on VCR Troubleshooting and Repair INDEX V.
Chapter 12) VIDEO PLAY AND RECORD PROBLEMS

12.1) Video playback problems

If the VCR works in all respects when tuning broadcast or cable channels but playing a tape results in no picture, a very snowy picture, or just a blue screen, there may be problems with the video heads, the lower cylinder, head preamps, or other video electronics. Testing most of these is beyond the scope of this document and will require a service manual and test equipment. However, you can do a decent job of determining if the video heads are likely to be at fault.

12.2) Video record problems

If attempting to record results in unexpected behavior, there could be a variety of causes depending on what you get for playback: Attempts to record are ignored by the VCR or cause the cassette to be ejected. This may mean the the record protect tab on the cassette is broken off or the record tab sense switch is dirty or bad.

Playback results in video snow. This means that the old recording is being erased (if there was one) but nothing or too weak a signal is being written by the video heads. This could be dues a variety of electronic faults as well as marginal or bad video heads.

Playback results in a picture but it has a wiggling rainbow pattern running through it. This is normal at the start of a recording made on top of an old recording if your VCR does not have a flying erase head. However, it should wipe down the screen in a few seconds and disappear. If it does not, then your full width erase head is not working.

Playback results in a flickering picture alternating between good video and snow at the frame rate (about 30 Hz for NTSC). This could mean that one of the heads used for record is dirty or defective.

Playback results in proper video but the previously recorded or no audio. The audio dub switch (if any) may be in the wrong position or the audio circuitry may be defective.

12.3) Snow on one or more speeds

Did the problem happen suddenly? Or develop over time? If suddenly, what were you watching at the time? A (literally) dirty rental movie? If this VCR has 4 or more heads, SP and EP may use a different set of heads, so certain heads may still be dirty or bad. If the machine tracks perfectly in EP, then alignment is probably fine - EP is more critical as to alignment as the EP track is 1/3 the width of the SP track.

Have the video heads been cleaned using the proper procedure (not just a cleaning tape - see section on video head cleaning). New video heads may fix this, though it can be caused by other problems such as weak read electronics. See the section on video head problems.

You should also check the backtension adjustment - if too loose, head to tape contact will be compromised. Try increasing it momentarily by pushing the backtension lever slightly to the left while the tape is playing. The usual way to adjust backtension without a backtension meter and service manual is to look at the image just before vertical retrace at the bottom of the screen - this is normally not visible unless you can reduce vertical size or play with vertical hold to get the vertical blanking bar to appear. Of course, most modern TVs don't have any such controls! This is the head switching point and when the backtension is properly set the image above and the bit of image below this break will be approximately aligned.

If increasing backtension helps, either the heads are marginal or the backtension was low. However, low backtension will usually show up as a waving or flagging effect at the top of the picture.

12.4) Jumpy picture in play

You have a VCR with known good heads that produces jumpy (vertically) video in play that cannot be stabilized with the tracking control. Perhaps you have attempted to adjust the mechanical tracking and maybe some other stuff. Some questions:
Did you replace the heads? Could you have gotten them 180 degree rotated from the correct position? I don't know what the implications would be on your model VCR, but there is a definite right and wrong on this. It would certainly show up as tracking being way out when attempting to play back tapes recorded on this VCR on another machine.

Exactly what adjustments did you touch?

Have you verified that the roller guides are fully engaged against the stops?

Have you checked backtension?

Did you touch roller guide height?
This is probably a mechanical problem, most likely an adjustment or fault related to tape path alignment. However, it could also be due to electronic problems with the video or servo circuitry. The vertical sync could be corrupted or the head switching point not set correctly.
The head switching point is 6.5 lines before vertical sync. If this ends up moving into vertical sync for some reason, you will get unstable video. The supply side roller guide height adjustment is also critical and would be the first thing to check mechanical alignment problems are suspected. However, don't overlook the obvious: your TV is marginal or misadjusted or you are attempting to play a bad tape.

12.5) Picture shakes or jumps or has snow in PAUSE/CUE/REV

Note that on a 2 head VCR, it is not possible to display a noise-free picture on a tape recorded at the SP or LP speeds. Therefore, for rental or prerecorded tapes, what you are seeing may be normal. A 2 head machine should execute these special effects perfectly fine with EP(SLP) recorded tapes, however. VCRs with 4 or more heads will usually have a V-Lock adjustment - either a knob on the front or rear panel, or sometimes 'conveniently' accessible from under the VCR. Sometimes, a special tool is needed to adjust this control. There may be separate adjustments for SP and EP(SLP) speeds as well. In any case, these settings are made while viewing a tape recorded at the appropriate speed in PAUSE mode.
For LP speed - which is being phased out by many manufacturers, at least for record - these special effects usually do not work well if at all. This is basically due to the nature of the sync signal alignment on tapes recorded at LP speed and would require complex circuitry to handle properly at anything other than normal LP play speed. (If you care, the sync tips between adjacent tracks align on the tape in SP and EP recorded tapes but are off by 1/2 line with LP recorded tapes. This results in the tearing seen in search modes with LP recorded tapes.) Since this tape speed is of little true value - it is a compromise anyhow - the added expense has been found not to be justified except on professional machines.
(From: Rob-L (rob-l@superlink.net)).
Some VCRs with +/- tracking buttons also use those buttons for adjusting the still-frame mode. If your VCR has them, try them. Other units may even have a little stem/knob you can turn for this. This assumes that you are talking about vertical jitter, which is caused by the two heads reading the same field on the paused tape and not lining the signals up. You actually get half as many lines as a moving picture. If the fields are off a lot, you get jitter.

12.6) VCR plays prerecorded tapes but its own recordings are noisy or jumpy (From: Richard (vcrtips@mail.vii.com)).

Almost all pre-recorded tapes are recorded at the SP speed. If you are like most people, you probably do your recordings at the EP speed (to get as much on the tape as possible). Do you have the same problem if you record at SP? Your VCR probably uses different heads for SP and EP. You may have dirty EP heads, defective EP heads, a head amp problem. Or, there could be a tape tension or other mechanical problem. (From: Frank D. Ralston (fdr@continet.com)).
Check the following:
Dirty (or worn) heads
Low back tension (common problem)
Tape path alignment (particularly input tape guide)

12.7) Incorrect frame alignment or bad video for part of frame

Symptoms like a picture which has a portion that is noisy or missing, or where the picture is split between top and bottom with the vertical blanking somewhere in between may indicate a problem with the PG sensor.
The rotational position reference for the video head drum is usually supplied by a pickup in close proximity to the edge of the lower cylinder (probably) which has a small magnet fastened to it. This generates the so called 'PG' pulse and is used by the servo circuitry to properly control the drum rotation and the head switching point. If this sensor is moved or if there is a fault in the PG circuitry, a variety of record or playback problems can result. Without this reference, the servo circuitry has no way of knowing where the A and B heads are at any given time. During record, this may result in recording video which is not properly lined up with the video tape - a track may consist of the end of one field and the beginning of the next rather than an entier fields as it should. During playback, the head switching point may occur at the wrong time resulting in a partially snowy or missing picture since a head that is not even in contact with the tape may be active. Similar problems may make look like your TV's vertical hold control is set incorrectly with the vertical blanking bar visible at an arbitrary point on the screen.
Beyond confirming that the pickup coil is in close proximity to the drum and that there are no bad connections or loose connectors, there is not much to be done for these problems without a service manual.
The definitions below are just FYI:
PG - pulse generator
The pulse is derived from the rotation of a magnet on the video head drum past a sensing coil. I suppose this could be done optically as well.
FG - frequency generator
This is a signal (sine or square) derived from the rotation of the video head drum. This may be phase locked to the PG pulse but can be a multiple of the frame rate. This could also refer to the capstan or reel rotation rather than the head drum.

12.8) Rainbow pattern in recordings made over previously recorded tapes

Unless your VCR has a flying erase head - located along with the normal video heads on the rotating drum - you will see a faint rainbow pattern near the start when recording over a previously used tape. The reason is that there is a separation of a few inches in the tape path between the video heads and the full width erase head. When you start recording at an arbitrary point, it takes several seconds (actual time depends on recording speed) totally erased tape to make it to the video heads. You are seeing an interference pattern between the old and new video signals. The pattern will slowly wipe from top to bottom as the diagonal tracks of new video intersect more and more of the erased tape. This effect will not occur (except possibly at the very beginning of the tape) as long as you record from start to end without backing up the tape at any time.

If the rainbow pattern is present whenever recording over previously recorded tapes and does not go away, then your full width erase head is not working. This could be due to an electronic failure or simply a bad connection to the full width erase head. Alternatively, a mechanical problem such as a broken or popped spring or gummed up lubrication might prevent the pivoting full width erase head from contacting the tape properly.

12.9) Flag waving

You have just loaded a videotape sent to you from your long lost cousin and you notice that the top of the picture is wiggling back and forth. First, if this wasn't the original complaint, make sure the flag waving problem exists with the TV that will actually be used with the VCR - it may just be your test TV or monitor that is unhappy.
(Parts of the following from: Andrew Morphitis, Andrew@andrewsm.demon.co.uk). This fault is sometimes known as flag-waving when associated with video recorders. If the tape back-tension provided by the tension arm and supply reel-table is not the same as the back-tension provided when the tape was recorded (possibly on another machine) then the field timing of the video tracks being played back will be inconsistent . Your back- tension can be checked using a back-tension cassette gauge (a typical reading would be about 35g-cm for VHS) or you could adjust the back tension using a known good test tape (or reliable pre-recorded tape) until the waving disappears. If your back-tension does turn out to be incorrect and you adjust it according to the manufacturers spec. then all of the tapes you have previously recorded will probably still exhibit this waving problem - adjust to spec. or to your tape library - take your pick.
Thats the theory - now the practice. Back-tension refers to the tension of the tape over the head drum, this is provided by the felt covered metal band (tension band) which is wrapped around the supply reel (left-hand reel from the front), the friction providing the tension. There are usually two adjustments associated with back tension and these can be found near the opposite ends of of this tension band, the tension arm operating position and the anchor point of the band. Adjusting the latter position will increase or decrease back tension (you will want to increase your back tension which has dropped due to excessive wear on the belt). If you do give it a bash then be aware that poorly adjusted back-tension can, at worst, give rise to premature head wear. Because of the differences between the back tensions of different machines, all modern TV's have a dedicated video channel button (usually channel 0) which has a shorter flywheel line-timing duration allowing the TV timebase to lock up more effectively to unstable video sources such as video machines. Are you using the video channel? - try playing the video through different channels on your TV.


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