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VCR drum head 'revving' with poor picture in play mode - Hitachi VT-M922E(UK)

Discussion in 'Electronic Repair' started by [email protected], Dec 5, 2013.

  1. Guest

    Hi.

    I'm trying to repair a Hitachi VT-M922E(UK) VCR, a 2-head model, manual dated 1992.
    The previous owner said it had been in the loft for a few years.

    On inserting a cassette, the tape is loaded around the heads, and the videohead drum sounds like it's speeding up and slowing down rhythmically (although the exact rhythm varies) with a cycle of just over one second, and is quite loud.

    In play, I can sometimes see a rolling snap of the picture, although it's mostly noise rolling up the screen in some sort of sync with the revving. In-picture PAUSE, REV and CUE modes yield a normal, stable picture except for the usual bars of noise I expect from VHS. In those modes the head drum speed is stable and it's quiet. In the transition back to play the picturecan be recognisably stable for 1-2 seconds before the revving and instability return.

    The machine goes through phases where, after operating for a while, engaging and disengaging REV, CUE or PAUSE can sometimes leave it running stable, until I either stop and restart the tape, engage a special play mode other than the current 'magic' one, or briefly knock the top of drum with a pieceof chamois leather on the top. Switch the machine off and come back to itthe next day, and that 'magic mode' no longer works. The magic mode differs each time - they're not interchangeable.

    As far as audio is concerned when, when the head is revving...
    Connected through SCART or RF to my only remaining CRT TV, the audio is choppy (again in sync with the picture rolling and the revving). I connected it to a year-old LCD TV via SCART and the audio was completely stable even though the video was still rolling and the TV was blanking the screen thanks to all the noise.

    I have tried the following:
    * cleaning out the dust from around the rear vents
    * repeated manual cleaning of all heads
    - although I've not done a manual cleaning of the video heads since removing the auto-cleaning roller which I understand could have been putting crud back onto them
    * removed the top drum, revealing a fairly fine blond hair stuck to the underside of it (which I duely removed)
    * thinking perhaps the drum may have been sticking slightly, I applied a drop of sewing machine oil in the bottom of the lower drum (I took the magnetic cap off the bottom, revealing the motor coils and the shaft that spins the upper head, and put a drop of the oil on the outside edge of that shaftand allowed it to drain in for a couple of minutes. I then swabbed up leftover oil)
    * fiddling with the backtension lever during playback to see if I can briefly eraidicate it - picture became slightly stronger with extra tension there but revving and picture-rolling continued.

    None of the above seem to have produced any noticeable improvements.

    The Auto Tracking feature doesn't seem to be affected - the 'TRACKING' light blinks for a while and becomes steady within half a minute. Adjusting the tracking manually via remote doesn't help.

    I have noticed also that in reverse modes, particularly REV search, the tape seems to push up the A/C head and behind a hook, and make crinkling noises. Unsure whether this is related, but either way I'd like to sort that out too!!

    The machine also smells like something's getting quite warm. A heatsink inthe PSU gets quite hot to the touch (no exact temperature readings, sorry)..

    If anyone has any clues as to what's going wrong, or suggestions as to how I might fix any of the symptoms I've described, I'd appreciate your help.

    Thanks,

    Nick
     
  2. Guest

    Well, the first thing I would check is the three small lytics across the motor coils. Looking at them on a print you almost can't tell what they're for but I think they keep switching noise out of the Hall effect sensors. When they open up they do very strange things to the servo circuit. Very unpredictable things.

    Exactly why this doesn't affect cue and review, forget about trying to figure that out, it is either the problem or it isn't.

    There are four reference frequencies for the heads, different speeds for cue, review, pause and normal play. That may be why is only play is affected.

    And I will tell you this, when a VHS has more than a certain number of hours on it, do not use review. The tape path wear causes problems and usually does cause the tape to ride up in review. Unless you want to replace practically everything, just forget it. I'm not sure exactly what wears out but Ithink it is the lower drum and the bottom of the ACE head, something like that. For some reason even on full loaders (ones that stay loaded in FF andREW) as long as the pinch roller isn't engaged it is fine, but you just can't get the friggin tape tension right top to bottom. Like I said I suspectit is due to lower drum wear and you don't want to try to find one of those. You can feel with your finger on most of these that the tension at the bottom of the tape is higher than the tension at the top. This normallly wouldn't make sense but I believe the problem is that the fine finish is worn off. That causes more drag. The ONLY other thing is could be is a lack of backtension on the takeup reel in review. Either way, it's just not worth dealing with.

    These things are so old and unreliable at this point their main function isto dub to DVD.

    One possible test to see if it may be those caps on the drum motor is to use your finger to put a little drag on the drum as it plays. If the speed stabilises, there is a good chance it's those caps. They're usually low value, like maybe 0.47 to about 3.3 uF so testing them with an ESR meter is largely inconclusive.

    Give it a shot, and if someone wants to use review tell them to dub to DVD and do it on that. Seriously, don't waste your time trying to get new performance out of a worn out tape path. That aluminum is so damn soft that whenyou get the really grungy heads and have to scrub with the coffee filter or whatever, the black stuff is actually the aluminum. The oxide from the tape is brown.

    Of course if you have to scrub the upper drum like that the coffee filter is stationary and you turn the drum by hand. ALWAYS COUNTERCLOCKWISE. Many have "swept back" heads and if you turn one of those the wrong way it'll catch and wreck it.
     
  3. Guest

    Thank you for your detailed reply.

    If I put quite a bit of braking force on the drum (I used a piece of chamois leather) the speed stabilised and the picture was stable albeit with a blue tinge down the right hand side of the picture, until I released it. Toomuch pressure and the video output was pure noise.
    I suppose the next step's to locate and replace those capacitors...
    I'll see how far I get with giving the whole path a good clean, and leave it at that then for the REV thing. I can make do without REV.
    Thanks for the heads up on that.

    Cheers,

    Nick
     
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