NEC VCR A/C head adjustment

[Ola Tuvesson]

I have a NEC VHS machine that I've been using for capturing video with a
computer. It's not a proper pro machine and probably around ten years old
but has been working very well for me. Until recently that is.

I started having problems with some tapes dropping frames during capture
(indication of a poor sync signal) and thought all it needed was cleaning of
the Audio/Control head and possibly re-adjustment (sound was a bit muffled
as well).

Since I had the machine open I took the oportunity to check and clean all
drive belts, the idler wheel and the tape guides. Everything seemed in
perfect order but still no sync from some tapes (no, they are not copy
protected) so I followed the guidlines in the Repair FAQ for adjusting the
A/C head. First I adjusted the height for maximum volume and then tweaked
the azimuth for the high end. Wow! The sound was suddenly much clearer and
louder!

I put everyting back together and hooked the VCR up to my computer. Still
dropping frames! And what's worse, it now only plays about seven seconds of
a tape and then goes into safe mode! This happens in both play and record
mode but everything else works fine. Indeed, I can even set it to play and
then hit pause before the seven seconds are up and it will stay on,
outputting a razor sharp (for VHS) still image. Fast forward, rewind both
with and without picture work fine and sometimes I can even get it to play
continuosly after setting it to play hitting rewind with picture and then
back to play.

Obviously it's having problems picking up the sync signal from the tape and
is clever enough to think that no sync pulse means the tape is stuck
(although take-up weel and capstan are spinning correctly) and then enters
safe mode. Since it does (very occasionally) play properly I can rule out
any electrical fault and it seems clear to me that the A/C head adjustments
I made are the cause.

I've also realised I will never get it to give off a clean sync signal from
all tapes and that that's a job for a TBC. The problem is they're damn
expensive and if I can't fix this I'll have to fork out for a new VCR as
well!

How do I adjust the A/C head to get the sync back? Has anyone experienced
anything similar? Is there a way to stop the VCR from going into safe mode
so I get a chance to make adjustments (seven seconds is not long!)? I don't
have access to a scope so I'm not able to see the (missing) sync signal but
maybe there is another way?

Here are some photos of the A/C head assembly:
http://213.210.1.2/nec_dx_3000k/top_view.jpg
http://213.210.1.2/nec_dx_3000k/ac_head.jpg

Any help greatly appreciated!

Ola Tuvesson

[Leif Navrsted]

Hello
Don´t make any adjustments, bring everything back.
Change Pinch Roller!
Leif
"Ola Tuvesson" <nos...@for.me> skrev i meddelandet
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[Ola Tuvesson]

Thanks for your reply Leif! Unfortunately I've already messed things up by
making the adjustments in the first place. What I need to know is how to get
back... :) I guess I should have listened to the old rule "Don't fix what
isn't broken" but I never thought lost sync would cause the machine to stop
playing... The internals of the machine are in pretty good condition as it
has been used very little, this includes the pinch-roller and the A/C head
which show no sign of wear (look at the pictures).

Tack i alla fall!
Ola

Leif Navrsted <leif...@telia.se> wrote in message
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[Pedro English]

To ballpark it, look closely for slight wearmarks on the face of head, put
into record (not relying on reading CTL pulses) and adjust for best fit. And
check the paint on the screws.
All these problems could be due tape-run errors due to pinch/ loose
guides/tension probs

Ola Tuvesson <nos...@for.me> wrote in message
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[Ola Tuvesson]

I've managed to get the machine re-adjusted and it's now picking up the
control signal correctly! This nightmare should be a warning to anyone
wanting to adjust the alignment of the A/C head for better sound; You may be
able to get much improved sound but if your VCR loses the control signal it
may refuse to play at all and getting back can be difficult! Thanks to
everyone who replied, your help has been much appreciated.

Ola

Ola Tuvesson <nos...@for.me> wrote in message

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