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Vintage Television and Video Vintage television and video equipment, programmes, VCRs etc.

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Old 1st Feb 2017, 4:19 pm   #1
stevehertz
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Default Video head gap?

I am in possession of a new old stock video head for a Philips 1702 machine. Apparently the unit does not work - someone else has tried it in their machines. On examination using a strong magnifying glass, all looks fine and dandy with the two head chips - they look new - apart from the fact that there does not seem to be a true gap? the thin slither of ferrite (?) material seems to go thinner where the gap should be, but no visible gap as such. There is a gap behind this as the windings are clearly there on each half. If only I could take macro photos.. So, what should a close up of a video head look like? Should there be a visible gap under strong, hand held magnification? I'm guessing that the gap is far too small to see even with a strong glass. Thanks.
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Old 1st Feb 2017, 4:31 pm   #2
Vauxfan2k
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Default Re: Video head gap?

All the heads I have worked on have a gap of at least 0.5mm. I'm guessing if yours has no gap at all the top drum may have been pressed down or dropped too hard onto the lower drum while they were trying it.

Wait for some more opinions before doing anything with it though

2 examples of mine


Cheers
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Old 1st Feb 2017, 4:31 pm   #3
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Default Re: Video head gap?

Ref post#1: Very very very tiny.

Lawrence.

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Old 1st Feb 2017, 4:50 pm   #4
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Default Re: Video head gap?

Isn't the gap filled by some sort of epoxy or glass material?
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Old 1st Feb 2017, 5:02 pm   #5
stevehertz
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Default Re: Video head gap?

Quote:
Originally Posted by Vauxfan2k View Post
All the heads I have worked on have a gap of at least 0.5mm. I'm guessing if yours has no gap at all the top drum may have been pressed down or dropped too hard onto the lower drum while they were trying it.
Ahh.. I think you're getting mixed up a tad, not that gap. The gap in the actual face of the video head chips - as in the the gap in any tape machine head.
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Old 1st Feb 2017, 5:04 pm   #6
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Default Re: Video head gap?

Can't speak for your Philips but the head gap for VHS would be around 0.3 u m.

Lawrence.
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Old 1st Feb 2017, 5:16 pm   #7
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Default Re: Video head gap?

Not visible then.

Is there any way of - to some extent - checking a video head for problems without putting it in a machine? The windings both seem to read ok, and the connections to the circular, inner PCB tracks seem ok.
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Old 1st Feb 2017, 5:18 pm   #8
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Default Re: Video head gap?

Ah sorry.
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Old 1st Feb 2017, 5:28 pm   #9
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Default Re: Video head gap?

I seem to remember you could detect worn video heads by measuring their inductance (presumably as the head wears and the gap get bigger, the inductance drops). In the case of the Philips machines this is hard to do as IIRC the head disk includes half of the rotary transformer so you can't separate the head from it (electrically) and test the head on its own.

In any case you would need to know the right inductance value (presumably by measuring good and worn heads)
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Old 1st Feb 2017, 5:28 pm   #10
ms660
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Default Re: Video head gap?

Quote:
Originally Posted by stevehertz View Post
Is there any way of - to some extent - checking a video head for problems without putting it in a machine? The windings both seem to read ok, and the connections to the circular, inner PCB tracks seem ok.
You could check that the heads haven't been chipped/damaged, the only real proof of how good they are is to fit the head drum into a known good machine, not sure if any eccentricity adjustment is needed on yours, I only worked on the original Sony Betamax machines, they could be adjusted for minimum eccentricity, bit of an art if not used to it.

Lawrence.
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Old 1st Feb 2017, 7:25 pm   #11
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Default Re: Video head gap?

1700/02 heads just drop in, I've done it loads of times. Problem is, I don't have a working machine at the moment. Heads are absolutely fine, no sign of damage.
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Old 1st Feb 2017, 7:30 pm   #12
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Default Re: Video head gap?

Quote:
Originally Posted by TonyDuell View Post
I seem to remember you could detect worn video heads by measuring their inductance (presumably as the head wears and the gap get bigger, the inductance drops). In the case of the Philips machines this is hard to do as IIRC the head disk includes half of the rotary transformer so you can't separate the head from it (electrically) and test the head on its own.

In any case you would need to know the right inductance value (presumably by measuring good and worn heads)
Curiously I admit, this is a new old stock head disc, was in a sealed box; hence, no wear, not the issue.
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Old 1st Feb 2017, 9:05 pm   #13
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Default Re: Video head gap?

Vauxfan,

Have a look about halfway down this page for some helpful pics: http://goughlui.com/the-vhs-corner/i...-cassette-vcr/
e.g. this one: http://cdn2.goughlui.com/wp-content/...7-1024x287.jpg
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Old 1st Feb 2017, 9:16 pm   #14
Vauxfan2k
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Default Re: Video head gap?

Thanks nick, I know about the head gap, just misread the original post. Thought he meant the gap between the drum edge and the head.

Nice article, read it a few times.

Cheers
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Old 1st Feb 2017, 10:28 pm   #15
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Default Re: Video head gap?

I thought you would have known that, but as you say, it's an interesting article about VHS basics
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Old 2nd Feb 2017, 1:15 am   #16
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Default Re: Video head gap?

Quote:
Originally Posted by stevehertz View Post
Should there be a visible gap under strong, hand held magnification? I'm guessing that the gap is far too small to see even with a strong glass.
From memory there should be a gap there you can see, it's obviously tiny but you should be able to make it out on the machine with a light and even without magnification if you have young good eyes.
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Old 2nd Feb 2017, 3:28 pm   #17
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Default Re: Video head gap?

The gap is equivalent to a crack with perfect alignment. In other words you would only just about make this out. It's not something you could slip paper inbetween.
As for damaged heads this IS noticeable and usually if it's bad enough a coil or two will be visible as well. On the ones I've seen damaged my their owners.

..Kevin
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