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Head Gap Width and the Track width of VHS Hi Fi head?

 
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Head Gap Width and the Track width of VHS Hi Fi head?

Post by Alastair » Mon Feb 02, 2015 11:06 pm

Looking to find possible suitable head-chips for my N1700 experiments.....

Philips used 15 degree slant-azimuth 80uM head width with a 0.6uM gap for the N1700 heads.

I remember that VHS HiFi heads are also 15 degree slant-azimuth, but cant find the track width or the head-gap for these Hi Fi type heads....
--Anyone any ideas on that?

 
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Re: Head Gap Width and the Track width of VHS Hi Fi head?

Post by Michael Watterson » Tue Feb 03, 2015 10:47 am

Isn't the angle set by the drum to tape? I thought the head gap is perpendicular to drum?
Narrower gap gives higher extinction frequency.

 
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Re: Head Gap Width and the Track width of VHS Hi Fi head?

Post by Alastair » Tue Feb 03, 2015 12:19 pm

Hi Mike,
No--Only on guard-band machines, such as open-reel and the N1500 have straight heads.

To eliminate the wasted tape guard-bands occupy, the angle of the head-gap is twisted from the drum shaft vertical by plus/minus 6 degrees for VHS video heads, and 7 degrees for Beta The original N1700 had plus/minus 15 degree slant azimuth offsets.

This azimuth angle is with reference to the head drum shaft and not the horizontal plane of the machine mech.

This makes head 'A' not in line/azimuth with track recorded by head B and so head A cannot pick up head B tracks.--They become sorta invisible to that head, the reverse is also true and this reduces the crosstalk between tracks
The tracks can then be recorded with no guard-band at all, or even slightly overlapped to maximise the play time of the tape.

Research suggests that HiFi heads may not be much good, as the head gap is greater for the Depth Recording they do but cant find what that head gap actually is.

I have found that both HiFi and video head gap depth (from front in contact with tape) to the back (where the gap opens up) is Only 30 Microns thick and 25 Microns in some cases.
--Its a wonder they last as long as they do with only 30uM thickness to 'wear' away before the gap opens up and they are stuffed!
This info was from a Mitsumi datasheet, looks like they were major makers of head-chips.

 
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Re: Head Gap Width and the Track width of VHS Hi Fi head?

Post by Michael Watterson » Tue Feb 03, 2015 2:39 pm

That makes sense.
However if compatibility is no issue then you just need a pair of heads.
A) What is FM carrier centre frequency and deviation on N1700 and VHS (for same N1700 Video bandwidth on both)?
B) Difference in head to tape speed?
That tells you if VHS gap is less or more than 1700, A smaller gap on a replacement head isn't a problem, but a larger gap will affect the HF end of the FM modulation.

 
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Re: Head Gap Width and the Track width of VHS Hi Fi head?

Post by Alastair » Tue Feb 03, 2015 4:19 pm

VHS Video head gap is (I think) 0.3uM and for Beta its 0.4uM as the beta drum is somewhat larger so higher tape to head speed.

N1700 is 0.6uM so a VHS Video-head with half the size gap should easily record the 3.0 to 4.4MHz carrier used. The record-current will probably need changing as the gap size will alter the available flux.

I was asking about HiFi heads, as I have a couple of drums (of unknown origin) with those heads on. They are 30uM tracks and the gap unknown. It is known that the gaps are wider to allow for a higher flux needed to record the HiFi tracks Deep into the oxide-coating as the top is overwitten then by the next Video track, which is laid down in line with it.....

--This is how (VHS) Hi Fi works. The audio tracks are recorded first at around 1.8MHz carrier plus/minus 200KHz FM, but deep into the oxide, with a wide gap head, and the video track is then overlaid On Top, partially erasing the HiFi track.
Multi-layer recorded you could say....

This is one reason why HiFi sound tends to fail well before the picture on HiFi VHS machines with worn out heads, That annoying frying sound is the lack of usable carrier from the HiFi tracks, particularly at the start and ends of the track....

More I find out about the HiFi head-chips, the more I don't think they will be usable as Video heads on an N1700 drum.

I think I'll put a shout out in the wanted section for a Basic Two-Head VHS drum, like would be found in a 3V16/22 or similar pre LP machines in new condition--See whats out there....

 
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Re: Head Gap Width and the Track width of VHS Hi Fi head?

Post by Michael Watterson » Tue Feb 03, 2015 5:17 pm

You'd probably want video heads.
I forget about the lower carrrier helical sound system.

I may have heads taken from a Philips S-VHS where the transport died. I have actually 3 x working VHS. One is only used as a video modulator for B&W CCTV camera!


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