How to detect head wear, protrusion?

[TCMullet]

For all the years I was doing serious video/VCR work in which I ended it with 7 VCRs for duplication (RCA VR610HF), two Panasonic AG-1970s and a JVC BR-S800U (which had replaced a Panasonic AG-7750), I don't think I ever understood head wear correctly. (Or if I did, I forgot about it over the years.)

My question weeks ago was "how can we objectively determine how bad the heads are worn"? But I ran across two items of great interest in my old AG-1970 notes: (I had traded both my AG-1960s for 1970s.)

1. In a service receipt for an AG-1960 dated 6-25-1992, the tech wrote, "Head protrusion at about 50 microns; no wear shown".

2. A note handwritten by me taken as dictation from the owner of the dealer/repair shop: VHS and S-VHS heads protrude 45 microns new. Should be replaced at 10-15 microns.

So there we have it. Head wear isn't simply based on how bad the picture is. (It could be DIRTY heads or a "head clog" caused by one bad spot on a cheap tape or maybe even something else.) It's a function of how much the head element PROTRUDES outward from the upper cylinder (really both cylinders).

So the real question becomes how can one measure head protrusion? What are the methods, and most importantly HOW MUCH does each method cost? I now have 11 VCRs and 3 units from a relative. I want to get back into some serious non-money making video production (for eventual sharing freely on Youtube). I need to always be able to check head wear. But am also very poor. Gimme the bad news in dollars! THANK YOU ALL!

[latreche34]

Not many tools are around anymore, So it will not be an easy task, You can try a machinist dial indicator if you know how to set it up, There are also RF signal meters using a factory test tape with a reference signal recorded on it after cleaning the heads. But none of these are easy to find nowadays as I mentioned.

[TCMullet]

Quote:

Originally Posted by latreche34

Not many tools are around anymore, So it will not be an easy task, You can try a machinist dial indicator if you know how to set it up, There are also RF signal meters using a factory test tape with a reference signal recorded on it after cleaning the heads. But none of these are easy to find nowadays as I mentioned.

What would a VCR repair shop do to evaluate head wear? Can you give a link to a "machinist dial indicator" seeing as you're familiar with that term? Thanks! I was thinking of some kind of one-sided micrometer to carefully hold against the upper cylinder and the protruding head element. Or some kind of laser-beam tool. I'll ask a physics buddy. (But I still hope LordSmurf and all other experts here (self-proclaimed or not) will chime in with their insights. This seems to me to be a pretty important topic for all of us.

[timtape]

I'm not a specialist in VCRs and dont own a protusion gauge. Magnetic heads in rotating drums are a special case but generally protusion allows the tape to 'wrap' around the head under some back tension so that the maximum tape to head pressure appears at the apex of the head which is where the critical head gap lies. It's this gap area which wears down first, and tends to flatten the design radius on the head face. To some extent intimate contact between tape and head gap (minimal spacing loss) can be maintained even with wear by increasing tape back tension, but I guess many techs would simply go with checking b/t was within specs and replace the upper drum when rf signal became weak and/or protusion was below recommended, if they had the tool to measure it. If good rf signal cant be maintained when everything else is good and back tension is within specs that suggests the head is worn. Increasing back tension might improve performance at least for a while until upper drum replacement becomes necessary. Another aspect is that VCRs use Ferrite rotating heads for their quality of resistance to wear and long life. Ferrite stationary
heads can exhibit normal radius and protusion but still fail due to losing their fine mirror like finish. But I'm not sure if this can apply to Ferrite heads set in rotating drums as in VCRs. But to be frank, so often, poor picture in otherwise good VCRs is due simply to dirty heads, drums and other components in the tape paths. Clean the %&$#@ thing first, and thoroughly. Many don't. I do a lot of cleaning.

[Bypassall]

Tentel made a Head Protrusion and Eccentricity Gauge (HPG-1) and wayback has a copy of the manual at
https://archive.org/details/Tentel-HPG-1-Instruction-Manual/mode/2up

They go on Ebay occasionally for anywhere between $30 and $120.

I'm still too terrified to open up my machines to play with mine at the moment because this crazy instrument looks dangerous (for the heads and my peace of mind).

[latreche34]

I'm not entirely sure that techs used dial indicators on consumer machines, They check the RF signal with a special test tape, Studio machines like uMatic and the Betas did use some mechanical tools, My suggestion of a dial indicator came out of desperation due to lack of tools. What does the service manual say? Do you have a copy?

[dpalomaki]

FWIW: The commercial/industrial gear service manuals I have on hand (e.g., AG-1970, AG-1980) list head replacement based on hours. However, the service manuals for consumer gear do not list a service/replacement interval or criteria. The assumption appears to be consumer gear will be replaced when head wear effects picture excessively.