Of Stripes and Recording ...

[Osi Osgood]

One thing I love about this forum is how, from time to time, I hear something completely new. Exactly, what is "relapping" sound heads, and, is it something that can be done safely at home, or is it something you must go to a professional for?

[Brian Fretwell]

I believe "Relapping" is replacing the worn tips of the head with new metal. A very precision job as you have to preseve the minute gap. Not something you could do at home as it would require expensive equipment to do it. There used to be a company that did it advertise in Movei Maker many year ago. I think Ged saud that Derann had thier recording machine heads relapped there.

As for the heavy wear fom paste stripe that could be made worse of the roller that should smooth out the paste before the mixture cures had the incorrect pressure and left a rounded profile so the centre of the head wore more quickly as all the epressure was on that. If you ran a film with a level stripe after not all of it would touch the heard surface.

[Tim Gillett]

Relapping magnetic tape heads has been going on for a very long time, perhaps since the beginning of tape recording in Germany in the 1930's. There will be fewer technicians relapping heads these days as the demand is smaller, but I know in the USA John French has been in the business for a long time and in the UK, Terry Summers of Summertone. There are probably others in Europe and elsewhere. If you visit a tape recording forum like Tapeheads, there's often discussion and close up photos related to head relapping and techniques. Relapping normally involves removing, not adding, metal from the head's face. Yes it's a delicate and skilled operation. Full relapping requires specialised machinery but a basic "hand" head relap can be performed using quite simple tools and supplies but again requires skill and patience for a good result.

As Brian mentions, the magnetic "gap" in the head is critical. A central aim in relapping is to allow the tape or stripe to contact that tiny gap intimately and consistently, at the right angles and at the right pressure. It's a very mechanical thing as well as magnetic.

[Brian Fretwell]

Yes at the home movie day I was told that 2" quad video head were often relapped.

[Steve Lee]

I remember years ago when we would run 70mm, it would depend if this was a long run or a one off screening to how we would show it. A one off would be done on changeovers and a long run with the film made up on a platter and always shown on the number one machine.

It was only one day while running on changeovers, that we noticed that there was terrible dropouts and the loss of the top end from the number one machine compared to the number two machine.

After inspecting the heads we found that they had worn down so much that they were flat compared to the convex ones on the number two machine, from showing a feature three or four times a day for weeks on end.

They ended up being sent away to be relapped, I remember this cost a arm and a leg back then, I can't imagin what the cost would be now.
We ended up installing more rollers to be able to take the film over to the number two machine, so it was possible to show the screenings on alternative machines.

It's a shame that all head manufacturers didn't follow Sony's Reel to Reel F&F heads that are finished with ferrite not only to the core but also to the guard, and achieves a head life approximately 200 times longer than that of conventional heads.

[Paul Adsett]

One of the great virtues of Elmo super 8mm projectors is that the magnetic sound heads seem to last forever. The Elmo heads used a special alloy called Sendust, and I do not know if this alloy was widely used in tape recorders or other film projectors or if it was propriatery to Elmo. By comparison Eumig heads are very soft and wear out in a few hundred hours of use, but they do produce amazing stereo sound quality.

[Tim Gillett]

One of the great virtues of Elmo super 8mm projectors is that the magnetic sound heads seem to last forever. The Elmo heads used a special alloy called Sendust, and I do not know if this alloy was widely used in tape recorders or other film projectors or if it was propriatery to Elmo. By comparison Eumig heads are very soft and wear out in a few hundred hours of use, but they do produce amazing stereo sound quality.

Thanks for that Paul. I didnt know they were Sendust but not surprised Elmo used Sendust. I know Sendust was used in upper range cassette deck heads. The Elmo heads have a very shallow face contour, almost flat, which perhaps also explains the relatively longer hours before problems develop. But they eventually develop problems. I saw an ST180 after many hours' use in a transfer shop and the record/play head had become quite concave at the gap. Very weak highs and lots of fluttery dropouts. I wasnt sure whether the head could be could relapped so I just informed the owner that after a good clean it was still playing back poorly compared to new performance.

Recently out of curiosity I got out my own ST 180 to test its playback performance. Compared to my modded Revox tape machine it was worse as expected but I decided to remove the head and inspect it. The head face had a minor divet at the gap so I went conservatively and just lapped down the surrounding face until level with the gap and polished the laminations areas to a mirror finish.

My standard test to evaluate playback of the treble on a stripe is to play back a bulk erased mag striped film. This gives me a "test signal" of bulk erased stripe hiss going up to the highest useable frequency. It's a very weak signal so is a severe test of the sound head as well as of the stripe's contact with the head gap. As well as listening, I monitor the hiss on a digital spectrograph display and note the hiss pattern. When all is well the hiss is clean, wide range and audible above the projector's own preamp's hiss.

I find the Elmo mag sound projectors have about the lowest hum compared to other sound projectors I've encountered. They also have enough easy access for cleaning around the sound head area and the capstan rollers without too much dismantling. Overall very well designed in my view.

[John Taylor]

Relapping heads is not something that could be done at home to my knowledge, I think if you replapped a eumig head it would disintegrate as Eumig heads are not particularly hard, good quality sound though but soft head.
A friend of mine reminded me, the process is similar to how a lens is professionally polished with a diamond paste and slurry, not too sure how this could be done at home, apart from that, just find a second head or a projector to salvage the head.
I must add though, playing with sound heads pre set at factory is one thing I would never attempt, once altered they are never the same. They have absolutely no reason what so ever to require alignment adjustment unless they are messed around with, a qualified engineer with the correct equipment is the only exception.

[Osi Osgood]

This has turned into a glorious, eye opening conversation, gentlemen ... Thanks! I have been fortunate with my Eumigs, the sound, when you have a really good soundtrack, still, to this day, sounds very good!

[Tim Gillett]

Relapping heads is not something that could be done at home to my knowledge,

I think that would depends on various factors including one's skill and the level of relapping attempted or achieved. Many very skilled people "work at home". Magnetic heads are small, even tiny. The basic equipment is not large or that expensive. Relapping heads "at home" is perfectly possible. It's probably the skill/knowledge/interest level which will be the determining factors.

I think if you replapped a eumig head it would disintegrate as Eumig heads are not particularly hard, good quality sound though but soft head.

I dont think you understand. In the factory, many magnetic heads are lapped. It's part of the manufacturing process. If what you said was true about the Eumig heads they could never have been made in the first place for they would have disintegrated. A relatively soft head will wear more quickly but it will not "disintegrate" any more than brass or Permalloy disintegrates when we machine or polish these materials.

A friend of mine reminded me, the process is similar to how a lens is professionally polished with a diamond paste and slurry, not too sure how this could be done at home, apart from that, just find a second head or a projector to salvage the head.

The very last stage of relapping is indeed fine polishing, and a diamond paste can be used as can a diamond dust coated plastic disc or other backing, (down to 1 micron size diamond particles) for a very fine finish. Some grind and polish with or without a suitable lubricant or other liquid to carry away the removed particles. It depends.

The problem with acquiring and fitting a used head from another projector is that it is used! It may be better or worse than the one, we are trying to improve on. In their heyday, magnetic heads were virtually consumables. They were readily available as a genuine spare part. These days the availability of new magnetic heads generally is vastly reduced due to vastly reduced demand but the remaining ones can command a high price.

I must add though, playing with sound heads pre set at factory is one thing I would never attempt, once altered they are never the same. They have absolutely no reason what so ever to require alignment adjustment unless they are messed around with, a qualified engineer with the correct equipment is the only exception.

It's standard good advice not to adjust a worn head. I dont do it myself and it's my trade. An exception I can think of is if someone has inexpertly adjusted head alignment screws, realized they'd made things worse and sought my help. If they didnt have the funds to have the head properly relapped or replaced I might recommend I just readjust the head back to where it was before the customer messed with it.

On the other hand when installing a new or relapped head, we start from scratch and we now must adjust all the angles - for maximum sound performance, care of the tapes/films/stripes and for maximum head life. There is no other way to do it properly.

[John Taylor]

So is there anyone who does this relapping work now, Tim, do you use jewellers rouge?
As for Eumig heads, they are a good quality sound unit but they are pretty soft,which is well know and they are not very hard wearing, nice compact projectors though,I think the Japaneese machine seem to have the best clearest and loudest sound.
I do know several collectors now thanks to this forum who have all said in all the years of collecting,some in excess of 40 years, they have never had to have a head machined in any way or form.

[Osi Osgood]

I wonder, how much brand new sound heads would cost now? The reason for the question is, if your lucky, you can get a used projector, perhaps even your make and model for spare parts, and when you look at all the parts available, just finding a replacement projector or not working projector, is great. I mean, yes, the heads may already be used, but could have a lot of use left to them.

[Tim Gillett]

I wonder, how much brand new sound heads would cost now? The reason for the question is, if your lucky, you can get a used projector, perhaps even your make and model for spare parts, and when you look at all the parts available, just finding a replacement projector or not working projector, is great. I mean, yes, the heads may already be used, but could have a lot of use left to them.

Here's one (below) from Wittner. They also advertise one for a range of Bauers but no price mentioned. Others from Van Eck also I think.

In a way it's a (sound) quality vs quantity issue. The most rapid head wear occurs initially on a new and unworn head, and then the wear rate slowly tails off, but so does the sound quality tail off, especially consistent sound quality. So if you're OK with less than optimal sound quality, leave the head alone and it will probably have a very long life until it finally fails completely. Relapping a head returns the head to better quality sound but at the cost of faster initial head wear, as with a new head. So being informed we can make a choice as to the tradeoffs we are prepared to make.


BOLEX SM8 Magnetic Sound Head, MKII version WCT SUPER 8 FILM SHOP (wittnercinetec.com)

[Lee McCaffrey]

The Bauer microprocessor studio models are an interesting case as they have 2 identical soundheads, one for recording and one for monitoring. Find a well-maintained model and you've got 2 soundheads at your disposal.
Another aspect that is often forgotten about is the pins/pressers that hold the soundstripe to the soundhead, these are plastic and will wear down and get flat, they can also end up with burrs or raised edges which can mark the edges of the frame on the emulsion side.

[Osi Osgood]

How would they get "burrs"? Bad splices on the film, broken sprocket edges? As a general rule, super 8 is usually not a Guage that has a lot of issues with being brutal to projectors.