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Author Topic: Transports and tape handling  (Read 10564 times)

Offline stellavox

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Transports and tape handling
« on: November 29, 2009, 01:11:48 PM »
Finished working on a few machines and got to thinking:

We've had discussions about tape recorder "sound" but this mainly has focused on the electronics. How about a discussion of the (mechanical) transports themselves, how they handle tape and any "sound that they may impart?

I'm particularly interested to hear from those of you who may have made MECHANICAL modifications.  What did you do, and what did the modifications do to the sound? As most of us seem to confine tape deck use to reproduction only let's concentrate on that end but if you've done some work on the record end also please let us know. 

Let me start off with scrape filters/rollers; Has anyone tried with / without these - can you describe the audible difference?  Then there's tape path guides; most of the mods I have read seem to suggest replacing fixed guides with moving rollers, or putting Teflon tubing over fixed guide surfaces to reduce friction.  Doc seems to have done the most - what have you found?

An area where I've come across big differences between transports is the ability to play "imperfect" tapes.  Those of you who collect pre-records know exactly what I'm talking about. A lot of these can be cupped or otherwise stretched such that they "wobble" across the playback head causing image "wander".  I've found that many machines lack what I'll call good "tape wrap" (maybe good tape tension or support is a better description - or a combination of the three); I'll define good tape wrap/support as forcing the tape to lie flat against the head.  This appears to be aided by rollers or guides near the heads coupled with the amount that the tape is allowed to wrap around the head over the gap.  And just fact that some transports have servo drives to keep the tape under constant tension doesn't guarantee that they will play imperfect tapes well if the tape path around the head doesn't have good wrap / support. 

I have not had an opportunity to listen to pre-records played on many different machines.  The 1500 seems to do a good job as does the Stellavox.  Lyrec - not so good. 

Charles

Offline ironbut

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Re: Transports and tape handling
« Reply #1 on: November 29, 2009, 05:01:27 PM »
Ahh, a thread after my own heart.

For the last year or so, almost all my efforts to improve the sound of my Technics has centered around refining the way the tape is pulled across the heads and other mechanical aspects.
Where to start?

One of the things that shocked me was the switch from the stock guides (just above the capstan) to the bearing/ceramic guides that's part of Doc's tape path mods. This resulted in an opening up of the high frequencies. Very easily heard and I guess the shock was, with the capstan and pinch rollers between the playback heads and this guide, I had to wonder how much influence this could possibly effect.

I also had some new pinch rollers made since (upon examination with a magnifier) I noticed that the surface of the stock rollers was covered with tiny cracks. When I received the new rollers, I installed them and adjusted the pressure to the factory spec. I gave them a couple of weeks on my machine and no matter how I adjusted the pressure or tweaked the moment of contact (and release), the new rollers seemed to add a few veils to the sound. Very disappointing! When I installed my old rollers, no matter how I adjusted the pressure, they sounded much better. But, I did find that adjusting the pressure a little higher than spec, the sound was the best. The improvements in sound were also in the highs.
I suspect that it's the shape of these rollers that make a difference on the Technics. Here's why;

There was a great discussion of the 3M isoloop machines on either the Ampex or Studer list a while back. This discussion centered around the deformation of the tape at the pinch roller's point of contact. The size and dynamics of this contact patch varies with the rollers tension and the surface of the capstan. This all got pretty technical but suffice it to say that the stretching of the tape by the roller alters the absolute speed as the tape crosses the head gap. One of the things that I took away from this discussion was that the shape of the business end of the rollers has to be a factor.
So, having two rollers, the Technics needs to avoid the stretch at both ends of the isoloop from fighting each other. I think that investigation of the reaction time of the servo might be useful.

Another thing I've addressed have been the bearings used in different areas of the transport. I found that the best bearings seem to actually sound inferior to cheaper, looser bearings in some areas.
I can hear the "What?" ringing off the walls out there on this one.
On things like the capstan and reversing roller, getting the best bearing out there reduces cogging, preventing speed variations and noise transmitted to the tape/head interface. But these applications have considerable mass so the highest tolerances are preferred.
But when it comes to the tape tensioner bearings where the load is very small, a bearing that's relatively sloppy and is loose enough to instantly respond to speed variations and presents the least resistance to the tape is better. My guess is that as minor speed corrections are made by the servo, any drag presented by the tensioner bearings amplifies tape tension variations which makes the tension control overshoot what it really needed to keep tension constant during these tiny speed corrections. Like I said though, just a guess.

One thing that's ongoing with me is tweaking how the tape moves through each guide. Ideally, I'm trying to keep the tape from ever touching either side of the guide. Of course, tape to tape variations makes this impossible to do consistently but having the tape move down the center of each guide much of the time reduces scrap flutter. This is where the word "Obsession" really applies.

Past the above, the things I'm currently working on (or think I'm working on) is improving the way the headblock connects to the machine, and reducing motor noise transmitted to the headblock.

You probably know that I hang out at some of the restoration/archivist sites and I believe that a machines ability to play damaged tapes can be a different kettle of fish. This depends on how badly damaged the tapes are. Machines that have a great number of on the fly adjustments like custom headblocks and speed/tension adjustments are a leg up on any general purpose machine. Some of the archivists have headblocks that have easily adjustable heads which have repeatable settings. Special adapters made for playing cupped tapes can be seated into adjustable swing arms to hold the tape flat as it crosses the gap. I've rigged up my own using squirrel hair water color brushes (insanely soft) taped to the machine face. The results vary widely.

Well, as you can see, I just love doing this kinda stuff but for anyone who doesn't know it already, the way the tape moves across those heads is huge!
steve koto
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Offline High and Outside

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Re: Transports and tape handling
« Reply #2 on: November 30, 2009, 03:32:58 PM »

I also had some new pinch rollers made since ... Very disappointing! When I installed my old rollers, no matter how I adjusted the pressure, they sounded much better.

Interesting. I had a similar experience. I have never had the time to go back to it and run down what was going on. But the stock pinch rollers have stayed on ever since.

There was a great discussion of the 3M isoloop machines on either the Ampex or Studer list a while back. This discussion centered around the deformation of the tape at the pinch roller's point of contact. The size and dynamics of this contact patch varies with the rollers tension and the surface of the capstan. This all got pretty technical but suffice it to say that the stretching of the tape by the roller alters the absolute speed as the tape crosses the head gap. One of the things that I took away from this discussion was that the shape of the business end of the rollers has to be a factor.
So, having two rollers, the Technics needs to avoid the stretch at both ends of the isoloop from fighting each other.

For those folks who aren't familiar with the 3M Isoloop transport, it's got one really unique feature. The capstan has two different effective diameters. Not easy to describe, but imagine if you will one of the 2" machines, so the capstan is 2 inches tall in the tape contact area. Instead of a smooth cylinder, imagine that there are five bands about 2/5ths of an inch tall. The first, third and fifth bands are the full diameter, the second and fourth bands are recessed, in other words they have a diameter that is just slightly smaller than the rest.  There are two pinch rollers in an isoloop design, one where the tape enters the head assembly and one where the tape leaves. In the 3M design the incoming pinch roller contacts the recessed steps, and the outgoing pinch roller contacts the outer steps. Because the outgoing pinch roller is running on a larger diameter than the incoming one, it is trying to drive the tape at a slightly higher speed than the incoming roller is. The tape can only be running one speed at a time, so something's gotta give. A little of it is explained by the tape stretching in the loop portion, but mainly it's slippage on the incoming pinch roller. In any case, this is how the 3M creates tape tension across the heads: the incoming pinch roller is trying to slow down the tape.

The Technics of course does not have a stepped capstan, so it is not using this trick to generate tape tension. In fact I still don't understand how the Technics creates tape tension in the head area, though clearly it does. If you start it up with a little extra length of tape in the loop, which you can do by hand, it takes up the slack pretty quickly and runs with normal tension across the heads from then on.
Paul Stubblebine
Managing Director, The TapeProject

Offline ironbut

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Re: Transports and tape handling
« Reply #3 on: November 30, 2009, 05:30:50 PM »

The Technics of course does not have a stepped capstan, so it is not using this trick to generate tape tension. In fact I still don't understand how the Technics creates tape tension in the head area, though clearly it does. If you start it up with a little extra length of tape in the loop, which you can do by hand, it takes up the slack pretty quickly and runs with normal tension across the heads from then on.

This is how I see the pinch roller/tape/capstan interface on the RS15xx machines.
 The Technics rollers have a taper on the outer diameter. When it's engaged, there's only about 1/32 of an inch contact on either side of the tape width. With a regular roller (like the new ones I have), since there is no taper, the roller contacts the capstan close to 3/32" and because of the compliance of the rubber, it makes solid contact with the capstan. So with the Technics rollers, the capstan drives the tape which drives the rollers. On a normal roller, the capstan drives the roller and the roller and the capstan drives the tape.
And because of the smooth surface of the capstan and the relatively light contact the pinch roller has with it, the tape can slip on the capstan allowing the reel motors to adjust the tension.
I think this is the source of the "hiccup" in speed that can be seen on the strobe. When the tension differences overcomes the friction between the tape and the capstan, the tension is corrected by the tape slipping on the capstan surface. (the hiccup can most easily be seen when the tape speed is a little off and the lines of the strobe are moving one way or the other).
I'm betting that an increase in the reel motor torque would spread this hiccup out more evenly .
There are no audible clues that I've noticed that this is happening but I'm also betting that increasing the reel motors' overall torque somehow would help in the "snap" factor of the sound.
steve koto
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Offline docb

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Re: Transports and tape handling
« Reply #4 on: November 30, 2009, 07:29:43 PM »
Charles asked about the Teflon we use. That is only on the standoffs that mount the headblock. When we remove the fixed guides from the headblock (to reduce tape scrape) there is a slight possibility of the tape running against the standoffs in fast wind on some machines. So we put a little Teflon tubing over the bare metal standoffs, just in case. Tape oxide running over Teflon is like 320 grit carborundum on aluminum - takes it down flat in no time! On some RS1500s the guide shafts are too big to allow the use of ball bearings in place of the fixed metal bushing guides above the pinch rollers. We use fixed Rulon bushings in those situations. Rulon is a more rigid and more durable form of flouropolymer than Teflon, that is favored over Teflon for use as self lubricating bearings. They will eventually wear a flat and need to be rotated just like the stock stainless bushings, but they are definitely more slippery than stainless - which is reasonably slippery itself.
Dan "Doc B." Schmalle
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Offline Tim

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Re: Transports and tape handling
« Reply #5 on: November 30, 2009, 11:44:04 PM »
Someone asked how tape tension is derived inside the tape head loop.  It comes from the reel table motor tension differential:  in this deck, back-tension is always 10 grams higher than the take up tension. This tension differential easily goes "though" the pinch roller/capstan contact area and results in tension inside the loop. For example, if you were to reduce the back tension by 10 grams to equal the take-up, then the tension in the loop would drop to near zero. Because of this design, the reel motors need to have regulated tape-tension.  If they were un-regulated, the 10 gram differential could not be maintained, because tape-tension would change as tape-diameter changed.

« Last Edit: November 30, 2009, 11:49:15 PM by Tim »
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Offline Tubes n tapes

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Re: Transports and tape handling
« Reply #6 on: December 01, 2009, 02:12:46 AM »
Quote
Because of this design, the reel motors need to have regulated tape-tension.  If they were un-regulated, the 10 gram differential could not be maintained, because tape-tension would change as tape-diameter changed.

I don't know about other members of the RS15xx series but the RS1520 reel motors do not have regulated tape tension. It is my understanding that the tape tension within the isolated loop assumes substantially the same tape tension that exist outside the loop due to a very small amount of tape slip when the tape tensions inside and outside the loop are not equal. A very even and well controlled tape slip is therefore of crucial importance with an isolated tape loop. The pinch rollers play the most important role in that process, which make pinch rollers on isolated loop decks much more critical than with conventional transports. Questionable pinch rollers on an isolated loop deck will diminish any benefit that the isolated loop initially may have had.

For an RS1520 with its unregulated reel motors the tape tension at the beginning and end of the tape is higher than in the middle. Still the tape tension will not vary as much as with a deck with a normal capstan arrangement and unregulated reel motors, which start with a low tape tension at the beginning of the tape and end up with more than twice the tension at the end.
Arian Jansen.

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VP of technology of the Los Angeles and Orange County Audio Society (LAOCAS).
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Offline Tim

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Re: Transports and tape handling
« Reply #7 on: December 01, 2009, 03:16:40 AM »
All of the Technics RS series reel motors do have a simplified type of tape-tension regulation (as stated in both the owners and service manuals)....it's not a servo-loop design which uses feedback from the tension-arms (the best way to do tape tension control). The tension differential the motors create is transfered to the head-loop via the pinch roller/capstan junctions (which I eluded to in my first post).  The pinch roller/capstan definitely need to be operating correctly for it all to work. It's one of the weaknesses of the "iso-loop" design: tape tension needs to "pass though" the pinch-roller/capstan junction in order to "get to" the heads.  This, in practice, causes tape skew within the loop, causing the tape to buckle, hence, uneven wear patterns on the heads.  If you look closely at the head-wear and guide-wear inside the loop, you will often see that it is quite uneven.  Wavy, not straight, wear patterns. This leads to premature degradation of the heads. 

 
« Last Edit: December 03, 2009, 03:58:57 AM by Tim »
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Offline ofajen

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Re: Transports and tape handling
« Reply #8 on: December 30, 2009, 05:50:17 PM »
One difference I noticed between the more commonplace transport of, say, an AG-440 versus the 3M Isoloop machines I've used is the much lower audio band scrape flutter in the 3M machines.  Pretty obvious difference in the sound of a 10K pure tone.

Cheers,

Otto

Offline Brian C.

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Re: Transports and tape handling
« Reply #9 on: December 31, 2009, 03:30:49 AM »
FWIW 3M M79s were Bruce Swedein's machines of choice for 'Thriller'.
We have an 8-track M79 one-incher in the studio and it is still a superb performer.
Sonically outshines the 2-track Studer A80RC.

Brian.
« Last Edit: December 31, 2009, 03:32:31 AM by Brian C. »
Brian Clark, One Inch Studio, London, UK.

Offline ofajen

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Re: Transports and tape handling
« Reply #10 on: December 31, 2009, 10:37:35 AM »
FWIW 3M M79s were Bruce Swedein's machines of choice for 'Thriller'.
We have an 8-track M79 one-incher in the studio and it is still a superb performer.
Sonically outshines the 2-track Studer A80RC.

Brian.

Yeah, I still kinda miss having the M-56 8-track, since it sounded so good.  The down side was that I work all in one room where the transport noise was a nuisance, and I need as much space as possible for me and my instruments.  My M-79s had much quieter transports, at least at 15 ips.  If I'd had an M-79 8-track, I probably would have kept it. 

Right now I'm tracking to a little 1/2" Otari 8-track which has a very quiet transport and is generally quite good.  If I can scrape up the funds, there is a Stephens 16-track in my future.  The Stephens machines can be configured to be very compact (and portable), since they have no I/O transformers and the electronics are simple and compact. 

Cheers,

Otto

Offline Brian C.

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Re: Transports and tape handling
« Reply #11 on: December 31, 2009, 11:34:35 AM »
A Stephens! I know someone who was involved in building a couple of those way back. Light portables for mobile use.
An interesting and unique optical method was used to ensure speed stability I understand.

Brian.
Brian Clark, One Inch Studio, London, UK.

Offline ofajen

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Re: Transports and tape handling
« Reply #12 on: December 31, 2009, 11:54:46 AM »
A Stephens! I know someone who was involved in building a couple of those way back. Light portables for mobile use.
An interesting and unique optical method was used to ensure speed stability I understand.

Brian.

The earlier machines actually used 3M transports, but eventually 3M got tired of him stealing their recorder business using their own transports.  His later transports are capstanless and completely driven by servo-controlled reel motors.  You can even spool tape directly from one reel to the other.  Same basic size and layout as your basic 3M transport, but with the tiny channel electronics installed right on the front face of the transport.  No transformers on I/O which saves a lot of size, weight and cost.  Basically, the whole machine is that little transport, a power supply and a sync/meter panel. The 16-track weighs 102 pounds in a console.  The pieces only weigh about 65 pounds.

Cheers,

Otto