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Author Topic: Oscilloscope for RTR Alignment.  (Read 30140 times)

Offline microstrip

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Re: Oscilloscope for RTR Alignment.
« Reply #30 on: August 29, 2009, 03:57:34 PM »
Although I can accept that a dedicated instrument is easier to use and faster, you have to consider that it can need calibration and service, something not easy to get nowadays and can be very expensive to get- sometimes an order of magnitude higher than purchase price.
I went through the excellent papers of Dale Manken at manquen.net, and after understanding the basics of his excellent paper on the ALTAIR T2DS I could emulate some of the measurements with a 24 bit 192 KHz sound card and appropriate software. Recently I have sorted some old and recent made pinch rollers to came to the conclusion that no recent one in my possession  surpasses in wow and flutter the original old Revox?s, although they look nicer! As soon as I go through the bearing tests I will post them to this forum - Arian suggestion for Revox was 100% accurate.
Do not forget that modern PC?s can perform very fast FFT?s with 131072 samples at 96kHz sampling rate with  .732 Hz resolution, more than adequate for this purpose.
All we have to do now is asking Doc to issue a 10 minutes reference tape at 15?recorded at 12.5 KHz in his excellent machines to make the measurements of scrape flutter much easier and more reliable. 
Francisco

Offline ironbut

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Re: Oscilloscope for RTR Alignment.
« Reply #31 on: August 29, 2009, 05:32:32 PM »
Recently I have sorted some old and recent made pinch rollers to came to the conclusion that no recent one in my possession  surpasses in wow and flutter the original old Revox?s, although they look nicer!


Hey, that's exactly what I came up with a while back.
The original rollers on my Technics are tapered so the contact surface is only about 1/16th of an inch on either side of the tape width (that's just a guestimate, it could be smaller). I believe the difference in performance is due to the amount of tape stretch that the deformation of the roller surface causes as it pushes  against the capstan. The wider (larger) contact patch of a new, more compliant or physically wider roller is in more intimate contact with the capstan surface around the tape or before and after the primary contact area. I believe that because the roller has a different diameter outside of the primary contact area, the capstan tries to rotate the roller surface at differing rates forcing the tape to either slip or stretch. Either way this causes speed irregularities which mimic the classic "stick/slip" of scrape flutter.
In a perfect world, a dual capstan/roller system might minimize this but in reality, it might be simply changing this from "stick/slip" to "slack/tight" which might also effect tension and tape/head contact too.
It seems that the amount of tape path friction could also have a part in this mechanism. The head profiles (contact area) and condition or any other guides seems to distribute the amount of tension at any given point differently and whether it's stick slip or slack, this tension determines where in the tape path these irregularities in speed are compensated.

Of course all these things are probably secondary to things like capstan surface finish and tape surfaces but, for the most part those are things that are design considerations and not really in our control to change.
That doesn't stop me from making home spun theories like the one I just outlined.

BTW For those who don't know better, these and many things I post here are the results of way too much time on my hands and a miss spent youth!
steve koto
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Offline 1audio

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Re: Oscilloscope for RTR Alignment.
« Reply #32 on: August 30, 2009, 09:34:09 PM »
I should have a signature below. I use the 1audio in many audio boards for consistency.

I just tried a close in measurement of a 1 KHz oscillator with an FFT running at 131K points and 96 KHz sampling. 10 averages didnt take long (but averaging might remove the flutter artifacts??) The frequenct resolution is there, I can see an artifact from something 20 Hz from the 1 KHz tone at -80 dB. It brings up lots of questions I have about digital systems since it should not be there. I'll dig out the analog spectrum analyzer and see if I can find it in the source (an analog oscillator). However its not easy to see from this if its a frequency modulation of the carrier or a tone from something else. I'll dig out the Mincom and see if it sees anything.

If you have lots of time you could look at the "violin string" resonance of the unsupported tape with an optical system. You might learn something. Fixing it would be the challenge. Dale Manquen suggests that above 12 KHz the tape itself damps the vibrations but below a lot can happen.

I have had to put in my own calibration resources. the ST1510 is a real pain to work on but seems to be very stable once its running. The Mincom also is very stable, self calibrating and seems to be built to run forever. Mine is at least 30 years old. Its an IRIG version with a lot of different carrier frequencies. But only the first few are useful.

The 3M design was based on stretching the tape with a stepped capstan and pinch rollers. It seemed to work very well. Some hate the design on philosophical grounds. I hope to have mine up by CES and I can get a better fix on it by then.

There was a process for resurfacing capstans with a satin finish that improved the grip and was supposed to reduce the flutter. That may all be lost art today.
Demian Martin
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Offline ironbut

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Re: Oscilloscope for RTR Alignment.
« Reply #33 on: August 31, 2009, 11:09:20 AM »
Hey Demian,
Yeah, the 3M machines don't get a lot of respect. I suspect that some of that is the whole parts availability thing but the stepped capstan isoloop does seem to stir up some debate. I came close to picking up a 4 track M79 a while back but just didn't have the room. Really interesting machines but a love/hate item it would seem.

I seem to recall it was the MM1200 that had problems with the capstan becoming polished and the tape slipping after a while. I read that some owners have taken to using emery paper to give them grip. That sounds pretty scary to me! I would think that a good machine shop (especially the ones that work on aircraft engines who work to insane tolerances) would be able to resurface a capstan with just a tiny bit better grip.
steve koto
 Sony scd 777es(R. Kern mods)> Vpi Aires>Dynavector XX-2mkll>Bent mu>CAT ultimate>CJ premeir 140>Magnepan 1.6qr(Jensen xover)Headphone Eddie Current Zana Deux>AT ad2000,HD800 ,Metric Halo ULN-2 (battery powered),
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Offline 1audio

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Re: Oscilloscope for RTR Alignment.
« Reply #34 on: August 31, 2009, 12:19:10 PM »
Keith Johnson first turned me on to the capstan refinishing thing. There was a shop in the Bay Area that resurfaced the capstan motors for the Ampex AG350's. It was some sort of bead blast finish. Rule 1 on AG350s, use the Bodine motors. The Ashland motors had design problems that limited their performance. Something about fields or heat, I don't remember what.

Keith also didn't like the stepped capstan. When I built up a 3M M23 for Reference Recordings (25 years ago now) he insisted that the pinch rollers be replaced with flat ones to defeat the differential drive. I'm not going to argue even though I'm not convinced. I would love to build a Steven's clone and ditch the pinch rollers completely. Or a machine like this: http://www.labguysworld.com/ChesterNewell_2.htm with no loop. But I have a real world to deal with.

If I get the 3M running well I'll invest in Manquen's servo upgrade for the drive.
Demian Martin
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Offline ironbut

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Re: Oscilloscope for RTR Alignment.
« Reply #35 on: August 31, 2009, 02:40:42 PM »
Yep, of all the machines out there, I'd love to get my hands on a Stephens. I believe that John Stephens started out modifying 3M M23's until the parent company stopped selling them to him (I wonder why? yuk yuk). His machines are still considered by many to have been the best sounding of any multi track although I believe he produced a 1/2" 2 track.

For any of those who've never heard of them, here's a link to a Stephens brochure;

http://www.dvq.com/hifi/images/stephens.pdf
steve koto
 Sony scd 777es(R. Kern mods)> Vpi Aires>Dynavector XX-2mkll>Bent mu>CAT ultimate>CJ premeir 140>Magnepan 1.6qr(Jensen xover)Headphone Eddie Current Zana Deux>AT ad2000,HD800 ,Metric Halo ULN-2 (battery powered),
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Offline microstrip

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Re: Oscilloscope for RTR Alignment.
« Reply #36 on: August 31, 2009, 05:21:21 PM »
My A700 has a capstan shaft with a highly polished surface, the A77, B77 and PR99 have a satin finish.
I can not remember anymore where I read it, but I had the idea that as the A700 is a constant tension transport, the best performance could be attained with a polished capstan.
MDIPrecisionMotorWorks can redo a capstan with a ceramic sleeving for around 175 usd.
Francisco

Offline ironbut

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Re: Oscilloscope for RTR Alignment.
« Reply #37 on: August 31, 2009, 08:42:51 PM »
Yeah, Fred Thal once told me that tape machines were designed as systems and changing things without taking the rest of the system into account will quite often degrade performance. I'm sure that Revox didn't spend the extra money to have that capstan polished for no reason so I'm sure you're right about the constant tension designs (it's the same on my Technics which is constant tension too).

I always wondered if continued design development had continued if they might have adopted pneumatics like on the big main frame computers at the time. That would have taken a total redesign of almost the entire transport/tape path. Maybe there are other reasons why it wouldn't have worked since video tape development continued and I don't believe they ever adopted pneumatics.
steve koto
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Offline 1audio

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Re: Oscilloscope for RTR Alignment.
« Reply #38 on: September 01, 2009, 12:49:40 AM »
I worked on those big tape drives briefly. There is no way they could be economical enough for use in anything but a mainframe. They were very complex. The vacuum columns were not for smooth drive, they were to permit reading and writing tape record by record. They could start and stop in a millisecond. The vacuum capstans had voice coil valves behing them to switch between air and vacuum. The columns had opto sensors to control the reel motors, which would push tape in and pull it out. Flutter was not a big issue.
Demian Martin
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Offline c1ferrari

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Re: Oscilloscope for RTR Alignment.
« Reply #39 on: November 21, 2009, 09:53:55 PM »
Utterly OT: ironbut (post 8/31/09) - thanks for the pdf...that cover is alright!!  1audio (8/31/09) - thanks for the link...loved it!  -Sam
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Offline ofajen

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Re: Oscilloscope for RTR Alignment.
« Reply #40 on: December 31, 2009, 01:20:49 PM »
Keith Johnson first turned me on to the capstan refinishing thing. There was a shop in the Bay Area that resurfaced the capstan motors for the Ampex AG350's. It was some sort of bead blast finish. Rule 1 on AG350s, use the Bodine motors. The Ashland motors had design problems that limited their performance. Something about fields or heat, I don't remember what.

Keith also didn't like the stepped capstan. When I built up a 3M M23 for Reference Recordings (25 years ago now) he insisted that the pinch rollers be replaced with flat ones to defeat the differential drive. I'm not going to argue even though I'm not convinced. I would love to build a Steven's clone and ditch the pinch rollers completely. Or a machine like this: http://www.labguysworld.com/ChesterNewell_2.htm with no loop. But I have a real world to deal with.

If I get the 3M running well I'll invest in Manquen's servo upgrade for the drive.

You might get a kick out of this.  This is my M-23 that I finally got up and going this summer.  A rack mount transport that I suspect came from a film sync machine and two electronics channels I got from JRF's machine, all housed in a little console I put together.  Once I sold my M-79 2-track, the M-23 restoration became more urgent.

http://i203.photobucket.com/albums/aa115/ofajen/IMG_8196.jpg

BTW, will Dale actually sell you the servo upgrade for the M-23 drive?  He's helped me a ton over the years in keeping my other 3Ms going and in getting this M-23 together, but when I asked about the servo upgrade, he said he'd got most of them back (mainly from the Rochester School of Music) and didn't want to sell any more because he didn't want to be responsible for supporting them.

Cheers,

Otto

Offline ofajen

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Re: Oscilloscope for RTR Alignment.
« Reply #41 on: January 08, 2010, 09:02:31 AM »
Yep, of all the machines out there, I'd love to get my hands on a Stephens. I believe that John Stephens started out modifying 3M M23's until the parent company stopped selling them to him (I wonder why? yuk yuk). His machines are still considered by many to have been the best sounding of any multi track although I believe he produced a 1/2" 2 track.

For any of those who've never heard of them, here's a link to a Stephens brochure;

http://www.dvq.com/hifi/images/stephens.pdf

I'm working on picking up a certain Stephens 2" 16-track machine some time this year.  We've agreed on a price, but the seller is in no hurry, which is just as well, since it seems to be taking me a long time to get the funds together.  The most compelling feature for me is actually the portability of the machine.  The guts (transport/meter panel/PSU) weigh a total of about 65 pounds and work nicely in two portable cases.

Cheers,

Otto

Offline ironbut

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Re: Oscilloscope for RTR Alignment.
« Reply #42 on: January 08, 2010, 01:25:26 PM »
That's a beautiful M23 Otto! Fantastic work there.

Perhaps you could give us a little detail regarding the servo upgrade you mentioned.

I seem to remember that Dale was working on a way to do away with the stepped capstan also.
steve koto
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Offline ofajen

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Re: Oscilloscope for RTR Alignment.
« Reply #43 on: January 08, 2010, 03:05:24 PM »
That's a beautiful M23 Otto! Fantastic work there.

Perhaps you could give us a little detail regarding the servo upgrade you mentioned.

I seem to remember that Dale was working on a way to do away with the stepped capstan also.

I just sent him an email to ask if he still has the kits and if he will sell them.

Here's the link:

http://www.manquen.net/audio/index.php?page=4

Cheers,

Otto

Offline ofajen

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Re: Oscilloscope for RTR Alignment.
« Reply #44 on: January 08, 2010, 04:55:00 PM »
I remembered correctly.  Dale says he will no longer sell the direct-drive servo kits for the 3M transport because he doesn't want to have to support them.  Too bad.

Cheers,

Otto

That's a beautiful M23 Otto! Fantastic work there.

Perhaps you could give us a little detail regarding the servo upgrade you mentioned.

I seem to remember that Dale was working on a way to do away with the stepped capstan also.

I just sent him an email to ask if he still has the kits and if he will sell them.

Here's the link:

http://www.manquen.net/audio/index.php?page=4

Cheers,

Otto