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Tape Machines => Reel to Reel Tape Machines => Topic started by: steveidosound on October 30, 2010, 09:52:59 AM

Title: Nagra T "stutter" issue
Post by: steveidosound on October 30, 2010, 09:52:59 AM
My friend and I were going to do a listening session with his Nagra T but it seems to have developed an issue where the left pinch roller  is struggling and clicking in and out rather than maintaining closed loop tape tension across the heads. The left capstan is turning and the right capstan and pinch roller are normal.
http://www.youtube.com/watch?v=mPOVyMWyTZc
Has anybody else experienced this with their  "T" and know what is going on?
Title: Re: Nagra T "stutter" issue
Post by: steveidosound on October 30, 2010, 10:36:12 AM
One more piece of info. In the "jog" mode the left pinch roller stays properly locked in and the machine will play normally forwards or backwards up to normal speed in response to rotation of the jog/shuttle knob. In normal play mode it seems to work properly at times then get into a mode where it "stutters" with the left pinch roller rapidly disengaging and re-engaging as in the video.
Title: Re: Nagra T "stutter" issue
Post by: docb on October 30, 2010, 12:17:56 PM
I don't now if this relates to your issue, but my T will stutter a lot only when the white leader we uses goes thru the tape path. Not sure if it is slipping for lack of oxide or what. Is yours misbehaving with a lot of different tapes, or just one? Any difference using 10" reels?
Title: Re: Nagra T "stutter" issue
Post by: steveidosound on October 30, 2010, 12:39:16 PM
I don't now if this relates to your issue, but my T will stutter a lot only when the white leader we uses goes thru the tape path. Not sure if it is slipping for lack of oxide or what. Is yours misbehaving with a lot of different tapes, or just one? Any difference using 10" reels?

Many different tapes both 7" and 10.5". It is not constant but will behave for short periods. I tried counteracting supply reel tension by hand, both adding slight drag or "pushing" it slightly to lower back tension, with indeterminate results. It seemed to help to change tape tension but not consistently. The dancer arms function properly to vary reel torque.
Is yours doing the same thing as in my friend's video on the leader tape?
Title: Re: Nagra T "stutter" issue
Post by: Ki Choi on October 30, 2010, 07:26:43 PM
Hi Steve:

It seems your friend's T is having tension control issue on the left side.  As I recall, the complex tension control scheme is bypassed in JOG mode and may explain why it is working in JOG and not in PLAY.

One of first thing I would recomend is to measure the tension on the left side to be around 80g when in PLAY mode.  There's a test point on A03 board (second from the bottom) where you can actually measure the voltage but can't remember the value off of top of my head.

I am traveling overseas this week and will return home near the end of next week and can suggest few more things.

Another thing to do is to clean the ruby tension sensor very carefully and check for clean tape path.  Does it misbehave in all speeds?

Ki
Title: Re: Nagra T "stutter" issue
Post by: steveidosound on October 31, 2010, 08:17:40 AM
Hi Steve:

Another thing to do is to clean the ruby tension sensor very carefully and check for clean tape path.  Does it misbehave in all speeds?

Ki

Yes, all speeds. If you look at the video, my friend states in the comments for it that he played a tape with sticky shed, but cleaned it  _VERY THOROUGHLY_  after that. Could the sticky shed tape have damaged the tension sensor? It obviously is working if it is disengaging the pinchroller because of sensing too high tape tension.

My friend suggests some sort of spontaneous logic failure.

The fact that Doc's machine does it on leader tape seems to be a clue that the machine can be tape sensitive. It has ceramic capstans. I do wonder if wear on those could exacerbate sensitivity if there is a bit of slippage on either capstan and pinchroller. It seems to be doing it with both reel sizes and all speeds on a variety of tapes.
Title: Re: Nagra T "stutter" issue
Post by: ironbut on October 31, 2010, 11:09:26 AM
Hey Steve,
I don't have a "T" but it seems to me that this stuttering is the result of uneven tension in the loop. In Doc's case, I wouldn't be surprised if the reason for this is the less than perfect slitting that's done with leader tape.
On your friends Nagra, my first suggestion would be to use a good light and magnifier and visually examine the tape path again. Almost every time I play a sticky tape and "thoroughly" clean the tape path, I find a speck that I've missed and have to re-clean it. As I'm sure you know, it doesn't take much to cause "stick/slip".
One thing that you haven't mentioned is "when" this condition began. Was the machine working fine before?
Title: Re: Nagra T "stutter" issue
Post by: steveidosound on October 31, 2010, 04:56:04 PM
Hey Steve,
I don't have a "T" but it seems to me that this stuttering is the result of uneven tension in the loop. In Doc's case, I wouldn't be surprised if the reason for this is the less than perfect slitting that's done with leader tape.
On your friends Nagra, my first suggestion would be to use a good light and magnifier and visually examine the tape path again. Almost every time I play a sticky tape and "thoroughly" clean the tape path, I find a speck that I've missed and have to re-clean it. As I'm sure you know, it doesn't take much to cause "stick/slip".
One thing that you haven't mentioned is "when" this condition began. Was the machine working fine before?

I agree that these things can be easy to miss and it doesn't take much "S/S glue" to literally gum up the works.
I did not want to initially prejudice the comments and he and I both inspected it and he has cleaned it a couple of more times since this started. The actual sequence of events was that it was playing properly, but dirty when returned from someone who borrowed it. We cleaned it thoroughly and put on a tape which we thought was OK, but in fact had sticky shed itself. We experienced the problem on that tape and another tape before we noticed the issue with the first tape we had tried. We cleaned everything several more times and tried other known good tapes (not my TP tapes) but the problem has persisted. My friend has lots of experience with servo systems in many applications and in fact built reel to reel machines for Scully much earlier in his carrier.
Title: Re: Nagra T "stutter" issue
Post by: steveidosound on November 07, 2010, 10:46:33 AM
So, my friend is now in search of a service manual. He wants specs and schematics and measurement values and service procedures rather than guessing about how it is supposed to be set up. There is probably a procedure for setting up the tape tension servo loop. He can just see what is out of spec. Parts are way too scarce and expensive to just "shotgun" things like capstan motors.
I looked at all info on here and saw conflicting info on whether a manual is still available from Nagra. Doesn't seem to be anywhere else.
Title: Re: Nagra T "stutter" issue
Post by: Ki Choi on November 08, 2010, 12:41:29 AM
Nagra USA had the operational manual they can order from Swiss when I orded mine last year.  It mainly covered the pushbotton panel functions with no schematics.  Unfortunately they only have the service manual for older Nagra T without TC functinos and older control panel without the numeric key pad in CD format.  Again, it has to come from Swiss and will take a month or so to get to you.

Title: Re: Nagra T "stutter" issue
Post by: docb on March 22, 2012, 06:36:26 PM
Arggh. Mine just started doing this. Cleaned the ruby sensor and everything else in the tape path carefully, no change. Not throwing any error codes. The left capstan motor growls when disengaging from play and also does so a bit in play. It seems to have a hesitation in its rotation, but the motor bearings spin free with no power applied. So I would guess it's an electronic issue and not a mechanical one. Steve, did your friend ever resolve it? If I figure it out I will share the solution...

Related question - when I hit play the cam position indicator under the circular inspection cover in the top center of the face panel sits at "right capstan". I don't find any setting on the machine that moves it to "two capstans", which is where it seems to move towards when the shuddering happens.
Title: Re: Nagra T "stutter" issue
Post by: steveidosound on March 22, 2012, 11:44:39 PM
Arggh. Mine just started doing this. Cleaned the ruby sensor and everything else in the tape path carefully, no change. Not throwing any error codes. The left capstan motor growls when disengaging from play and also does so a bit in play. It seems to have a hesitation in its rotation, but the motor bearings spin free with no power applied. So I would guess it's an electronic issue and not a mechanical one. Steve, did your friend ever resolve it? If I figure it out I will share the solution...

Related question - when I hit play the cam position indicator under the circular inspection cover in the top center of the face panel sits at "right capstan". I don't find any setting on the machine that moves it to "two capstans", which is where it seems to move towards when the shuddering happens.

He did not resolve it. Finding proper documentation and procedures for troubleshooting and setup proved to be difficult and expensive.
I let him know about your post, and perhaps he will register and jump on here tomorrow.
Title: Re: Nagra T "stutter" issue
Post by: Ki Choi on March 28, 2012, 02:01:46 PM
Hi Doc B:

I am a bit late in seeing your post.  One of the first thing to check would be the two signal outputs that are 90 deg out of phase from each other from the left Tachometer with a scope.  It's circuit is in 8.5-1 section.

There's also a known issue of Nagra T's Tachometer etching rubbing off over time and not producing consistent pulses for the capstan servo motor to follow.

If this is the case, your left capstan will be jerky and eventually fault out (or run away...)

Ki
Title: Re: Nagra T "stutter" issue
Post by: docb on March 28, 2012, 04:29:10 PM
Thanks Ki! This has been a gradually worsening problem, so the tach etching sounds like it might be the problem. Unfortunately I don't have the full service manual and the manuals I do have have been on loan and are in transit back to me. So I'm flying a little more blind than usual at the moment. Where is the etching?
Title: Re: Nagra T "stutter" issue
Post by: Ki Choi on March 29, 2012, 12:29:25 AM
Actually the tacho feedback for the capstan motor might not be an encoder disc but a ring.  Either case, the tacho signals should be checked since they directly impact capstan motor.

The other possibility is that your battery is on its way out and may have already lost its charge and corrupted the RAM.  One of the members I had helped last year also had numerous issues with his T including shuttle mode problem similar to yours.  The RAM was refreshed by removing the battery for about 10 min for the capacitor to drain and reconnecting a fresh battery and powering it up to reload the firmware onto RAM and the shuttle worked properly again.

I will put a copy of the service manual CD in the mail tomorrow.
Title: Re: Nagra T "stutter" issue
Post by: docb on March 29, 2012, 01:05:16 PM
Ki, you are a gentleman and a scholar. Thank you! I considered the battery being the issue, though the machine hasn't thrown any error codes. The problem has come on gradually. Thought it might be due to a bad cap somewhere in the speed control. I tried rolling tape right after starting it up in the morning and then comparing performance after the machine sat powered up for many hours, with the idea that the cap might form up after a while,  but the problem remained the same. So it doesn't seem to be a fading cap or battery that recovers with time. But it's sure simple to try the battery thing and I will give it a shot. I'm sure it could stand to be replaced.

I do wonder if maybe there is a bunch of crud inside the capstan housing that is causing problems. Looks like a bear to take it all apart....
Title: Re: Nagra T "stutter" issue
Post by: Ki Choi on March 29, 2012, 07:00:36 PM
Thanks Doc for your comment and maybe a judge of good beverages of adult kinds...

You should have the service manual tomorrow, and it has good illustrations of the measurement points for the capstan motor/tachometer without removing the assembly.
Title: Re: Nagra T "stutter" issue
Post by: docb on March 30, 2012, 10:26:55 AM
T'would be my pleasure to fulfill your desire for an adult beverage, anytime. In fact if you can make it over for our open house tomorrow I'll keep your glass full. We have a backup rig in the system - Shawn's tricked out MX-5050BIII running with my Tube Repro. And I'll be demoing a killer phono cartridge tomorrow too, the Haniwa HTCR01, playing through our Tube Phono (the one that Jonathan Valin admired in his coverage of the new Magicos a few months ago).  Best vinyl sound I've ever had, by far.

I got my borrowed schematics back and luckily found scope traces and adjustment procedures and voltages for the control circuits printed on them. I'll need to rig some long test leads for a couple of test points as I don't have extender cards, but hopefully I can get some measurements done today. I'll keep you guys posted on what I find.
Title: Re: Nagra T "stutter" issue
Post by: steveidosound on March 30, 2012, 11:15:59 AM
I thank you in advance for anything you might come up with to help my friend Dave out. He really got nowhere without proper set up and test procedures and also in getting any documentation of same.
This machine, or at least this issue with it is far too complex to just guess and tweak around with, hoping you might stumble upon the cause.  Again, my friend does not work on Nagras or Studers or other  high end tape machines for a living, but is a very well rounded machinist / industrial  designer, who is very familiar with microprocessor and other logic based servo control systems as used in CNC machines  and other automated process equipment for industry, which  he has been involved with for years.
He did want me to ask if there was anything regarding even light mechanical shock from moving the "T" around that might have occurred right before failure. Rumor is they don't travel well even in a passenger vehicle. Put another way, is there something that mechanically tends toward failure in these machines if transported in a normal, safe, manor (not dropping them) like for instance, capstan bearings?
Title: Re: Nagra T "stutter" issue
Post by: docb on March 30, 2012, 01:47:37 PM
Arrgh, the test points and adjustments are at the back of the A03 card. So without an extender this is going to be a PITA. So far what I have found is I am supposed to be seeing 4V in park and 0V in skip or play at test point TP203 on A13. I'm seeing 7.5V in any mode. The equivalent test point for the right capstan (TP204) reads correctly at 0V in play. So we definitely have something out of whack in the left capstan stabilizer circuit. I'll keep you posted as I gather more info.

Update - I now have the proper 4V at TP203 in park (by adjusting RP202) and a fluctuating 0V +/- .1V in shift ( by adjusting RP201). But now I see about -4V in play at all speeds rather than the expected 0V. The stuttering is reduced quite a bit, but there is still a bit of grumbling and a distinct growl when the motor spins up and runs down. The capstan itself spins quietly and freely. Starting to suspect some sort of issue with the motor itself.

Update - Just the got the service manual. Holy smokes, 576 pages! Thank you so much Ki! Starting to look like I need to investigate the capstan motor clutch adjustment. Cross your fingers...
 
OK, the left capstan motor clutch was very loose! There is a brass nut around a set screw at the back end of the motor, under the shield can. You're supposed to adjust it such that you get about 180g of resistance when pulling on a 4" allen wrench stuck in the set screw with the motor locked in place. I didn't have all my tools and just snugged it a bit for now. The growl is mostly gone, probably need a little bit more of a tweak when I get my tension meter and the proper wrenches here. Went back and reset the stability test point voltages by trial and error (it's a balancing act between RP201 and RP202, I hate that kind of interactive adjustment...)and got pretty much the spec voltages. Seems to pull tape well now and goes into the two capstan mode on the cam in play. Hopefully it will hold together for tomorrow. Still got the backup rig, just in case.
Title: Re: Nagra T "stutter" issue
Post by: Ki Choi on March 31, 2012, 12:15:41 AM
Hi Doc B:

Although I feel a little uneasy about not knowing the exact cause for the issue, it looks like you are making good progress.  When you run into a critical demo situation and your T is still sick, remember you can borrow one of mine with direct head wired to temporary RCAs that you can mount your VU meterbridge with the Tube Repro for show & tell.

Steve,

Please send me an email.

Ki
Title: Re: Nagra T "stutter" issue
Post by: steveidosound on March 31, 2012, 12:28:00 AM
Arrgh, the test points and adjustments are at the back of the A03 card. So without an extender this is going to be a PITA. So far what I have found is I am supposed to be seeing 4V in park and 0V in skip or play at test point TP203 on A13. I'm seeing 7.5V in any mode. The equivalent test point for the right capstan (TP204) reads correctly at 0V in play. So we definitely have something out of whack in the left capstan stabilizer circuit. I'll keep you posted as I gather more info.

Update - I now have the proper 4V at TP203 in park (by adjusting RP202) and a fluctuating 0V +/- .1V in shift ( by adjusting RP201). But now I see about -4V in play at all speeds rather than the expected 0V. The stuttering is reduced quite a bit, but there is still a bit of grumbling and a distinct growl when the motor spins up and runs down. The capstan itself spins quietly and freely. Starting to suspect some sort of issue with the motor itself.

Update - Just the got the service manual. Holy smokes, 576 pages! Thank you so much Ki! Starting to look like I need to investigate the capstan motor clutch adjustment. Cross your fingers...
 
OK, the left capstan motor clutch was very loose! There is a brass nut around a set screw at the back end of the motor, under the shield can. You're supposed to adjust it such that you get about 180g of resistance when pulling on a 4" allen wrench stuck in the set screw with the motor locked in place. I didn't have all my tools and just snugged it a bit for now. The growl is mostly gone, probably need a little bit more of a tweak when I get my tension meter and the proper wrenches here. Went back and reset the stability test point voltages by trial and error (it's a balancing act between RP201 and RP202, I hate that kind of interactive adjustment...)and got pretty much the spec voltages. Seems to pull tape well now and goes into the two capstan mode on the cam in play. Hopefully it will hold together for tomorrow. Still got the backup rig, just in case.

LOL ! _Always_  check your clutch before adjusting your pots !
I am quite sure this info will give Dave a place to start probing.
Thanks !

Ki - did so. Thanks !
Title: Re: Nagra T "stutter" issue
Post by: docb on March 31, 2012, 10:22:46 AM
Hi Doc B:

Although I feel a little uneasy about not knowing the exact cause for the issue, it looks like you are making good progress.  When you run into a critical demo situation and your T is still sick, remember you can borrow one of mine with direct head wired to temporary RCAs that you can mount your VU meterbridge with the Tube Repro for show & tell.


Thanks Ki, I appreciate that. The slipping clutch was the exact cause. Looking at the cutaway drawing of the capstan/motor connection it appears that there is some sort of soft spacer that functions as the clutch, and I suspect it's just thinned with age. In classic Northern European form the service manual says something to the effect of "The clutch adjusting nut is secured with Locktite 601. It never needs adjustment, but when it does need adjustment, here is how to do it" with a detailed illustration. I love that.

The only thing left hanging is that I need to measure the clutch resistance with my tension gauge. I'm bringing the necessary tools from home this morning, to check it. Meantime the voltages that I found were off when the clutch was slipping are all spot on, it engages both capstans nicely and it sounded great last night when I tested it.

With that amazing service manual (easily as comprehensive as an ATR or Studer manual) I will go through and check everything, as I found a few voltages that were off just a smidgen. Adjusting some of them is a PITA because some of the pots are at the back of the card. So the drill is pull the card, tweak the pot, put the card back in, measure with test leads snaking back to the test point, pull it again, tweak a bit more, until you hit the numbers. After seeing a set of card extenders go for $400 on the bay I think I will try to find the right connectors and fabricate my own.

Title: Re: Nagra T "stutter" issue
Post by: steveidosound on March 31, 2012, 10:41:18 AM
Doc,
 Glad your machine is happy again. Hope your demo day goes well and is fun.
I will post here again regarding what transpires with Dave's T.
Title: Re: Nagra T "stutter" issue
Post by: flez007 on June 03, 2012, 08:46:12 AM
Hi DocB/Ki - I do have exactly the same issues with my Nagra T, Ki was kind enough to help me out and restoring it from scratch - my deck was playing fine for about three months but started to behaive just as the OP mentioned. I contacted Bartels at Nagra and sent ne a new roller tension piece that will be replaced shortly (I also needed to help the transport by hand to RWND properly) - I do hope this fixes the problem. I wi?l share my findings once I have it back from the shop.

Fernando
Title: Re: Nagra T "stutter" issue
Post by: docb on June 03, 2012, 10:17:39 AM
Just don't tighten the screws too much or this might happen! ;^)>

http://www.facebook.com/photo.php?v=392552707447151 (http://www.facebook.com/photo.php?v=392552707447151)
Title: Re: Nagra T "stutter" issue
Post by: flez007 on June 03, 2012, 11:05:54 AM
Thanks DocB!, I will have that clutch checked with an advisory note on the screws! :)
Title: Re: Nagra T "stutter" issue
Post by: docb on December 12, 2012, 07:06:57 PM
I had the privilege to get to fix this sucker again today. I was having a problem with a very bad wow in play. Turned out it was due to a couple of things. Primarily it was that dadgummed clutch on the left capstan motor getting loose again. I never adjusted it with any precision and thus never locked the thread of the clamping nut. This time I took the clamping nut off the back of the motor and pulled the screw that it threads onto. Cleaned the fairly well petrified old threadlock off with alcohol and put it back together with Loctite, adjusting the clutch tension to the manual spec.

I also took some time to really, thoroughly clean the pinch roller sled and rails. There was quite a bit of grunge in all the narrow gaps between the rollers and the rubies, under the sled itself and in the rails. That plus cleaning the top and bottom edges of the pinch roller as well as the face made a huge improvement in the movement of the sled and the locking up of the pinchrollers against the capstans. 

So she's running better than ever. Now I can get back to my A80 1"2 track project. That one is up and running, with an 8 track head with bands 1-4 wired in series for track 1 and 5-8 wired in series for track 2. FR is amazingly flat thru a Tube Repro, down maybe 1/10 dB at 50Hz and dead flat at 15kHz. I did the math on how much track area I am missing due to the guard bands between 1-2, 2-3, 3-4 and 5-6, 6-7, 7-8, and I have thus decided to call this my 3/4" two track, until I get some custom 1" 2 track heads made. Even with the less than optimal head it sounds really good playing one of our running masters. I've been listening to 1/4" pretty exclusively for the past several months and it's always an eye opener to put up a 1".
Title: Re: Nagra T "stutter" issue
Post by: docb on January 17, 2017, 06:03:21 PM
Well, it's been four years, and the stutter problem came back. This time it was the takeup side capstan clutch rather than the supply side one. I couldn't get the clutch to get any purchase at all, even with the brass nut very tight. That scared me, didn't want to strip the nut or round the flats on it. So I took it apart. I mentioned in a previous post in this thread that there is some sort of soft washer inside the assembly that I though might be the clutch plate itself. Nothing so complex. The thin little SS washer right under the brass nut is the clutch. I am amazed this thing works at all! Cleaned the nut and the top of the cap that the clutch runs against with alcohol. More importantly I went over the thin washer/clutch plate on both sides with a Scotchbrite pad soaked in naptha, which took off a ring of gray schmutz (guessing it was a buildup of graphite dust off the motor brushes). Rinsed with alcohol, put it all back together and was able to tighten the nut down to get the spec'd 180g of resistance. Medium locktight holds the nut, which I am waiting to dry as I type this. Fingers crossed...

I should also mention that I found the brakes slightly out of adjustment as well, which was causing a bit spill when I put the deck in load mode. That is worth checking while you have the deck open and the tension gauges out.

Followup: all is good, deck running like old times. I recently acquired some extender cards, so now is a good time to go back through and do a full CLA.
Title: Re: Nagra T "stutter" issue
Post by: Ki Choi on January 20, 2017, 02:36:14 PM
Good work, Doc B!

You have inspired me to play my favorite TP tapes over the weekend on my last remaining Nagra Audio T.

Ki