Nagra T "stutter" issue

[steveidosound]

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?

[steveidosound]

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.

[docb]

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?

[steveidosound]

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?

[Ki Choi]

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

[steveidosound]

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.

[ironbut]

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?

[steveidosound]

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.

[steveidosound]

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.

[Ki Choi]

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.

[docb]

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.

[steveidosound]

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.

[Ki Choi]

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

[docb]

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?

[Ki Choi]

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.