Nagra T "stutter" issue

[docb]

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....

[Ki Choi]

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.

[docb]

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.

[steveidosound]

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?

[docb]

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.

[Ki Choi]

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

[steveidosound]

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 !

[docb]

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.

[steveidosound]

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.

[flez007]

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

[docb]

Just don't tighten the screws too much or this might happen! ;^)>

http://www.facebook.com/photo.php?v=392552707447151

[flez007]

Thanks DocB!, I will have that clutch checked with an advisory note on the screws! :)

[docb]

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".

[docb]

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.

[Ki Choi]

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