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Nagra 4.2 tension issues


bottlegardener
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Hi all,

I was attempting to do a health checkup on a friend's Nagra 4.2 and before I could even run an MRL tape to check response and azimuth I noticed that the tension was acting very strange.

The tension rollers for both the supply and take-up sides are bouncing up and down adding a horrible wow to playback. The motor mode on the meter shows the current wildly fluctuating in time with the rollers if that gives any clues. The issue seems to eventually go away while running at 15ips but persists at 7.5 and 3.75ips. The tape stock isn't sticky and the tape path is very clean. The machine had been in storage for at least five years though. 

Just wondering if this is a familiar problem and if anyone has any ideas of the cause?

I'm still searching for a proper service manual but it would be great if I could get some clues in the interim. 

Also, does anyone know if there's any easy way to determine the EQ curve of a 4.2? I own a Nagra IS-LT and a "NAB" sticker was attached to one of the PCBs. 

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The Nagra does that "idler-dance" when the back tension is really out of whack.  It kind of requires some experience and a tension gauge to get it right.  You might want to have a well-equipped expert do this part of your service, even if you are adept at the electronic aspects of the machine.   In the absence of a label or marking this person could also run the freq scan to determine where the EQ is at.

p

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The fix for this skittish performance, as Philip and Llloyd have said, is good maintenance and adjustment.

But, it's usually possible to smooth out the transport by just reaching out with a finger to stabilize the bouncing roller. They start bouncing at start-up but will usually run properly afterward if you intervene. This ought to permit you to run some other tests and evaluations before sending the machine off to a shop.

David

(Apologies if this amounts to a statement of the obvious)

Edited by David Waelder
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Thanks for the great information folks. 

As it turns out I may have been too impatient. After leaving it powered for another 24 hours the 'dancing idlers' seems to have calmed down at all speeds. Maybe the felt pads Lloyd mentioned were a bit stiff from disuse?

Update: Just as I was about to post this the issue came back. I was taking measurements with the Tentelometer and the idlers started bouncing again. David's trick of smoothing them out by touching didn't seem to work in my case. But switching from normal to speaker playback rapidly did seem to fix it. 

I guess I'll leave it on the bench for a couple more days and see if the gremlins will work themselves out through use.

Thanks again! 

 

 

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