Tandberg model 15-41 clutch

[saxmaniac]

I have been restoring one of these and the only remaining problem out of many! is the clutch on the take up turntable. Take up and fast forward work fine and the mechanism is adjusted and all in good condition but when stopping from rewind the braking effect is too weak and tape loops out from the take up spool. The friction is by felt rings on plastic washer. Spring tension seems ok but I can't work out how to increase torque here. Any tandberg experts out there?

[ricard]

The only adjustment that can be made in this area is the lifting depth of the pulleys which doesn't affect the friction in any way.

If and when dismantling the clutches often there is what appears to be a greasy residue on the mylar disks that mate with the felt clutches, and one is tempted to clean these with meths or other solvent. However, in my experience this always reduces the friction. I therefore never clean the clutches on sight, only if there appears to be too much friction on that particular clutch, and then first with a dry rag, and only if the friction is still too high with some solvent.

If winding in both directions work fine (also with an almost full reel to the right?) it could be that you have too much friction on the left hand reel, and that cleaning that clutch might help (I usually put the machine in FREE mode, then try to slip a rag in between the mylar disc and felt pad, then back to STOP mode, and slowly turn the reel table and plastic pulley while keeping the rag stationary in order to clean the clutch without dismantling the reel table.)

If for any reason I've gotten too little friction, I've resorted to smearing the felt pads with damping grease (I have some samples from Nye labeled Flourocarbon Gel 868VH. It looks like white lithium grease but in fact increases friction. There are other similar products such as Kilopoise from Rocol; the big problem with these products is that it is hard to get them in small quantities as a consumer). It seems to work well provided enough is applied, and seems to be stable over time - I've got a couple of machines which I worked on several years ago which still work fine (when I actually get around to getting them out...).

My theory, not substantiated by anyone (in fact, anyone I've talked to who used to service these machines back in the day claims there was never any residue on the mylar discs), is that the surface of the felt initially contains some minute amounts of soap due to the felting process used to manufacture it, and that the soap accounts for a certain amount of friction when the pads are new, and then gradually migrates to the surface, eventually leaving the residue on the mylar discs. Cleaning the discs and pads removes all remaining soap, thus reducing friction. If this is true, it might be possible to recover the friction using some sort of soap (I'm thinking ordinary dishwashing detergent here such as Fairy Liquid) but I have not tried this myself nor would I recommand anyone to do it unless they're prepared to completely ruin the machine as I have no idea when harm could come of it (for instance if it migrates into one of the bearings).

[saxmaniac]

It sounds like I have got to get creative. I have already cleaned both clutches and the only dirt in there was a bit of brown powder. There is a large difference in torque between the feed and take up, though winding is fine in both directions. Removing some torque from the feed side may help but the braking effect on the take up spool is very poor so I think I still need more on that side. I think I might experiment with different materials than felt to get some friction

[DOFFERY]

Assuming all other aspects are OK, you do have an adjustment (plastic) screw on both turntables, at 6 o clock these should be adjusted to give you a compromise on all mechanical functions.

Colin.

[ricard]

Colin, doesn't that screw basically just set the distance between the pulley and felt disc when the clutches are disengaged, but has nothing to do with the actual pressure between the two when the clutch is engaged?

Later Tandberg models use various forms of rubber to get increased friction, and cork is a material that tends to have appropriate friction properties. However as the model 15 (and other machines with similar design) were intended to operate with felt pads I would try and stick with that to start with.

There could also be something wrong (like a part binding) with the spring mechanism that pushes the pulley against the felt pad. The take up one is more complicated than the supply side as the pressure applied depends on whether the machine is in PLAY or WIND/STOP mode (in play mode, the tension is less than when winding). On the supply side the clutch is either engaged or not. The difference in mechanism is the reason for the different diameter of felt pads on the two reel holders.

[saxmaniac]

The adjustment screw doesn't affect the fast wind torque, the only things affecting it are the spring tension and the friction of the felt. The spring design makes it very hard to adjust tension so friction is the only thing to alter. In other applications felt has been used as a bearing so it doesn't say much for its friction properties.

[DOFFERY]

Hi Ricard,
As I mentioned the screws I mentioned are to enable a compromise (as in so many cases) in the various functions, next time you have a 15 or similar check it out.


Saxmaniac,
Does your comment in post 6 mean you have tried to adjust said screws or just your opinion?

Colin.

[ben]

One thing to remember: I have found with these units that you MUST use reels of the same size or you get looping. Apologies if you've already eliminated this possibility!

Have you got the service manual?

[saxmaniac]

Hi Colin
I have adjusted the screw according to the manual but on fast wind it adjusts the position of the upper clutch arm so that there is approx. 0.5mm clearance, ie it is doing nothing on fast wind. There appears to be nothing else adjustable. I am going to experiment with cork rather than felt to get better friction.
Ben, I have been using same size spools and the relative fullness of the spools affects things due to momentum but this problem on this machine is more than an occasional small loop and can't be ignored. The difference between the two sides is huge but reducing the torque on feed spool is as difficult as increasing it on the take up spool!

[af024]

Felt replacement perhaps?

[saxmaniac]

After messing around with cork and causing more problems than solutions I have gone for the quiet life and accept this fault as a quirk of the machine! Thanks for the replies. Just out of interest I was able to make an acceptable pinch wheel from a rubber suspension bush from a scrap washing machine. The rough work was done in the lathe with a very sharp small tool but I could only get it to run true enough by gently scraping the surface with a new Stanley knife blade while running in the machine. I held the blade at right angles to the wheel, then finishing with fine wet and dry used wet. It was a fairly hard grade of rubber. I am so glad to have avoided the cost of a professional re rubber!

[DOFFERY]

As I mentioned in a prior post, compromise essential, the manual is only a guide "when a machine is new"

I am just undertaking a similar problem on a model 15 .

Colin.

[ricard]

It would be interesting to hear about how your 15 comes along in the end, Colin.

[DOFFERY]

Completed Ricard, not as per manual, as stated a compromise, works well on all functions.

Colin.

[ricard]

So you only needed to adjust the small screws in front of the reel holders then?

The problem I find with the Tandberg figure-8 drive method is that say you're rewinding and have almost reached the end of the tape, when you stop the fast-spinning right hand reel carries quite a bit of momentum and if the clutches grip too firmly it can create a tape loop by forcing the left hand reel to reverse direction. If on the other hand there is not enough friction, accelleration in wind mode will be slow or even non-existent if at the end of a tape. Thus the friction coefficient is more critical with this drive, on the other hand, the design does seem to be fairly stable to start with.

[DOFFERY]

Not just the adjustment you mention Ricard, as I have often stated it is a compromise on a deck of this type, each one is different.

Colin.

[ricard]

Yes, that's why I was curious as to what else you needed to do.

[saxmaniac]

I don't think there is anything else that you can do to adjust the friction on fast wind. The as I understand it the screw adjusts the position of the upper clutch arm to disengage the stronger spring used for fast wind and allow take up to be purely controlled by the light central spring. When I experimened with cork and other attempts to generate more friction I caused rough running on takeup and no cure for the original problem. Now the machine has run a few hours using felt it is only showing the looping problem with the take up spool nearly empty on rewind, and I think this is probably inherent in the design and not a huge problem. If anyone comes up with a mod that works do post it here!

[ricard]

I agree, the screw only adjusts the clutch position, hence I was curious what else Colin had done with his machine.

When the friction is right, and appropriately balanced between the left and right clutches, these models shouldn't spill tape under any conditions, as long as the reels are the same size. I usually resort to carefully cleaning the mylar disks if the friction is too high on a clutch, or smearing Nyle 868VH damping grease on a clutch whose friction is too low, which after a couple of attemps usually gives fully satisfactory results.

[DOFFERY]

Ricard,
Each problem is resolved on a "compromise" basis, no two sets require the same adjustments, I have only been beaten once & that was due to problems with the shaft bearings fitted to the chassis.

Colin.