Quote:
Originally Posted by Michael Maurice
It would be one hell of a coincidence if the same clutch pulleys on both machines were warped and or the decks were misaligned in the same way. Of course not impossible.
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I would rather say that after this time, it would be the rule rather than the exception that the plastic pulleys have warped in some way, even if it is seldom detrimental to the proper operation of the macihne. Whenever I've dismantled a clutch assembly, it has struck me that there are usually fine cracks near the hub of the pulley, and there is often some warping. I think the plastic used is not completely stable over time. I suppose the good thing is that the cracks tend to alleviate some of the stress built up in the plastic which will help avoid it breaking completely.
And then the second part of the puzzle would be that the movement induced by the warped pulleys injects energy into the belt, which is slightly stretched and hence has a natural resonant frequency, ergo, the belt will tend to vibrate at its resonant frequency no matter exactly what the excitation frequency might be.
Tandberg have a specified how the belts should cross over in the service manual, I've found that sometimes that makes a difference, but mostly not.
On some of the older machines at least, there's a little loop of piano wire I think bolted to the speed changing assembly, which goes either side of the belt, seemingly avoiding it coming off the motor pulley should it vibrate too much. So I'm wondering if this was a problem seen even in the factory.