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Thursday, May 15, 2014

Beomaster 6000 4-Channel: Rebuilding the Motor Unit

Oh well, another setback!!... While messing with the output amplifier and doing the tuner dial string etc...I had to witness the degradation of the Buna N O-rings that I installed a month back into the Beomaster 6000's motor unit. I could literally see how cracks in the Buna N rings developed that increased in depth every day in a noticeable way. I think in another month the rings would have broken and fallen off by themselves. Here is a picture of the damage:

Pretty uncool! Anyway, this meant I needed to redo everything, and I already had the indicator bands installed.
I studied up on O-rings and the materials they are made from. It turns out that the best material for 'drive belt' uses under tension is EPDM rubber (ethylene propylene diene monomer rubber). I found the "Parker O-ring Handbook", and guess what, it has a chapter on "Drive Belt Applications" (section 3.17). There it suggests material "E0751-65" as ideal for drive belt applications. A bit of internet sleuthing then yielded that this is EPDM...unfortunately it is very difficult to buy Parker products as a private person...it seems they have a distribution system that is loth of modern purchasing methods, i.e they want you to go to a local handyman location to pick up the O-ring...crazy! So I went to the O-ring store, and ordered six of their E70032 model, which is a 1-7/8 inner diameter 1/8" thick ring made from EPDM. I also went through a learning curve with regard to the inner diameter - the original rings measured 2" diameter, and I installed the E70033 model first, which has 2" diameter, only to learn that two of the potentiometer drives slipped with these due to a bit higher resistance in these pots...so I had to do it a 3rd time with the 32 size...live and learn!

Anyway, I was able to finally complete the repair, and I made another video about the process. This video shows the entire process: Removing the indicator bands, putting in the new rings, and putting the bands back in. There is also a demo of the rebuilt unit in action at the end. You can find it on YouTube:

********Please, note that the video does not make clear how to properly position the indicator bands relative to the potentiometers. See this post for details.**************************************

Another interesting issue about this repair procedure was the brittleness of the V-shaped plastic clips that hold the indicator bands to the purple drive pulleys on the potentiometers. The original clips immediately broke when I tried to squeeze them a bit when reinstalling the bands:

So I took measurements of one of the still intact ones, and made a model in Autodesk Inventor. Then I exported the finished model as an STL file and printed it on a Makerbot II in yellow PLA (I thought the yellow would work well with the purple drive wheels, adding to the colorful interior of the Beomaster 6000....;-).
Here are the new clips:

Pretty cool! But I had to manually Dremel the grates off at the side that connected to the plate of the printer to prevent damage to the indicator bands when sliding the clips in.

This worked very well. They can be installed by squeezing them together with needle nose pliers and pushing them in from the top while bunching the band ends together into the 'crack' (sounds more complicated than it is - watch the video posted above - it shows this process in detail).

In a second step, the clips are pushed into the wheels until they seat against the wall:

And that is pretty much it. I really like the yellow-on-purple theme...;-). Watch the video for a demonstration of the rebuilt unit and the clutch mechanism.


  1. Pretty cool experience of replacing the wear belt. For your information Darcoid deals in Parker O Rings and you can order the custom makes from their website

  2. Awesome! Congrats to your repair! Do you have a part number /link to the belts at Darcoid?
    My belts from the O-ring store seem to hold up well, but Parker is oaf course a 'good name'...;-)


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