My recollection is from reading this info from:
http://www.theturntablefactory.com/dual.html
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A NOTE TO OWNERS OR PROSPECTIVE BUYERS OF THE MODELS: 502, 510, 601 and 1249:
When these first came on the market, there was a major problem. When used in countries that utilized 50Hz line frequency, they worked fine. However, when used on the US 60Hz current, MANY of these units had a motor that would "double speed". Upon either pushing the start lever or manually moving the arm off the rest to start the machine, if it was set at "33", the unit would actually spin at 66! Back then, the only cure was a replacement motor. At that time, Dual extended the warranty period to cover this problem. Fast forward 30 years, and the problem still exists on MOST of these units.. so, a word of caution.. If you intend to buy one, ask the seller if the one you are interested has ever exhibited this problem. Not all did, but more than enough did (and do) that this warning is necessary. Since this could have been an "intermittant" problem, OR a constant one, you'd need to ask a seller to turn the machine on at least 10 or more times.. The chances are, if it doesn't double-speed at least once in 10 or 20 tries, it's OK. Another thing to look for: The original motor was a model # SM-840. The replacement is an SM-860. If it's been replaced with the 860, you're "home free". If not, the chances are all-too-good that the machine will exhibit the double-speed syndrome. The cure now, as then, is motor replacement.. $100 "back in the day", CONSIDERABLY MORE, now!
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So, you can see it is not an issue to ignore. You will certainly be able to hear and see the difference between 33 and 66 rpm at start-up <G>! Plus, the stobe will not sync up with the too-fast turning platter.