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DavidM
07-11-2004, 02:06 AM
I have a Pioneer PL-120 turntable which must be at least 25 years old. Have recently replaced the belt with a generic 23.5" belt. I now find that the turntable is running 2-3 percent faster than before (based on comparing quoted track times and actual running times). There appears to be two sets of speed adjusting points. Both points are marked up to adjust 33 and 45. One of the adjusting points is close to the motor pulley, and the other point is further along the rim. Looked in the handbook and no reference is made to these points. Can anyone tell me please how to proceed with the speed adjustment? Have located some strobe discs to assist me.

Thanks

skeptic
07-12-2004, 03:38 PM
Since nobody can or will reply, I'll take a stab at it. First, I don't know anything about this turntable.

The only way to judge if a turntable is going too fast or too slow is with a strobe disc. Use it in artificial light only. It will not work in sunlight. Assuming you live in the United States, use the pattern indicated for 60 hz. You are most interested in the 33 1/3 speed.

The speed of the rotating platter is affected by 1) the motor speed 2) the diameter of the pulley and 3) the diameter of the part of the platter the belt is wrapped around. Often it is time to change belts when they stretch to the point where they slip on the motor pulley. This makes the turntable turn too slowly. If particles of the belt have worn off and stuck to the motor pully, this will increase its effective diameter making the turntable turn too rapidly. If this has happened, remove the belt and clean the pulley with a cotton swab Q-tip dipped in alcohol. Don't put the belt back on until the pulley is dry which should only take a minute. Some turntables had a variable diameter pulley shaft like my old Empires did. Althought the motor speed was locked in synchronization with the power line frequency, the speed could be adjusted by a thumbscrew which tilted the motor allowing the belt to ride higher or lower on the pulley. I think this kind of adjustment is rare. Turntables with brushless DC motors are often driven by an electronic circuit with an adjustment potentiometer. Some turntables have no speed adjustment at all. To check the speed and assist you in adjusting it, place the strobe on the platter after dark and watch it in incandescent light. When the speed is exactly correct, the lines will appear to stand perfectly still because the time interval between one cycle when the light is on and the next is exactly the time necessary for the lines to travel so that each one is in exactly the same spot as the one before it was. If the turntable is too fast, the lines will appear to "walk" or move slowly in the direction of rotation. Too slow and they will appear to walk backwards in the opposite direction. The greater the speed error, the greater the rate of walking. Adjust the speed until the lines appear to stand perfectly still. For even greater precision, put a record on and the strobe on the record. Put the tonearm on the disc. This will allow any adjustment to compensate for the additional drag the record and stylus impose.

good luck.

DavidM
07-14-2004, 12:56 AM
Thanks for the reply. I found it very helpful. I have obtained some strobe discs via the net but I'm not 100 per cent sure that they are accurate, but they are probably good enough for my cloth ears. In the UK, so I used the 50 hz settings. Have got it set up now so that the running times approximate the published runnings times, and the strobe lines do appear to be standing still. After watching them for a time, I start getting a headache. Just wanted to get the thing set up reasonably so that I could transfer my vinyl collection to computer. Once again, many thanks for your help.

skeptic
07-14-2004, 08:40 AM
"I have obtained some strobe discs via the net but I'm not 100 per cent sure that they are accurate"

The strobes are very accurate. As long as they are not slipping on the turntable, they will indicate exactly the right speed when you achieve it. BTW, the frequency of the line voltage is also extremely accurate. The number of cycles in a 24 hour period is adjusted by the power company to be correct and the generators are all locked together in a grid and turn at the same rate. There is no reason to believe that a properly working high quality hysterisis synchronous motor isn't perfectly acceptabe for first rate phonograph turntables.

Garrardman
07-15-2004, 12:34 AM
DavidM, just a thought - if your turntable appears to be running faster with the new belt and the strobe disc says the speed is OK, it was probably running too slowly with the old, stretched belt and you had just grown used to it!!
Adam.