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Old 8th November 2005, 04:11 AM   #1
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Default Grounding / hum problem with Pioneer table

I just found a Pioneer PL-200 turntable with an old M91 Shure cartridge and it seems to work great except for this hum problem. When the table first kicks in there is a noteable poping noise (louder than I'm used to) with the first jolt of electricity and as it warms up the hum gets louder until it is very noticeable and unpleasant in the overall mix. I try to roll off the high end but it is still there, like a loud electric whirring noise. Does anyone know how to open this thing up and where to look to fix this issue? I'm guessing that it has something to do with the power supply or something since it seems to be a "warming" problem of some kind. Any help is appreciated!

-jes
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Old 11th November 2005, 01:35 PM   #2
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Iíd say that the initial popping noise is probably due to a faulty, possibly open circuit capacitor across the mains switch. Many manufacturers use something like a 0.01 uF high voltage capacitor here to suppress clicks and pops when the switch operates, but they frequently fail, due to the high voltages. In your Pioneer turntable, the switch is most likely associated with the tonearm, so that when the tonearm moves away from rest, it switches on the motor. It should be replaced with another high voltage capacitor, such as a class X2 275VAC suppression capacitor. The other noise may be either a grounding problem, hum being induced into the cartridge, or may also be a problem with the suppression capacitor across the motor.

To minimise hum, make sure you keep the turntable well away from your amplifier, as they have quite an electro-magnetic field from the transformer(s), which is easily induced into cartridges, and then, because the cartridges produce such a low output level, the high level of amplification needed amplifies the hum to annoying levels. You also should make sure that you have connected the earth wire from your turntable to your amplifier, to earth the tonearm and other parts in the turntable.

Most turntable motors have a suppression capacitor across them to suppress the back EMF at switch on, and this may also be faulty, and would require replacing. There could also be a problem with an earth loop caused by multiple earth returns from the cartridge to the amp, which gets a current flow around the loop induced by the electro-magnetic field from the ampís transformer. A good way to test for this is to switch the amplifier off, and then slightly remove the RCA plugs from the turntable where they plug into the amp, so that only the RCAís central signal contacts are making contact, not the outside earth contacts, then switch the amp back on and test the turntable. If you lose sound, or the hum get worse, plug them back in, after switching the amp off, as the earths arenít redundant. If the hum is lowered, it is an earth loop, and you should leave the RCA plugs slightly unplugged, so that only the central signal contacts are making contact, as the earth is redundant, due to a secondary earth somewhere completing the circuit.

If it is mains hum that you are describing, then it will actually be at a low frequency, either 50 or 100 Hz, or probably 60/120 Hz for you North Americans. That will only be fixed by rolling off the bottom frequencies. Iím just going by what youíve described as hum. However, if itís a high frequency noise, such as a whistle, or a squeal, then it may be caused by something else.
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