1964 Zenith Console Stereo with Microtouch 2G Changer

[vortalexfan]

Originally Posted by RJMiranda

As the original set had a transformerless power supply, the platter ground was probably floating, that is, not connected to the amplifier´s B-. Maybe only thru a 0,05 microfarad condenser in parallel with some high value resistor.

Do you hear the hum always, or only while the turntable´s motor is running?

I hear the hum all the time, but it's worse when the turntable is turned on.

[RJMiranda]

The instructions that came with the old-time radios used to say that you could reverse the line plug (back then they were not polarised) and leave it where the hum was minimal. But today the only acceptable hum level is the inaudible one.

If the hum persists even with the turntable unplugged from the wall (not just turned off), there must be something wrong with the cabling. Either the RCA-to-1/8" cable, or the shielded cable within the platter.

Sometimes a RCA-to-1/8" can have a bad ground contact, even when new. (I assume you put two RCA jacks somewhere in the plinth to plug the RCAs end of your cable). Try unplugging both RCAs from the turntable. I think the input impedance of the computer will not be so high as to pick hum from the cable while it is disconnected, so if the cable is OK, you should have no hum.

I suppose you are using the original flexible cable going from the cartridge, along the tonearm, and down to the RCA jacks. Check if there is some connection between the shield of this cable and the turntable's ground, maybe thru a condenser. Try removing this connection. Many old time cables have the shield exposed along the metallic pick-up arm, so it will be not an easy task to do this.

A quick test would be to disconnect the cables from the cartridge, and connect the computer's line input directly to the cartridge terminals, using a temporary cable. This cable need not go thru the tonearm, just leave it hanging between the cartridge and the computer's line input. You don't need to play a record, because you will be just troubleshooting the hum. Check for hum a) with the turntable disconnected from the mains, b) connected but turned off, and c) while the motor runs.

If you still have the hum, it is coming from the cartridge itself.

If the above three tests are hum-free, the trouble may be in the original shielded cable that goes inside the tonearm: either a high resistance (or open) in the inner conductors, bad contact with the cartridge pins; or an high resistance or open shield.

If the resistances check OK, and the test with the independent (provisional) cable was OK, there must be some connection between the shield and the platter's ground that should be avoided. If you need to substitute the original cable you will need a very flexible one, so it does not interfere with the side movement of the tonearm.