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Phono Hum Troubleshooting


If you experience a hum when using a record player, the problem may be caused by a lack of shielding (allowing noise to enter) or by a ground loop (usually a redundant connection between the system's ground and earth). Perform the following tests and you will likely solve the problem.  (The table in this example is a Linn Sondek LP12, but this will likely apply to most any table.)


Disconnect the Turntable from the Hi-Fi system, and from the mains (the power outlet on the wall) or outboard power supply. You may now begin.


Check to make sure that the cartridge tags are correctly connected to the cartridge and the headshell (see "Continuity Test" below for the proper headshell configuration) and that none of the cartridge wires' ("tags" we call them) connectors are touching the headshell or the cartridge body. Also make sure that none of the tags are caught between the cartridge and the headshell, or beneath the head of a cartridge mounting bolt.


Earth Tests: With an ohm meter on it's lowest scale (200 ohms on most units), check to insure that all desired earth continuities exist and are no more than about two ohms, as follows:

1) Unplug the cable from the arm and check that the tonearm earth lead spade (which attaches to the preamp's earth {"ground"} post) is continuous to the center pin of the connector which plugs into the tonearm. If you have a problem here, the ground lead may have broken internally due to stress. Cut about an inch off of the lead and test it again. If it's still bad, the arm cable will have to be replaced.   If it's ok, plug it back in.

2) If you are dealing with an LP12, check from the earth spade to the turntable as follows:
bulletThe stainless steel top plate (use a bolt head so you don't scratch the plate).
bulletThe subchassis and main bearing (if you have continuity to the record spindle then you also have continuity to the subchassis).
bulletThe earth pin of the mains lead (if applicable; Basik and Valhalla power supplies only).

3) Check from the earth spade to the tonearm as follows:
bulletThe main pillar (try the bare threads of a screw hole).
bulletThe arm tube (try the little screw which holds the nylon "thingie" on to the bottom of the tube where the counterweight threads on, or scrape a little spot of paint off of the bottom of the tube at the rear).
bulletThe headshell (the cartridge bolts usually cut through the paint and make contact with the headshell, but if not just scrape a tiny bare spot on the bottom of the headshell).

If you find a problem here, check the set screw that holds the din connector into the arm and makes the earth connection (it's the one which is 7/8" up from the bottom of the main pillar). It should be very snug.


Continuity Test: Disconnect the cartridge tags from the cartridge. With an ohm meter on it's lowest scale, check to insure that all desired internal tonearm and cable continuities exist and are no more than about two ohms, as follows:
bulletLeft channel RCA plug positive (pin) is continuous to the White cartridge tag (upper left headshell pin).
bulletLeft channel RCA plug negative (sleeve) is continuous to the Blue cartridge tag (lower left headshell pin).
bulletRight channel RCA plug positive (pin) is continuous to the Red cartridge tag (upper right headshell pin).
bulletRight channel RCA plug negative (sleeve) is continuous to the Green cartridge tag (lower right headshell pin).


Ground Loop Test: With an ohm meter on it's highest scale (20 Meg-ohms on most units) and the cartridge tags still disconnected from the cartridge, test that there is no continuity (measurement is greater than 20 Meg-ohms) between the tonearm earth lead spade and any of the arm (and cable) or cartridge contacts as follows:
bulletLeft channel RCA plug positive (pin).
bulletLeft channel RCA plug negative (sleeve).
bulletRight channel RCA plug positive (pin).
bulletRight channel RCA plug negative (sleeve).
bulletLeft channel cartridge positive.
bulletLeft channel cartridge negative.
bulletRight channel cartridge positive. *
bulletRight channel cartridge negative. *


Most moving magnet cartridges will be shielded and as such one channel's negative pin will be connected to the body of the cartridge. The body should not be continuous to the cartridge's mounting ears (the tabs through which the mounting bolts pass). If it is, a ground loop is created and a hum will result. In such a case the cartridge must be replaced.


LP12 Motor and Supply Short Test: With an ohm meter on it's highest scale, test to insure that no part of the turntable's electrical system is continuous to the turntable's chassis. Check from the tonearm earth lead spade to:
bulletThe four motor wires (one blue, one red, and two gray).
bulletThe two power conductors of the mains lead (black or ribbed is Live = 120v, white or plain is Neutral). (In the UK. and Europe, brown is Live and blue is Neutral = 0v.)
bulletIf the turntable has an early Basik supply on which the components are connected via a two-row terminal strip, check for any continuity to any terminal on the strip.

If you have continuity to any of the above, do not use the turntable until the source of this continuity has been found and corrected!


Misc.: If all of the above test out all right, we suggest that the trouble may be:

* The earthing arrangement of the associated electronics.   Experimentation here as to what is earthed and where may help.  (Do not alter the earthing arrangement of any Linn equipment.)

* Video or antenna ground loop.  See the Ground Blocker section of Hi-Fi Tips.

* Hum pickup from the power transformer of the associated electronics: Temporarily turn off and disconnect from the system any equipment unnecessary to the task of listening to records. One at a time, move the preamplifier and power amplifier (and turntable power supply if it's an outboard unit) to a different position relative to the turntable. (At low volume settings some equipment continues playing for a few seconds after the power is switched off. This is very handy. If after switch-off the music remains but the hum stops, the transformer in the component you just switched off is most likely the cause of the hum. Warning: Some preamplifiers emit a very large "pop" when switched off or on with the power amplifier on. This could cause amplifier or loudspeaker damage.)

 

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