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AR 'The Turntable' Sudden Hum in Left Channel

milanomike

milanomike

Well-Known Member
OK, I've had this table a long time with the Sumiko (Jelco) MMT arm.

I have rotated 3 cartridges each on their own headshell (Signet TK7, Ortofon 2M Blue & Nagaoka MP110) over the past couple of years. Unit plays and sounds great, never had any hum or any that was noticeable. But....

A hum developed in the left channel some months back. I thought I traced it to my Hafler DH110 preamp, but it really looks like it is my AR turntable as it is present when using a different preamp. I have been running the DH 110 with my Thorens (and periodically switching which phono input I use) and no hum or other issues with this preamp (preamp was refreshed a year ago) and amp (DH 500) combo. If I put the AR back in, the hum issues start again.

Hum is negligible with the Signet, worse with the MP110 and much worse with the Ortofon.

I checked (using multimeter) from the tonearm pins to the RCA connector. This seems like it should verify the wiring between at the front of the tonearm thru the RCA plugs that goes into the preamp.

I seem to have continuity where I should, and don't where I should not, but not sure this is the best check. The tonearm is old, was thinking maybe it needs a rewire, but not sure that it does at the moment.

And is there anything else that could be driving this hum, like the start cap?

Thanks for any help to solve this one.
 
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milanomike said:
OK, I've had this table a long time with the Sumiko (Jelco) MMT arm.

I have rotated 3 cartridges each on their own headshell (Signet TK7, Ortofon 2M Blue & Nagaoka MP110) over the past couple of years. Unit plays and sounds great, never had any hum or any that was noticeable. But....

A hum developed in the left channel some months back. I thought I traced it to my Hafler DH110 preamp, but it really looks like it is my AR turntable as it is present when using a different preamp. I have been running the DH 110 with my Thorens (and periodically switching which phono input I use) and no hum or other issues with this preamp (preamp was refreshed a year ago) and amp (DH 500) combo. If I put the AR back in, the hum issues start again.

Hum is negligible with the Signet, worse with the MP110 and much worse with the Ortofon.

I checked (using multimeter) from the tonearm pins to the RCA connector. This seems like it should verify the wiring between at the front of the tonearm thru the RCA plugs that goes into the preamp.

I seem to have continuity where I should, and don't where I should not, but not sure this is the best check. The tonearm is old, was thinking maybe it needs a rewire, but not sure that it does at the moment.

And is there anything else that could be driving this hum, like the start cap?

Thanks for any help to solve this one.
Click to expand...
Not familiar with that table, but make the headshell contacts are immaculate?
 
I just went through a similar ordeal with my Thorens TD-160. Left channel hum appeared out of nowhere. After determining the hum was indeed the TT and trying different headshells, all with clean and tight contacts, I flipped it over and peeked inside. The ground on the TD160 is combined with the tonearm wiring and connected to the chassis. It was still soldered but hanging by a thread. Just seemed a little loose. I'm no electrician but I do have some crude soldering skills. I just heated up that ground point and added another dollop of solder for good measure. I did the same to left channel connection. Hum disappeared entirely. Dead quiet now.

Sometimes the solder connections just need a refresh. Maybe try that as a last resort.
 
Probably time to take the unit down to my bench for a closer look and see if anything obvious jumps out.

The Thorens (a 145) plays fine, real fine, so not an emergency (ha, 1st world problem), but want to get it going.
 
milanomike said:
I checked (using multimeter) from the tonearm pins to the RCA connector. This seems like it should verify the wiring between at the front of the tonearm thru the RCA plugs that goes into the preamp.

.
Click to expand...

Does this MMT arm have a head shell? Did you measure from the head shel wires or from the arm pins with the head shell removed? It might be the sprung pin in either the head shell or arm might not be making good contact . check to see if when pressed the pins spring back .

Athanasios
 
Nashou66, Yep, I checked from the arm pins with the headshell removed. Pins seemed OK, in that if I pushed on a pin it sprung back. Probably needs a closer look.

Coolhandjjl, will take a closer look at the headshell.

I have 3 headshells I use, 2 are MMT (or Jelco) headshells and the 3rd was a recent purchase. The worst offender is the Nagaoka on the original Jelco headshell.

Thanks.
 
I finally got some time to study this table a bit. I was thinking must be a ground issue, but all seems good with wires, solder joints and ground connection at the t-bar.

Recall, this is a Sumiko (Jelco) arm and the cable at the arm is Jelco. I checked again and the cable seems fine as in not shorted. See photo for ground, it comes off the cable and attaches to the subframe, connection is clean and tight.

AR Ground.jpg

The unit has a start and a run capacitor. I checked their reading with my Fluke only because I was there.

I believe the start cap is 600 VDC and 0.022 mfd and it checks perfectly, but....

The run cap is 400 VDC and 0.25 mfd and it reads overload, checked both 10X. I did not remove either cap from the board, but will at least remove the 0.25 mfd cap to verify the OL reading.

AR_Run_Start Caps.jpg

Searching tells me the motor will start and run, but voltage could be messed up at the motor. I think need to verify 100% the cap is dead, but it sure seems dead.

The question is, can a bad run cap cause additional hum?

The AR has never really made any hum in a bunch of years, and all of a sudden it started to hum in 1 channel.

Thanks in advance for any thoughts on this one.
 
Don’t think you would get hum from a bad cap on the motor. Bad cap on the motor would cause it to run hot or not at all or the motor would start rotating in the wrong direction.
I would try removing the ground from the subchassis just to see if you get more hum from both channels. Weird that it’s one channel and worse with the Nag cartridge. I would consider replacing the Din interconnect cable at this point also.
 
sfrost, thanks for input.

Searching around a bit, it seems there are some threads suggesting this can lead to some issues. I can replace, but it seems to be an odd rating.

Anyone got a recommendation on what to use for this capacitor?
 
Rather than shot gun this, it seems replacing the cap is a no brainer and like almost free, next is the phono cable and worst case is a re-wire of the arm. There just doesn't seem to be anything else here that might cause an issue.

I don't see any 0.25 mfd capacitors with 400 volt or higher rating. How critical is the capacitance here? Parts Express has a 0.22 and a 0.33 both in 400 volts. They are local, so I could get on Monday. Either one, OK?

Thanks.
 
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