AMI D-40 tonearm cable

[wdln]

I'm working on a D-40 that has been converted to run 45's.  The tonearm cable from the mech to the amp is missing sections of the cloth insulation and the amount of hum in the audio changes as I move it around; the hum is unacceptably loud even at its best.  Most of the cable I've seen sold online is either 1) way too short, and often terminated for a specific application and/or 2) crazy expensive.  The run requires 3 or 4 feet of cable to get to the amp.

The juke is mono, so the cartridge is just a 2-pin output.  My thought was to find a 2-conductor lightweight cable with a braided shield.  Ground the shield to chassis ground and the two signal cables might reject the hum a little better.

I suppose I could use a shorter run of the lightweight cable just to get down the tonearm and away from moving parts, then transition via a splice or plugs to a more normal cable.

Thoughts?  Any leads on a source for tonearm cabling?  Anyone ever use an isolation transformer between the tonearm and amp to get rid of hum?

[jukeboxmarty]

You can use short lengths of tone-arm wire (for flexibility) spliced to shielded audio cable for the remaining run.

I would not ground it to the tone-arm or the mechanism chassis to prevent ground loop and hum. I would try grounding it only at the amplifier.

EDIT: Unless you need to ground it to the mechanism chassis to enable muting. My memory fails me.

[robbytherobot]

Got tonearm wire from Stamann www.jukebox-world.de/en/Cartridges/Tone-arm-wire-stereo.html
I know it's 4 wires, but it's very light, and work for me on Ami B.
You have to solder it to old plug.

I had to ground the tonearm with the shield to get rid of the hum.
What cartridge are you using ? Some cartridge need to be grounded.
If unsure use a loose wire and try grounding tonearm, or mech, or cartridge...

[robbytherobot]

Dave, check here for wire www.needles4jukeboxes.com/tone_arm_wire.htm

[Hildegard]

There is a mono tonearm wire also available: www.jukebox-world.de/en/Cartridges/Tone-arm-wire-mono.html

Hildegard

[wdln]

Thanks all! Great leads and information. To answer your question Robby, the cartridge is a P51-3, replacing AMI's original F-2112 cartridge that was completely worn away, well up into the plastic.

Different question: there does not seem to be an adjustment for counterweight on the tonearm that I can see nor that's in the book. I'm a little concerned how much weight the tonearm puts on the stylus and record. 78's require and tolerate more pressure, but 45's don't need that much. It seems to play just fine, but should I be concerned for the long term? Is the stylus designed to take the extra pressure without damaging the record?

[Hildegard]

Cartridges P51-1, -2 and -3 are good for ca. 8 g tonearm weight.
But when playing 45s in your D-40 you should not use a 3 mil stylus which is made for 78 rpm records.
P51-3 would say that a stylus for 78 rpm is mounted.

The tip sizes are different for 45 and 78 rpm records. You would need the stylus with max 1 mil, better 0.7 mil tip in case you play also stereo records.

For old mono records from the 1950s era the 1 mil stylus would be best.

Hildegard

[robbytherobot]

Which tonearm do you have ? Is it alloy and short or metal with a long rod ?
With 78 tonearm (alloy), I got reading way above 10g for cartridge pressure.
I went to the trouble to make a new tonearm specifically for a GE RPX cartridge and be able to adjust pressure with weight.
But you can't go under 10g pressure, or it will skip at the end of record, too much friction in tonearm pivot on mech to activate "end switch".
I wouldn't consider it safe to play valuable 7"

[wdln]

The tonearm is tubular and straight with a stamped steel head. My tonearm scale maxes out at 5g so it's definitely heavier than that. I thought about gluing some washers or something to the butt end but there isn't much room back there before it would run into the records, and the pivot point is very close to the end of the tonearm so I don't think it would do much good.

Maybe I'll just keep records in there that I don't care about much.

[robbytherobot]

Forget about the weight on original tonearm, as you guess, too close to pivot which would require a very heavy weight and impossible to fit it.
You can try a spring at the rear of tonearm, in direction of the base pivot, but very unlikely to find the correct spring and keep enough tracking pressure.
Tonearm is totally unbalanced so very hard to modify it.

[wdln]

Looking at it again, the rotating "table" where the tonearm is anchored is quite long. It would not be terribly difficult to fab up a new bracket to move the vertical pivot point forward into the tube section of the tonearm, allowing the block at the back to become a counterweight itself and/or a place to add more counterweight if necessary. If you mounted it to the adjustable part on that flat table, the set-down point would still be adjustable, the existing tonearm height adjustment would still work, and I don't think it would change the geometry of the way the tonearm tracks across the record.

I think I'll file that away under build-a-better-mousetrap and just enjoy it the way it is for now. ;-)

[robbytherobot]

That's the idea. But geometry must be adjusted for correct tracking.

[wdln]

Just following up... I replaced the tonearm cable with some raw tonearm cabling I bought. The stuff I bought had a braided shield on the outside, so I used what was left of the original cloth covering to insulate most of the length, then did the rest (where nobody could see it) in heat shrink tubing. I did some probing and found that the larger pin of the 2-pin input on the amp was ground, so I hooked the shield to that and the other pin to the interior wire. I also hooked up the existing ground jumper from the mech to the amp. Somewhere with everything I did, all the hum went away.

I've been messing with it most of the day today and now have the amp sounding great, all the lights working with new fabbed-up color tubes, set-down and take-off points adjusted, and everything lubed up and reassembled. It sounds and looks great... definitely the coolest one in my collection. The only thing left to do is replace the felt on the turntable, which will probably require revisiting the tonearm height. The felt is functional but it looks awful. I might also fix the lid latch so that the coin mechanism works correctly... coins miss the slot when you insert them because the lid is not closed all the way when it's not latched. But someone before me had already added a free credit button so that's a low priority.

Thanks all for your help.