SL-65B - Irritating hum

the jewel in the crown
amagasakii
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Re: SL-65B - Irritating hum

Post by amagasakii » 28 Oct 2019 06:26

DSJR - I was all excited about the missing fifth wire (easy fix!) and then a bit of Google Image searching showed no other SL-65B with a fifth wire either-- boo. (Maybe it doesn't need a ground wire since the carrier is metal, rather than plastic like on my 40B?)

Does this sound like a good list of things to check?
  • Disconnect cartridge
  • Install cartridge sled and verify minimal resistance from each cartridge lead to corresponding solder joint on terminal block
  • Verify continuity between tonearm tube and green ground wire/chassis ground
  • Verify no continuity between cartridge positive leads and tonearm tube/green ground
  • Measure resistance between each neighbouring pair of cartridge wires (B/W, W/R, R/G) to ensure no wipers are touching
Assuming this all checks out, I'm tempted to short together the existing tonearm wires, then attach new secondary wires to the terminal block, and connect the cartridge to those to test.

At this point I think it has to be one of three things:
  1. Bad ground connection at tonearm or terminal block
  2. Bad sled wiring
  3. Bad tonearm wiring (short or break in insulation)
I'm hoping that the tonearm just needs better grounding, but if I have to rewire the tonearm (I'm not against it; I like a new challenge) I'm tempted to bypass the sled connector altogether and just solder cartridge tags to the tonearm wires

DSJR
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Re: SL-65B - Irritating hum

Post by DSJR » 28 Oct 2019 09:11

Like I said, the contact tabs/fingers in the headshell need aligning and checking, especially the L- 'black' one, as it's not original.

The C2 carrier connections in the headshell were always a difficult aspect, but checking with a continuity meter is a good thing to do - disconnect the cartridge first though. As the exit cables to amp are basic RCA ones I believe, it wouldn't hurt to check these too, as cheaper ones lack decent screening and may just play a part.

amagasakii
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Re: SL-65B - Irritating hum

Post by amagasakii » 29 Oct 2019 22:44

Did a little poking around late last night:
  • Less than 1 ohm resistance from each RCA plug to the headshell connectors, including the questionable replacement LG lead.
  • Less than 2 ohms resistance depending which two points of the chassis I tested; less than 1 ohm from terminal block to chassis.
  • Unreliable readings when testing tonearm tube grounding. Far as I can tell, the ground is made through the vertical point bearings -> tonearm carrier -> tonearm tube mounting screws; this doesn't seem to make good contact anymore.
Tonight I'll measure resistance with the cartridge sled in place with cartridge unclipped, and also try with an additional wire connected to the tonearm tube mounting screw.

amagasakii
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Re: SL-65B - Irritating hum

Post by amagasakii » 30 Oct 2019 04:11

Good news: Excellent connectivity from cartridge tags to RCA plugs and everywhere in between.

Bad news: New tonearm ground wire didn't help, but I did discover that someone has previously stripped the bolt holes in the tonearm tube and driven the screws in at an angle, which explains why the cartridge azimuth is off (it was on the list of things to fix).

Out of frustration, I pulled the 40B out of its all-in-one plinth and swapped the 65B in.
42460

Lo and behold, now the 40B hums, and the 65B installed in the all-in-one is quiet. So even though I tried more than one set of patch cables (including a new set of "premium" shielded ones) it looks like it's a patch cable problem after all. Annoying.
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DSJR
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Re: SL-65B - Irritating hum

Post by DSJR » 31 Oct 2019 12:50

What a pita!

Hope you get it sorted.

amagasakii
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Re: SL-65B - Irritating hum

Post by amagasakii » 01 Nov 2019 04:08

On a whim I picked up a beefy set of RCA cables from the thrift store— no more hum. Lesson learned. I'm a tad annoyed that the new shielded cables I bought (and returned) didn't fix the hum. Not so premium nor well-shielded, I'd guess.

Thanks everyone! If nothing else, all this disassembling and tinkering brought to light a few new things to fix.

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Re: SL-65B - Irritating hum

Post by amagasakii » 13 Nov 2019 04:16

Just wanted to say thanks to everybody for helping me work through what turned out to be a very simple problem!

I spent a few nights last week building the SL-65B the new plinth I promised it over a year ago (finally!) The mitres aren't perfect since I only have access to a mitre saw with questionable bearings. Work with "what ya got". :)

I've bypassed the old Amplok connector and replaced it with a Molex/Mate-N-Lok connection; as part of the process I installed a standard "figure 8" power socket. I added external RCA jacks and replaced the separate chassis ground by soldering a wire between the terminal block and the left RCA ground lug, as this proved to have the least hum (and I like the convenience).

Thanks again, I'm really enjoying finally putting this thing into regular use.
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AsOriginallyRecorded
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Re: SL-65B - Irritating hum

Post by AsOriginallyRecorded » 13 Nov 2019 05:32

Glad I checked back on this thread, and pleased to hear it has all had a happy ending. For the most part, most of the equipment truly worth restoration can be redeemed using fairly easy, straight forward adjustments and repairs. Often, it is a case of simple maintenance that has been neglected over the years, sometimes sitting in a dusty, dark corner somewhere. Every one of these success stories is very satisfying to follow,and serve as a running manual for others with similar problems. Thanks for keeping us all posted on your project and your ultimate solution (hum is one of the main problems members encounter). Congratulations on the finishing of your completed Blondie TT...looks great.....now spin some discs and enjoy! Cheers! =D> =D> =D>

DSJR
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Re: SL-65B - Irritating hum

Post by DSJR » 13 Nov 2019 08:38

That plinth looks amazing - well done! I can tinker around with mechanisms well enough (and set up iffy springy sub-chassis audiophile turntables professionally), but making plinths like that - sadly no way..

amagasakii
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Re: SL-65B - Irritating hum

Post by amagasakii » 13 Nov 2019 20:32

Thanks to both of you! This was a fun project and a another great learning experience; I'm happy to contribute to the running manual any way I can. :-) I've grown really fond of these Autoslim-derived machines. They're robust, easy to tear down and reassemble, and very forgiving about years of neglect.