Loud intermittent hum from turntable

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Nnicola
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Loud intermittent hum from turntable

Post by Nnicola » 22 Feb 2019 21:33

Thought I would post this for some Help before I start taking apart my turntable.

I recently purchase a vintage Dual 504 TT and shortly after setting it up I started to experience a loud low-frequency hum from the right channel. The hum is so loud that when The volume is turned up on the receiver I am afraid of speaker damage. The hum starts unexpectedly and can occur anywhere from the start to the end of a record. The hum usually disappears after returning the tonearm to rest or when touching the tonearm.

To isolate the problem I've done the following:
Changed receivers, then cartridge shells, changed sources, like FM tuner and CD player to pinpoint the source, and it's definitely the TT. The TT was grounded on two different receivers and still occurred, so if it's a grounding issue, the grounding issue is isolated to the TT.

Any ideas?

Woodbrains
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Re: Loud intermittent hum from turntable

Post by Woodbrains » 22 Feb 2019 21:46

Hello,

Sounds like a signal wire (red) in the tonearm is shorting to earth (ground). It might be at the headshell connection, or a break in the insulation somewhere, perhaps were it exits the bearing spindle. If it is intermittent, it is possible the wire is rubbing on the chassis, and the position of the tonearm makes/breaks the contact depending on where it is. I had such a short last week, and it presented the same loud hum you describe. Luckily mine was an easy fix.

Mike.

Spinner45
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Re: Loud intermittent hum from turntable

Post by Spinner45 » 23 Feb 2019 00:21

Woodbrains wrote:
22 Feb 2019 21:46
Hello,

Sounds like a signal wire (red) in the tonearm is shorting to earth (ground). It might be at the headshell connection, or a break in the insulation somewhere, perhaps were it exits the bearing spindle. If it is intermittent, it is possible the wire is rubbing on the chassis, and the position of the tonearm makes/breaks the contact depending on where it is. I had such a short last week, and it presented the same loud hum you describe. Luckily mine was an easy fix.

Mike.
While the issue is of bad connections, true, but using the term "shorting" is incorrect.
If a real "short" would happen, it would cease allowing any sound.
What I believe you're saying is an "open" connection, not a short.
Open connections to the ground wire, or signal wires, would cause a hum.

Woodbrains
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Re: Loud intermittent hum from turntable

Post by Woodbrains » 23 Feb 2019 00:30

Spinner45 wrote:
23 Feb 2019 00:21
Woodbrains wrote:
22 Feb 2019 21:46
Hello,

Sounds like a signal wire (red) in the tonearm is shorting to earth (ground). It might be at the headshell connection, or a break in the insulation somewhere, perhaps were it exits the bearing spindle. If it is intermittent, it is possible the wire is rubbing on the chassis, and the position of the tonearm makes/breaks the contact depending on where it is. I had such a short last week, and it presented the same loud hum you describe. Luckily mine was an easy fix.

Mike.
While the issue is of bad connections, true, but using the term "shorting" is incorrect.
If a real "short" would happen, it would cease allowing any sound.
What I believe you're saying is an "open" connection, not a short.
Open connections to the ground wire, or signal wires, would cause a hum.
Hello,

That is what I mean. I thought of it as a short circuit to earth, but if the correct term is open circuit, then I stand corrected. Luckily, in my case, the hot right signal wire had a bit more bared conductor than necessary and was touching the metal headshell, which is ultimately earthed (grounded, sorry I'm British). I just shortened the wire and resoldered leaving no bare wire showing. I have known broken insulation around the bearing pillar to do the same, and unfortunately meant a rewire!

Mike.

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Re: Loud intermittent hum from turntable

Post by Spinner45 » 23 Feb 2019 01:50

Woodbrains wrote:
23 Feb 2019 00:30
Spinner45 wrote:
23 Feb 2019 00:21
Woodbrains wrote:
22 Feb 2019 21:46
Hello,

Sounds like a signal wire (red) in the tonearm is shorting to earth (ground). It might be at the headshell connection, or a break in the insulation somewhere, perhaps were it exits the bearing spindle. If it is intermittent, it is possible the wire is rubbing on the chassis, and the position of the tonearm makes/breaks the contact depending on where it is. I had such a short last week, and it presented the same loud hum you describe. Luckily mine was an easy fix.

Mike.
While the issue is of bad connections, true, but using the term "shorting" is incorrect.
If a real "short" would happen, it would cease allowing any sound.
What I believe you're saying is an "open" connection, not a short.
Open connections to the ground wire, or signal wires, would cause a hum.
Hello,

That is what I mean. I thought of it as a short circuit to earth, but if the correct term is open circuit, then I stand corrected. Luckily, in my case, the hot right signal wire had a bit more bared conductor than necessary and was touching the metal headshell, which is ultimately earthed (grounded, sorry I'm British). I just shortened the wire and resoldered leaving no bare wire showing. I have known broken insulation around the bearing pillar to do the same, and unfortunately meant a rewire!

Mike.
All is well Mike.
It's just that I've heard people use the term "shorted" or "short" when describing something, and I know it's an incorrect term for the problem.
I suppose due to my formal electrical training of decades ago - we always used correct terminology to describe things. :lol:

Nnicola
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Re: Loud intermittent hum from turntable

Post by Nnicola » 23 Feb 2019 04:20

So if I understand this correctly, somewhere between the cartridge shell and the base of the tonearm, I should be looking for a portion of bare or uninsuleted red wire somehow grounding to the chassis of the TT.

Is that correct?

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Re: Loud intermittent hum from turntable

Post by Spinner45 » 23 Feb 2019 06:48

Nnicola wrote:
23 Feb 2019 04:20
So if I understand this correctly, somewhere between the cartridge shell and the base of the tonearm, I should be looking for a portion of bare or uninsuleted red wire somehow grounding to the chassis of the TT.

Is that correct?
Continuity - tight cartridge connections - the "ohms" at the RCA plugs should read essentially the same.
And each wire (4) should read "0" ohms to each RCA plug (tip and shield) from the cartridge.
The ground wire should read "0" ohms from chassis to amp connector.
Get your meter out and do some troubleshooting.

Nnicola
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Re: Loud intermittent hum from turntable

Post by Nnicola » 24 Feb 2019 22:55

Given the age of the TT and the fragility of the extremely light gauge wire, I'm wondering if it might be easier, and I might be better off if I just rewired the entire cartridge/tonearm assembly instead of looking for a fault. I can upgrade the wiring in the process, eliminate the chintzy Dual cartridge plug in wire system, and switch to heavier gauge RCA cables while I am at it.

Feedback and advice, please?

Spinner45
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Re: Loud intermittent hum from turntable

Post by Spinner45 » 24 Feb 2019 23:08

Nnicola wrote:
24 Feb 2019 22:55
Given the age of the TT and the fragility of the extremely light gauge wire, I'm wondering if it might be easier, and I might be better off if I just rewired the entire cartridge/tonearm assembly instead of looking for a fault. I can upgrade the wiring in the process, eliminate the chintzy Dual cartridge plug in wire system, and switch to heavier gauge RCA cables while I am at it.

Feedback and advice, please?
You don't want to disassemble the tonearm neddlessly.
That's a massive job and working with delicate assemblies as well.
Do a continuity check of the wires as I mentioned previously, that would narrow down the issue.
If after that, the wiring is indeed an issue, it can be done without taking the arm apart.

Woodbrains
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Re: Loud intermittent hum from turntable

Post by Woodbrains » 24 Feb 2019 23:10

Nnicola wrote:
24 Feb 2019 22:55
Given the age of the TT and the fragility of the extremely light gauge wire, I'm wondering if it might be easier, and I might be better off if I just rewired the entire cartridge/tonearm assembly instead of looking for a fault. I can upgrade the wiring in the process, eliminate the chintzy Dual cartridge plug in wire system, and switch to heavier gauge RCA cables while I am at it.

Feedback and advice, please?
Hello,

I would look for a fault, it might be easy to fix. A tonearm rewire might be difficult; I'm not familiar with your Dual, but some tonearm rewired can be extremely taxing, some are near impossible. It is unlikely the insulation has gone bad inside the arm tube. I like look at either end, where connections to the headshell terminal or at the base of the horizontal bearing where the wires might have been stressed or rubbed on something because of the bearing shaft movement. If you can see some exposed conductor, it is feasible to insulate it with some heat shrink tube.

Spinner is right, though, if you haven't a multi meter that can indicate continuity, get one. I'm sure an inexpensive one will suffice, but will make things easier, even if you plump for a complete rewire.

Mike.

Nnicola
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Re: Loud intermittent hum from turntable

Post by Nnicola » 27 Feb 2019 05:01

Thanks, everyone for your input.

After reading the responses I decides against a re-wire and proceeded to check the existing wiring with a multi meter but was unable to find a problem. I did notice, however, that there was a connection for tonearm grounding that was not being used. The 5 tonearm wires on the Dual 504 all connect to a terminal box with 5 crimp style male-female connections. The box is the male part of the connection and the middle male connection was labeled as a ground that had no wire from the chassis running to it. I fashioned a ground cable and wired it to an available screw on the chassis. While I had the turntable opened I figured that I would replace the chintzy RCA cables with higher quality ones and wired them to the the remaining four terminal connections.

The tonearm wires on the 504 that run from the terminal to the cartridge are an incredibly thin hi-gauge wire set that are impossibly delicate, probably somewhat due to age.While checking for faults I had to untape that wire set from the chassis and inadvertently disconnected two of the wires from the terminal box. I resoldered them right after re-wiring the replacement RCA cables.

After reassembling the turntable I played three album sides without getting a hum. I think I may have somehow fixed the problem. (Fingers crossed that it does not reappear tomorrow...) Perhaps the new tonearm grounding did the trick? Anyone want hazard a guess what fixed it?

While researching this project I read that tonearm wires are by design and necessity very thin high-gauge wiring. Does anyone know if replacing them with slightly thicker lower gauge wires would have compromised the performance of the turntable?

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Re: Loud intermittent hum from turntable

Post by Vinylfreak86 » 27 Feb 2019 10:58

Dual turntables use DIN connectors. There can be a problem if you only use DIN-RCA adapter, this is not enough, you need special version with a groundwire. It is even more problematic if someone removed DIN connector and didn`t professionaly replaced it with RCA and groundwire cables.

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