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How do I reduce hum - Dual CS521

cadillacmike

cadillacmike

New Member
OK, so I'm getting back to listening via the HiFi equipment and the turntable produces varying amounts of hum - sometimes. Turntable is a Dual CS521, receiver is Harman Kardon 930. With the Micro Acoustics 530MP cartridge, it was barely there. However It is more pronounced with the MA 2002e cart. I don't know why. I do have the TT ground wire attached to the back of the receiver. When I touched the phono outputs while they are attached to the HK the hum changed and it also changes when I hold the ground connector. When I pulled and replaced one of the TT phono inputs to the HK, the hum stopped. ???

Do I need to clean the connections on the TT cables and the back of the HK?

Should I add another ground?
What?
 
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Removing the ground wire is easy enough. I'll also clean the RCA contacts both ends.

There is no ground wire in the Dual 521 tonearm.

I don't think the MA cartridges need grounding - never had bad hum before. They are connected to the headshell by the screws and I always make sure the 4 pins on the headshell connect securely to the tonearm.

Why would the TT come with a ground wire if it grounds via the RCA connections?

How should I test for a ground loop? Should I be checking for voltage or resistance? and between what points? - the RCA and receiver ground connection? or somewhere else?
 
Even though one thinks everything is fine with the headshell sleds connection to the tonearm, it may be worth another look an try do clean both sled pins and the mating surface on tonearm once more. I use pencil eraser (IPA or deoxit don't work so good here).
Also check that the spring function on the sled pins are ok.
I had to do this a couple of times on my Dual 601 to get rid of a hum problem.

If it's not this, I agree with advice to measure cartridge connections for continuity to the rca connectors and figure out if a groundloop is happening.
Good luck sorting this!
 
cadillacmike said:
There is no ground wire in the Dual 521 tonearm.
Click to expand...
There should be! IIRC it's olive green. Look around the mute switch. You should be able to see it.

H2yvoKi.jpg


Note this configuration is the default RCA scheme as opposed to the German DIN scheme, which is a little different. If you have the DIN configuration, there may be some tweaking to do with the default ground jumpers, so make it known. A pic might help.

The Dual headshell includes one grounding tab (in addition to the 4 audio tabs from the sled) which is attached to a metal plate that's grounding to the armtube. Internally, from this point, the (olive) tonearm grounding lead runs down the armtube all the way to the ground tab on the mute switch. These connections are a known culprit. The copper rails where the sled pins make contact must be clean and good conductivity is obtained only when the finger lift lock nicely secures the sled in place. You may want to give it a gentle scrub and clean with a toothbrush and contact cleaner. Use the meter's continuity test mode (basically a low resistance test) to verify you get a beep with one probe on the headshell grounding tab and one probe on any metal part of the chassis (or on the external ground lead). With the external ground lead configuration you should have absolutely no continuity with any of the channels leads (RCA's). This is tricky to test with the mute switch shorting so you're gonna have to manually cycle the platter until the reeds disengage in order to emulate play mode.
 
Yes, I pulled the left channel RCA and re-attached it and hum went away (or was greatly diminished).

I cleaned everything with deoxit. Even the 4 contacts at the end of the tonearm where the pins connect. I cleaned the pins too. The minus side (outside of the RCAs) of the receiver inputs were, shall we say, not very clean. They are much cleaner now. The RCA outputs from the tonearm had been replaced when the TT was serviced for a new belt and lubrication and are now gold. They got a little deoxit gold on them now.

The diagram pictured must be at the end of the tonearm where is goes into the turntable itself. This was all checked by the servicing company a couple years ago, and I don't want to take it apart which will probably only break something else.

There is a metal tab, underneath, across the top of the headshell (not the sled) with a wire going inside the tonearm. This must be the cartridge / headshell ground. The strange thing is that where the sled contacts this, it is all plastic. The only protruding metal on the sled are the 4 pins that go to the end of the tonearm where thet contact a small circuit board with 4 matching contact areas.Hmmm. or should I say Hummm.

There is no mute switch on this model.

Now for the fun part. I realize that one does not usually do this. When the system is running, by my merely touching either of the TTs RCA plugs where they go into the receiver introduced some hum. Touching the metal case of the receiver at the same time cuts this hum out. So yeah, don't be touching the contacts, but body contact introducing hum - these TTs must be VERY sensitive to even that low powered electrical signal coming off the body. Touching the ground wire where it connects the TT to the receiver doesn't seem to make any additional hum. I'm leaving the ground wire there. Dual put it there for a reason. This is like my old 1968 Cadillac - all these little things seem innocuous make a difference.

I can try disconnecting the TTs ground wire to see if that helps. That's easy enough.
 
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When I disconnected that TT ground wire I got a hum ding danger of a hum. I re-connected it. No hum. Or course I Always powered everything OFF when making changes. I put the sled with the MA 2002e back on and still little hum on that one but the wires on that sled are not right. That was one of the broken sleds and it's not going to be usable. I'll wait for the new sleds that I ordered. No hum with the MA 530MP.

All this stuff is two wire (no ground plug) and the TT is plugged into the unswitched socket on the back of the HK receiver. Should I plug the TT into it's own socket - it will still be on the same surge strip as the receiver.
 
I mentioned that in the beginning. The 2002e had some hum but the 530MP did not. And just for good measure the Grado was only outputting on one channel. The Grado had the red wire broken so that was obvious.

Then I broke a wire on the sled that held the 2002e trying to remove it from the sled.

Then I got it repaired and I THOUGHT it was ok, but I still got some hum. with both the 530MP and the 2002e. That when I first posted this thread.

Today, after cleaning and experimenting with the ground wire I got no hum with the 530MP. So I put I put the sled with the 2002e back on and there was some hum but not a lot. Still, it was more than with the MA 530MP which had no hum after the cleaning.

After the experimenting with touching the connections and chassis, all with the 530MP on, I went to put the sled with the 2002e back on and noticed that the small rectangular plate that covers the spring tip was missing. It was on the sled earlier or those pins would have gone all over the place. I found it in my cartridge case. It must have given up the ghost. It was removed to repair the broken wire and I guess a retaining tab was broken.

I was able to seat the pins and tape the plate back down, but there is still a little hum. I now attribute the hum with the 2002e to be something wrong with the pins or wires on the sled that is holding the 2002e.

I'm now waiting on the new sleds that I ordered earlier this week to arrive so I can use known good parts.

I guess the Golden rule of network troubleshooting applies to HiFi as well. Do NOT change more than one thing at a time.
 
Very often the hum is caused by a bad connection. Make sure the headshell sled contacts are clean. Deoxit them and install and remove it a few times. If that doesn't help I would open it up and see if it has a muting switch. If it does clean the contacts with some 1500 grit emery cloth. Then a good dousing if deoxit or equivalent.
 
I think I have it clean now. I did clean the contacts on the sled and on the arm where they connect. The MA 530mp is not giving me hum. I'm waiting on the new sleds to mount the other cartridges. I don't want to take the 530MP off of its sled, I broke two wires this weekend on the other sleds.

Listening to that Micro Acoustics test record is a trip. The voice keeps bouncing from left to right.
 
Two of the new sleds arrived today. I'll mount the MA 2002e and 282e on them and hopefully that will be the end of it.

The new stylus for the AT97E does not fit though so back to LP gear to have them send the correct one.
 
Got another sled in today. That makes 4 available to mount so I'll get to it later this evening. I hope I have enough of the right size screws.
 
When all the obvious causes of buzz and hum are eliminated (poor grounding/loops, faulty cartridges/sleds, tonearm issues, etc.), I have found through experience that the audio cables and the audio connectors under the turntable chassis should be looked at carefully.

I had a Dual 1229 that I recently restored and it worked flawlessly... except for a slight, yet persistent buzz that although barely noticeable, was unacceptable to me. I knew my tube amp wasn't the problem (many other "buzzless" turn tables have been connected to it at one time or another), but I could find no obvious wiring problem or ground loop that could cause this annoying sound. So, I decided to replace the cheap audio cable that connected the 1229 to the amp, but the short distance between the left and right channel jacks of the audio connector board beneath the chassis, precluded using a high quality cable with large RCA plugs on the ends. This problem was solved by removing the original RCA jacks from the chassis bracket (actually I just used a spare "white metal" 1219 bracket I had lying around) and replacing them with BNC panel connectors (with grounded outer shell). Moving the connectors to the outermost position on the bracket increased the distance between the jacks, so BNC/RCA adapters fit right on. Then, I could've just used a cable with BNC connectors on one end and RCA plugs on the other to connect the turntable to the amp, but in this case, I was easier to just use a higher quality OEM audio cable with RCA plugs on both ends.

The turntable now plays so quietly that you can't even tell it's on until the music starts. Here's a photo of what it looked like after the BNC retrofit.

BNC Connectors.jpg
 
Since I replaced the RCA jacks on my Dual 1229 with BNC connectors (because I always prefer locking connectors), I decided to fabricate a replacement RCA jack stand for some of my other 1219 & 1229 record changers (I have quite a few). This still uses RCA jacks, but they are higher quality (gold plated) than the originals and are spaced further apart to allow for high end audio cables to be connected. This is a less expensive alternative to using BNC connectors and cables.

The material is a short piece of a steel channel bracket that I salvaged from an old Magnavox tuner chassis. It's very rigid and does not require any more support than the single screw (one of the 2) that used to secure the old stand and the wide base on the bottom portion of the bracket provides a good chassis ground for the audio jacks.

Dual 1229 Fabricated Phono Jack Stand.jpg
 
Chip2020 said:
When all the obvious causes of buzz and hum are eliminated (poor grounding/loops, faulty cartridges/sleds, tonearm issues, etc.), I have found through experience that the audio cables and the audio connectors under the turntable chassis should be looked at carefully.

I had a Dual 1229 that I recently restored and it worked flawlessly... except for a slight, yet persistent buzz that although barely noticeable, was unacceptable to me. I knew my tube amp wasn't the problem (many other "buzzless" turn tables have been connected to it at one time or another), but I could find no obvious wiring problem or ground loop that could cause this annoying sound. So, I decided to replace the cheap audio cable that connected the 1229 to the amp, but the short distance between the left and right channel jacks of the audio connector board beneath the chassis, precluded using a high quality cable with large RCA plugs on the ends. This problem was solved by removing the original RCA jacks from the chassis bracket (actually I just used a spare "white metal" 1219 bracket I had lying around) and replacing them with BNC panel connectors (with grounded outer shell). Moving the connectors to the outermost position on the bracket increased the distance between the jacks, so BNC/RCA adapters fit right on. Then, I could've just used a cable with BNC connectors on one end and RCA plugs on the other to connect the turntable to the amp, but in this case, I was easier to just use a higher quality OEM audio cable with RCA plugs on both ends.

The turntable now plays so quietly that you can't even tell it's on until the music starts. Here's a photo of what it looked like after the BNC retrofit.

View attachment 1769394
Click to expand...

I by-passed the RCA sockets on the bottom of my 1229. The original Dual cables were bad and had to go as soon as I refurbed the machine, but later I figured I could dispense with cleaning those sockets, if I just soldered in my interconnects on the other side of them. The replacement cables aren't anything special or high-priced, it's just that they work just fine. My machine didn't have a ground wire for some reason, so I connected one to a nice sturdy screw in the underside metal frame.

An additional note about cleaning connections in the audio line: More than once I've had to clean spots I thought I had cleaned well enough a second time, when trying to get rid of a slight hum, distortion, or diminished signal on one side. Also, if the muting switch isn't closing or opening fully when it is supposed to (because of being bent, or being dirty) that can cause an audio problem.


.
 
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illinoisteve said:
I by-passed the RCA sockets on the bottom of my 1229. The original Dual cables were bad and had to go as soon as I refurbed the machine, but later I figured I could dispense with cleaning those sockets, if I just soldered in my interconnects on the other side of them. The replacement cables aren't anything special or high-priced, it's just that they work just fine. My machine didn't have a ground wire for some reason, so I connected one to a nice sturdy screw in the underside metal frame.

An additional note about cleaning connections in the audio line: More than once I've had to clean spots I thought I had cleaned well enough a second time, when trying to get rid of a slight hum, distortion, or diminished signal on one side. Also, if the muting switch isn't closing or opening fully when it is supposed to (because of being bent, or being dirty) that can cause an audio problem.
Click to expand...

Bypassing the RCA connectors is certainly a valid way to eliminate poor connections to the audio cable (the European version of the 1219 & 1229 came from the factory just like that), but I wanted to retain the option of easily changing out the audio cables if necessary. I also wanted to provide a good chassis ground right on the shielding of the audio cables for each channel (GL & GR) to prevent any ground loops. A ground wire was also added to the molex AC power connector, which is something that most US versions of the 1229 already had.

1229 Added Ground Wire.jpg
 
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