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the ground loop hum is going to kill me

Started by Wardok, December 16, 2010, 12:34:38 AM

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Wardok

Okay guys, I've read numerous other threads on evil 'hums' and have posted in another thread but not received any joy, so please forgive me starting another topic.

My rhodes (mark I 1975) has had an annoying buzz since the day I got it . It sat unloved for a couple of years but now I'm going thru the restoration process and basically ready to voice it. Quite an annoying task with the buzz that I've got happening.
The buzz is not due to my PA, speakers, or cables. It's coming right off the RCA harp and It's a ground loop.

It's not the Rhodes pre-amp, seeing as it's bypassed.

It's NOT being caused by damage to the aluminium tape, as fas as I can tell. I took apart the entire harp and also put on some new tape where it had worn somewhat.

I don't believe it's being caused by a 'bad' pickup, since all the pick-ups sound as they should when playing their respective keys.

I don't believe it's the 'curly' wire - although I could be wrong on this. There's no crackles, buzz, or anything when I shake this wire, which makes me think it's OK - although, It could also mean it's completely shot.

I'm certainly no electrician, so If someone has a surefire way to locate this buzz (a more detailed description of how to use process-of-elimination on the pickups etc?) I'd really appreciate it - I'd love to have my piano sounding great, but would equally love to avoid dismantling the harp and pick up rails AGAIN if this is at all possible - you know how time consuming those damn tonebars are.

LOVE,
Penny!!

Cormac Long

Penny,
   based on your other post (http://ep-forum.com/smf/index.php?topic=6126.msg30701#msg30701), I'm suspecting that the ground shielding is not properly connected to the tonebar and/or pickup strips.

If you have a multimeter handy to test the continuity you could probably verify this. You should be able to make contact with the strip at the lowest bass tonebar and test if it has a connection to the outer metal band of the RCA. Test the same with the pickup strip.

My piano is also a Mk1 '75, so I'll try and photograph the harp wiring in detail tonight in case there's a difference you can spot.

My piano also had an unshielded ground cable about a foot in length with a female spade connector. It was hanging loose when I bought it. There's nowhere on the harp or elsewhere where you can obviously connect this. However I've read that these were usually connected to the aluminium damper rail. If your piano has one of these, see if you can connect it to the damper rail to test if reduces the noise.

Lastly.. nearby fluorescent or transformer-based lighting and other electrical equipment can induce noise on a Rhodes harp.. they are like antennas for this kind of interference. So make sure you are eliminating all possible sources of interference. I have a retractable fluorescent desk lamp that I use to shine light over the piano and harp when working on it.. it induces noise on the monitors when the Rhodes is connected. It can be retracted and 2 feet away and still induce this noise. Disconnect the amp from the Rhodes and the noise is gone.

I've seen similar issues with a laptop being close to the harp.
Regards,
   Cormac

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sean

#2
Well, there is no reason you have to live with a buzzing Rhodes.  We will find the problem.


The first thing to confirm is that your cable is a shielded audio cable, and not a speaker cable.

Step two is buy a cheap multimeter and a soldering iron.  (Alligator clips are handy too.)

#3:  Suspect the RCA jack, but check the ground connections all over the harp first.

Make sure that your wiring on the treble end of the keyboard looks like this:


- Note that the red wire from the last pickup and the jumper from the tape under the tonebars must both connect to the black squiggly wire.  

- The black squiggly wire runs over to another terminal lug at the RCA jack.  The ground connection of the RCA jack should be in direct contact with the foil tape that runs under the pickup mounting tangs.

- At the RCA jack, the red wire from the first pickup is soldered directly to the center pin of the RCA jack.

- All the pickup mounting tangs, and one tonebar mounting screw from each tonebar should come into contact with the foil tape on top of the harp.


If all of your wiring matches this, then you should double your suspicion and contempt for the RCA jack itself.  Corrosion of the RCA jack can cause a nastly layer of oxide, and the result can be low signal or buzzing or both.

If you don't feel like replacing the RCA jack, it is perfectly fine to clean it with sandpaper.  Roll up a tiny piece of fine-grit sandpaper and stick it into the center hole of the RCA jack, then twist and turn and scrub until you get nice clean metal again.  Also give the male connector that you plug into the RCA jack the same cleaning treatment.
    -  -  -  -  -

If the buzz doesn't go away, then you have something sneaky that looks like all is well, but isn't.   Hmmm....

Take your cheap multimeter, and set it to Ohms at one of the low ranges, like zero to 200 Ohms.

You should be able to touch one multimeter lead to the ground lug at the RCA jack, and get a zero or near-zero reading from any pickup mounting tang, any tonebar mounting screw, any tine mounting screw, or any place on the foil tapes.  Sometimes you have to push hard and scratch through the surface layer of crud on the screws.

If you find any portion of the harp that is supposed to be near ground, but does not show less than 1 Ohm resistance from the RCA ground lug, then you need to repair the break.
    -  -  -  -  -

Double check that the portion of the foil tape that you already replaced is making good contact with the other part of the original foil tape.  If you folded a section of tape over at the end, sometimes this makes good contact, sometimes it doesn't.  If you did not fold a section of tape over at the end, you need to electrically connect the two tapes with a jumper wire, or with some other foil tape origami.
    -  -  -  -  -

If still you have buzz, then use one of your multimeter leads to make a temporary ground connection from the RCA jack ground lug to the surface of the damper bar.  If the buzz goes away, then create a permanent gound connection to the damper bar.
    -  -  -  -  -

Check the circuitry on the name rail.  Plug the sandpaper-cleaned RCA plug on the grey cable into your harp's RCA jack, and use your multimeter to confirm that you have a low-resistance connection from the ground lug at the RCA jack to the sleeve of the 1/4" jack on the name rail.  
    -  -  -  -  -

If after you complete all this, and you still have a buzz, then please reply with pictures of everything.  We should be able to sort this out.  I assume you have already seen the threads below:

http://ep-forum.com/smf/index.php?topic=6126.msg30175#msg30175
http://ep-forum.com/smf/index.php?topic=4594.msg22223#msg22223
http://ep-forum.com/smf/index.php?topic=3711.msg16468#msg16468

Best regards,

Sean







sean



I just ran downstairs and poked around with a meter, and discovered that it is very hard to get a connection to the aluminum mounting tang on the pickups.  I had to scratch very very hard to get through the outer layer of corrosion.  Same problem with the pickup mounting screws too.

Also, some of the tonebar mounting screws that perforate the foil tape are NOT electrically connected to the foil tape.  Sometimes the mounting spring makes the electrical connection, sometimes it doesn't.

So sometimes a tine and tonebar assembly might not be connected to ground.   

I assume the same is true for an occasional pickup mounting tang.

Maybe these connections are not as critical as I thought.

Also, I wonder if your replacement foil tape has a conductive surface.  It might have a layer of "protective" plastic that would prevent it from working as intended.

Sean

Cormac Long

#4
Did some poking myself....

I got continuity on every single pickup mounting tang. For the tonebars, I'd say about 15 of my 88 didn't seem to be making contact with the aluminium strip.

Some images of the wiring...










Regards,
   Cormac

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sean



Dear Penny,

Don't let Cormac's photos freak you out.  The 88-key piano is wired very differently from your 73-key piano.

On the 88, the black wire is not squiggly, because it is a shielded conductor that brings the ground and hot audio signals from the far end of the piano.  This is because the top dozen (baker's dozen) notes are segregated off and sent through a capacitor to remove some of the ka-whack sound that the hard wooden tips make against the tiny short treble tines.

Your 73-key piano does not have the terminal strip and sprague orange-drop capacitor at the treble end.

Best regards,

Sean




Wardok

#6
Big hello everyone and thanks for the informative replies.

I got myself a cheap multimeter, and assuming i'm doing everything right, here's whats happening:

The 'normal' reading on my rhodes is approximately 2 to 3 ohms. Ok, so maybe this is a problem in and of itself, but for now let's just accept that as "ok". I find that my pickup rail is good; ie: gives this reading on all pickups. I also get this reading on the metallic tape on the pickup rail, at both ends. I get this reading on the metallic tape at both ends of the harp rail, too (where the tonebars are.) So far, no too bad. The problem is on the tonebars themselves, where a significant amount of them give readings between 60 and 100. Some of them are even more erratic - they seem to change! When I've removed some of these to check; the tape underneath is not damaged. I've tried cleaning the springs with steelwool and sanpaper to reduce their corrosion, with inonsistent results.

The screws themselves are all new, no corrosion. I also dismantled the RCA jack and although it didn't really seem corroded, gave it a going-over with the fine sand paper.

I guess I'm learning a little, but I'm very frustrated still. I've learned that part of my noise problem is that the audio I'm getting is extremely low anyway. I hadn't really noticed through the pa, but when plugged straight into the ol twin, well, it's pretty damn week.
I'm assuming a healthy rhodes should have a signal of comparable hotness to a guitar? Because this one is definitely not.

thanks everyone for the help thus far... and in advance for any further advice :)

PW!

sean


But you have made progress!!

The reading of 2 Ohms is much too high.  This is especially bad if it is a consistent reading from the vicinity of the RCA jack to other places that should be "ground" connections (like the other end of the black squiggly wire, the nut around the 1/4" jack, etc.).

Maybe you have a bad solder joint at the RCA jack.

You should inspect the solder joints at every terminal lug, and re-solder them to ensure a solid bright-silver-color low-resistance connection.


================

The Rhodes signal output is less than what you will get from an electric guitar.  You will need a tiny bit more gain for the Rhodes, but the guitar amp should be completely adequate.  You don't need any preamp before the Twin, just set the volume a little higher.

sean

Cormac Long

Would a cheap and cheerful test be to use a crocodile alligator clip or whatnot to quickly bridge the RCA bracket to the first pickup tang or even one of the hex mounting bolts?

Maybe even a manually held/taped down wire to connect the two might yield results. Aside from any noticeable improvements in audio, a test would be to repeat the resistance test and see if the 2 Ohms can be lost.
Regards,
   Cormac

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Wardok

Hey everyone,

- I decided it must have been a bad RCA. I've replaced with a shiny new one, the difference is negligible. Some crackles are gone, but the buzz is still there.

- Bridging the ground tab to pick-up tangs with a temporary wire made no difference whatsoever.

- temporarily connecting the audio tab on the RCA with a length of wire, to anything in the ground path, shorts the audio out.

- Reassembled, with the non-shielded wire attached to damper rail, things are definitely better - but they always were in this scenario. The buzz is still there. It just becomes even worse when you take things apart to WORK on that buzz!

- FYI it's a high-pitched buzz, not a low hum.

- I've tried turning off the lights and all other electrical items in the vicinity with no difference to the buzz sound. I even removed the nameplate just in case somehow its proximity was giving off some freaky unexplainable radiation! no dice.

- The only way to completely remove it is STILL to lift the harp assembly away from the keybed and be touching it with my hand. In this scenario the tines sound clearly and there is no buzzing at all. bliss!!! But hardly practical.


I'm starting to lose a bit of heart on this project! I should be playing the darn thing by now!
again, thanks for your interest though and your helpful suggestions.

P.W.

Cormac Long

Is it the amp and input impedance?.

The Rhodes output is akin to that of a guitar.. high impedance. It needs to be fed to a high-Z amp input or first run through a DI or preamp to correct the issue. Failure to do this causes humming noises in the amplifiers. Guitar amps or those with High-Z inputs avoid this. I must admit I ruled this issue out here day one when you asserted that it was not an amp issue etc.. but now I'm not so sure.

I used to use an old hifi to amp my keyboards art home... It worked fine... no noise.. But I had to get a preamp to get it to work with my Rhodes. When I connected it, it worked.. volume felt lower and there was this higher-pitch buzz. I was initially convinced the wiring was dodgy. But I corrected the issue by getting a mixer with a built-in mic preamp and used that to act as a DI for the high-Z signal and also provide much needed boost. Since then, I've found that a DI box will quickly solve the impedance issue but you still need the preamp. So I now use a dedicated preamp at all times. The output signal is essentially now a standard line signal and literally usable by any amp.
Regards,
   Cormac

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Wardok

#11
I don't want to pretend to know a LOT about impedence and all that kinda thing, but I've swapped out leads, amps and speakers in my efforts to locate this noise.

when I hook up using a normal audio cable to a Fender Twin amp, I get the noise. When I hook up through a pre-amp and mixer it's the same thing. (also I'm not using speaker cable. It's normal grounded audio cable, with a mono jack plug on one end and an rca male on the other. It's wired correctly.

My feeling is that there's poor shielding or something going on my piano. I can't see what else it can be - All the pickups sound good.
How do I check that each of the pickups is electrically sound, with my trusty (but dirt cheap, and possibly inaccurate) multimeter?

i seem to have a solid ground path. touching wire from the rca audio lug to anywhere in the ground path mutes the audio. Maybe the ohm reading of '2' is just a sign of how cheap the multimeter is, and is within its margin for error.

Here's something else: I can DEFEAT THE HUM by running a temp wire from the ground path, directly to a wire lug on a pickup! Of course, however, this has the effect of muting all audio for the pickups further down the audio path ...  Is that a clue to something??
could that mean what I'm doing is effectively bypassing a buzzing pickup??




If that's not the answer, them I'm really, really downhearted at the moment!! . But thanks everyone, too.

best,
Penny :)

Cormac Long

Penny,
   owning a Rhodes is like a Liz Taylor and Richard Burton marriage.. full of love and hate at the same time. Some months back, I got a validation email off one of the forum members where we posed the question.. "What is a Rhodes piano?" His reply was that it was a "magical instrument designed by the devil himself". He was not wrong. You are experiencing the low points at present.. we're here to give you therapy... we'll do our best!

Unfortunately I'm not in Ireland.. in Dresden, Germany for Weihnachten mit dem in-laws. I'm over 1300Km (800+ miles) away from my Rhodes at present so I can suggest but cannot verify.. hopefully Sean et al can assist more.

OK, so assuming its not the amp or impedance. Then its got to be connection. I made the observation on my rhodes that the pickup tangs were well connected to the aluminum strip. When you tried the simple short between the RCA ground and pickup tang, it made no difference. It honestly should have had this been a poor connection.

Based on your original post in http://ep-forum.com/smf/index.php?topic=6126.msg30701#msg30701 ....
QuoteHello all, I too have a horrid ground buzz that I just can't beat. The only way I've worked out of eliminating it is to have the harp raised, and to touch the metallic frame of the harp at the front, and touch the tonebar rail as well.
.. makes me wonder about the damper rail.

The next suggestion I would make is to run a wire from the RCA ground to the damper rail. I'm not sure if all Rhodes had this (mine does). It would seem the damper rail was capable of causing interference and grounding it along with other conductive components helped to avoid this by making it part of the shield. Not that I can recall if my RCA ground makes contact with the harp frame, why not also include that in the connection. The reference to canceling this by touching it and a tonebar suggests it may be a contributing factor.
Regards,
   Cormac

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geyster

Quote from: Wardok on December 22, 2010, 04:27:59 AM

- The only way to completely remove it is STILL to lift the harp assembly away from the keybed and be touching it with my hand. In this scenario the tines sound clearly and there is no buzzing at all. bliss!!! But hardly practical.

Here's something else: I can DEFEAT THE HUM by running a temp wire from the ground path, directly to a wire lug on a pickup! Of course, however, this has the effect of muting all audio for the pickups further down the audio path ...  Is that a clue to something??
could that mean what I'm doing is effectively bypassing a buzzing pickup??


Hey mate,

Have you come over your grounding issues?
I have the EXACT same problem with my Stage 1972 and the 2 "workouts" you're stating in the quote above do remove the hum on mine as well, so it seems we have exactly the same issue!

I also made numerous tests with my cheap multimeter, I even replaced 1 or 2 pickups that sounded slightly weaker than the others (but still working good)...
BUT the WEIRDEST thing:
Owning another Rhodes (Stage MK1 1979 that is extremely quiet, no buzzing or hum at all!) I went to swap the harp of my 72 to the harp of the 79 (and vice versa) to see if it made any difference...and guess what ? The exact same hum was also present with the 1979 harp !!!! And the 79 with the "presumably faulty 1972 harp" was quieter than ever !!!

So my conclusion is: the hum has nothing to do with the harp (hence nothing to do with pickups, foil tape, RCA connection, tonebars...) !

It has to do either with the whole action shell (maybe the 79 rhodes metal frame harp support vs the wooden one on the 72 makes the difference in some way?) or maybe the damper bar is made from a different material on the 79 rhodes and creates a "natural" shield with the pickups, therefore eliminating any hum or buzzing that could be created by approaching the pickups to the damper bar ?

Well, let me know if you've worked it out.
Thanks!

Geyster

Cormac Long

The damper bar is indeed looking to be the culprit here!
Regards,
   Cormac

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