Rhodes MK I Stage Ground Hum

[ggengras]

I have a Rhodes MK I Stage (88 keys, date stamp 0277) and am pulling my hair out trying to get rid of a ground hum. The hum is coming straight off the harp (I have an RCA to 1/4" going direct into an interface) and becomes significantly louder when I touch any of the pickup terminals. After reading a bunch of posts on this forum, I tried to diagnose the problem to no avail. Here are some things I've tried:

• The RCA jack looks clean and seems free of corrosion and the quality of the sound / hum doesn't change at all when I wiggle things around there so I thought it was safe to rule that out for now?

• I'm unable to move my Rhodes but to rule out the hum coming from electronics in the room I covered the inside of the lid in foil tape which successfully insulated the harp from a laptop charging cable among other things that I tried bringing near it, so that seems to be taken care of.

• There is a solid ground connection to all of the pickup mounting screws. The tone bar screws have various levels of resistance to ground (0 to 100 ohms or so) and some don't read as grounded at all - however, after inspecting the metal tape beneath some of the 'ungrounded' tone bars everything seems to be fine so I chalked this up to corrosion or build up on the screws. Could this be an issue?

• All of the dampers seem to be grounded along with the rail that sits on top of them and the metal frame beneath the harp that holds everything together. There actually doesn't seem to be a wire connecting the RCA ground to the sustain rail? But when the harp is screwed down it seems like the sustain parts are properly grounded and bridging this connection doesn't do anything new.

• The solder joints all seem solid and no amount of wiggling things around seems to affect the hum.

.... what am I missing??? I can try replacing the RCA jack but it looks clean + like it was replaced already (some time in the last 40 years  )

I was just listening to a solo Rhodes track by Helen Sung recorded completely analog (so no de-noising!!) that is totally free of any buzz or hum so I know it's an achievable goal. Please help -- thanks!

[Jenzz]

Hi + welcome :-)

Give it a try to connect the damper bar to harp ground. The damper bar is electrically floating, since the axles are running in those nylon bushings. All noises that the bar recieves are then capacitive coupled into the pickups, which are right above, esp. on the bass side.

The older models had such a connection right off the factory...


Jenzz

[ggengras]

Hi Jenzz! Thanks for the reply.

After checking to make sure the damper rail was grounded I did also try alligator clipping it to the harp ground and unfortunately that didn't help 

[Jenzz]

Hi :-)

Did you plug directly from the harp RCA, while having all other connetions (preamp) disconnected?


regards, Jens .-)

[ggengras]

Yes I did! So it seems like the noise is coming straight from the harp rather than any of the front rail electronics.

[Jenzz]

Any transformers nearby?

Remember that the aliminum foil will only shield against static / capacitive coupling, but not against magnetics...

Jens

[ggengras]

I figured out the main source of the problem! In one of the pictures I included you can see a capacitor that someone had thrown in. Shorting this got rid of the majority of the ground hum so I just removed it altogether. There is now basically no noise coming off of the harp!

... however there is some more troubleshooting to do. My Rhodes has the front panel of a Suitcase but without the speaker so I have a Vintage Vibe cheek block power supply that powers the front panel and provides stereo output. Running the harp signal through the electronics introduces another ground hum and a thumping noise that happens when the vibrato is turned on.

It seems like there is an easy fix for the thumping but the extra ground hum could be a challenge.

I'm just running straight from this Vintage Vibe situation into an audio interface with no regard for impedance - could a DI box provide any noise reduction? I know very little about DI boxes but have seen some talk about this. It's also worth mentioning that the noise is about 30 dB below the level of a single note, I don't know how that compares to what others can achieve in terms of SNR.

[Jenzz]

Hi :-)

The additional cap is original on an 88 piano. It forms a high-pass for the last upper 12 notes.

Do all of the upper 12 pickups work?

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Jens