Repairs, Maintenance & Upgrades > Preamps, Modifications & Upgrades

Rhodes 54 preamp level very low compared to harp level

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musomarc:
Hi all Rhodes lovers,
Working on restoring a Rhodes 54 stage and have just about finished BUT am getting a very low signal from the jack out on the preamp rail compared to from the harp ( via RCA).
It's probably X3 the level from the harp compared to from the jack out on the piano preamp rail. I've looked everywhere but can't seem to find a schematic circuit diagram for a Rhodes 54 preamp amp ( the one with 2 slider pots one is volume the other is tone).
Does anyone have a schematic for this preamp?  Possibly there may be a problem from the preamp wiring or components!
Or any suggestions to boost the output level from the jack out rather than use the rca Harp output. I know some people use external preamps e.g VV Tone bomb, but am wondering if there is a simple alternative to check my preamp and level or to boost it.
Great forum and always helpful advice Thanks!!!!

Cormac Long:
Stage passive controls..



There's no actual preamp here as its unpowered. I'm not familiar with the 54 in person but its the same passive wiring used in other stage models. Aside from sliders/versus circular pots, there may be fluctuations in actual resistance values used given the shorter set of pickups and wiring variations of the 54 harp.

Also attached to this post is a simplfied image someone posted way back on how the circular pots for the stage models were wired. Its the same wiring as shown above.

See if you can following the wiring and confirm its as shown. We can go from there then in troubleshooting the signal drain.

David Aubke:
I just pulled the Tine Bomb out of my Fifty Four. It works fine but doesn't do anything for my tin ear.

I referred to a drawing I made when I first installed the preamp to restore things to their original state but now it doesn't seem to work properly. My volume increases slowly then swells drastically during the last ten percent of the fader's range. Also, my tone seems to be non-functional. I'm feeling quite certain I've put it back the way it was but if anyone has any thoughts, please chime in.

Anyway Mark, here's a drawing of what I believe to be the correct wiring. Does anyone know the values of the two faders?

Cormac Long:
David,
   in your wiring, the capacitor needs to be shorted out on the tone/bass boost when its up full. So the pickup hot runs serial through the capacitor onto the volume pot and the tone is merely shorting out that capacitor, negating the bass filtering. Based on your image there, I suspect you're getting no bass.

As for the non-linear volume, it could be an aged pot, but equally a facet of the amplifier. There's a balancing game being played with a volume pot in that its 2-way setup to add resistance between the jack tip and RCA signal while you reduce volume while at the same time reducing resistance between the ground and jack tip. The logic with that is that it tries to maintain a continual resistance over the ground and signal out. That helps avoid buzz if the impedance goes too high... otherwise you'd get a buzz increasing as you turned volume down.

The original resistance rating should be on the sliders and I'd start with trying to source a modern equivalent from an electronic parts shop.

You can also get one readymade from VV...
http://www.vintagevibe.com/shop/rhodes-54-note-passive-electronics/

musomarc:
Hi Cormac and Dave,
Thanks so much for your speedy response and great help with the wiring details, diagrams etc!
Dave, when you used your 54 without the VV tone bomb preamp did you also only get a very low level of output from the piano?
Regards :) ;)

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