Rhodes Suitcase Janus Slider Preamp: Vibrato pots broken

[sopranojam85]

Hello everyone! First post here, and I hope I am putting this in the correct location. I have a Rhodes Mark I Suitcase piano which suffered a fall on stage. Everything was fixable, except that the vibrato speed and depth pots were damaged - their lugs sheared in half right where the lugs meet the pot's body, making them completely trashed.

I'm looking for replacements, and am having a heck of a time finding ones of the same size.

These are 25k and 100k reverse pots. The body of the pot is 24mm in diameter, and it has a 6mm-diameter shaft. The total shaft length is 25mm, but the tricky thing is that the threaded portion of the shaft is about 16mm long.

I understand why the threaded portion of the shaft is so long; it is used to structurally support the preamp chassis behind the nameboard. The threaded portion must protrude through the preamp chassis, AND through the nameboard.

I managed to find a few pots with a 25mm shaft, but none which had a threaded length even close to 16mm. (8mm was as long as I could find.) Reverse pots are the most limiting factor here, I think. They just are not as plentiful as standard pots. I've lost track of the number of suppliers I have checked with. I remember for sure that I've checked with Mouser, Digikey, Newark, Vintage Vibe, a few eBay sellers, and also Alpha. Alpha can make this pot if I provide them specs, but a minimum order size of 1,000 is required... 

I'd love to find an identical replacement without actually buying a junk preamp or Rhodes to do it. Any ideas?

If worse comes to worse, I will have to make a few mods to the preamp chassis (NOT the nameboard.. just the chassis behind it) to accommodate the body of a shorter-length-shaft pot, and come up with a way to support the chassis elsewhere.

[goldphinga]

I think a few pics of the front panel of your Rhodes will assist here. Any chance of seeing some?

[sopranojam85]

Having issues with attachments, so going to try one photo at a time. This photo is the name board, and preamp chassis behind it. Obviously the pots themselves are not installed, but you get the idea of their positioning and hole size.

[sopranojam85]

Here's one of the two victims. When I desoldered the pot, it just fell right off, and the break was evident.

[sopranojam85]

They're both this same physical size, but one is 25k, the other 100k. My ruler is in inches, but converting to mm, you can get the idea of the measurements.

I found a metric ruler and here's the measurements:

23mm pot diameter
6mm shaft diameter
25mm total shaft length
9mm knurled length
16mm threaded length
Terminals 7mm apart

Like I was saying earlier, the threaded length is so long because it goes through both the preamp chassis hole, and the name board hole, each having its own mounting nut to fasten it. It's both a functional pot, and structural support for the preamp chassis.

[sopranojam85]

I have finally finished this repair. Some of you may hate me for doing modifications to the preamp chassis, but everything I did was done to preserve the outer appearance of the piano, and to preserve its electronic functionality.

In a perfect world I would have been able to find the pots I needed, but it was impossible to find reverse log pots of these values, and of this size. So, I ended up buying small-ish pots of the proper values, but which were smaller bodied. They were 16mm diameter, with a 6mm shaft that was knurled (so it fit the original knobs just fine.)

(Another option here would have been to buy a separate Rhodes or vintage preamp module, altogether, but that was cost prohibitive.)

Since the new pots were way smaller, they could not serve the purpose of being a structural support for the preamp chassis. I had to construct a new support for that side of the preamp chassis. What I did was disassemble one of the old pots and remove just the threaded neck of the shaft. I shaved the bottom surface of it with a Dremmel, and epoxy'd it to the back of the name rail.

I then cut a hole in the preamp chassis at the exact place where this threaded neck was to protrude through that hole, and screw on with a nut. This worked really well, and had no visible impact on the front of the name rail.

In the photo, the brass threaded segment is this new support I'm speaking of poking through the inside of the preamp chassis.

Next, I had to attach the new pots. The shaft was smaller, so the holes that were there were too large. I also needed room for the new pot terminals, to make sure they didn't short against anything. I used a stepped drill bit and Dremmel to cut out a hole sort of shaped like a large key hole to accommodate the new pots. I CA glued rubber insulation to the metal chassis edge that was close to the pot terminals.

To make the fit of the pot shaft more secure, I CA glued a couple of medium sized washers to the front of the name rail. This was not my first choice, but it was the most sound choice structurally, and the washers are concealed by the pot knobs when the knobs are on.

Finally, I had to construct small extensions for the pot lugs to reach the PCB. I had some good quality tinned wire handy which I used to shape L-shaped extensions and solder them in. The photo shows these extensions. These extensions are a LITTLE bit flexible, so if the piano ever gets roughed around or dropped again, it will hopefully not break anything major, as there's some flex to these wires.

Everything is reassembled, and tested 100%. I'm very happy with this repair, and I am confident that it will never need to be repeated. But, heaven forbid, if it does need to be serviced, it is much easier to remove or replace these pots now that the necessary mods have been made.

While I was at it, I replaced the damper rail bushings, the RCA jack, and realigned the harp to adjust the striking point.

[sopranojam85]

Somebody has private-messaged me asking for more detail about what I did. I felt it was proper to place the information here. Sorry if some of this is redundant, but here goes...

The original two pots were 25k, and 100k reverse log pots. They were large, and had a long threaded neck. Finding pots of this value and size was impossible. At every turn, either the shaft length was incorrect, or the pot value was improper. The only option (aside from buying a spare Rhodes preamp altogether) was to buy correct-valued pots, but of a smaller size.

The original pots served as a structural support, as well. The new pots were not long or wide enough to still serve as structural supports, so a new structural support was created off to the side of the pot hole by epoxying a threaded neck (taken off of the original pots, I believe,) to the back side of the name rail, drilling a hole into the preamp chassis, and mounting the preamp to the name rail. (Photo attached.) See in the photo two holes marked in pencil. One on the left is the location of the hole I drilled for just the old pot neck, as a structural support. The one on the right is the hole to be drilled for the pot itself.

Since the shaft of the new pots was also smaller, and to prevent the leads shorting to the chassis, three measures were taken: 1) Holes of chassis were made larger, with room routed out below holes for pots terminals. 2) Insulation pieces glued to edges adjacent to leads. 3) electrical tape pieces placed on back of name rail where pot leads were present, just to prevent any shorts.

The name rail holes were not made any larger, but they were too large for the pots. So, I CA glued two medium-sized washers to the front of the holes on the name rail.

Finally since the lugs of the pots did not reach the PCB, extensions were crafted with tinned wire and soldered in to place.

I am sorry but I do not know what exact model pot I bought, but it is made by Alpha. I made sure they were 25k reverse audio, and 100k reverse audio.

[sopranojam85]

One more thing to note...

A few months after I made this repair, this same EP suffered some water damage from a leaky ceiling. The moisture caused some of my epoxy joints to fail, and the washers I had installed to fit the new pots no longer were attached firmly. I had to re-do some of this work.

I was able to save the piano though after a lot of work. And here it is after it was all said and done.

[Tim Hodges]

Whilst I haven't had a look around for those values I find that if a pot has broken like this I usually open it up and remove the carbon track board and replace it with another of a similar value and size.

Last one I did was for a Wurlitzer 200a, the volume / power switch is a pain to get if you're not in the US.

The resistance track was completely destroyed so instead of purchasing a new one for $25 and then the P&P of $34 I bought a new CTS 10k pot and used that as a donor to put into the original Wurlitzer pot. Total cost £2.50.

[sopranojam85]

Whilst I haven't had a look around for those values I find that if a pot has broken like this I usually open it up and remove the carbon track board and replace it with another of a similar value and size.

Last one I did was for a Wurlitzer 200a, the volume / power switch is a pain to get if you're not in the US.

The resistance track was completely destroyed so instead of purchasing a new one for $25 and then the P&P of $34 I bought a new CTS 10k pot and used that as a donor to put into the original Wurlitzer pot. Total cost £2.50.

This was what I tried doing initially, as I recall. But, the tracks were a different size. The diameter of the pot shaft does NOT always equal the inner diameter of the carbon track, so it is impossible to know by just studying pot data sheets for new pots. You just have to open up a few scrap pots until you find the one that fits.

[Tim Hodges]

Yep, I count myself lucky with that Wurlitzer one as CTS over the years seem to have stuck to the same size over time.