RMI 368x part 1: voltage conversion and clean up

Years ago I bought a pair of RMI 368-X Electra-piano electric pianos for cheap. Like sub $100 for the pair cheap. Caveat – they were both 220V versions. Other caveat – neither was in good condition. Upside? The old guy I got them from said he bought one of them in the late 70’s from a touring British prog rock band. “The one with the wood.” he said. He couldn’t remember the bands name and these were popular enough with rock bands in the 70’s that it would be hard to deduce who it might have been.

In 2020 I bought a big house on 9 acres and after getting my life set up I resolved to get all my music equipment in one place and working. Low on the list was the pair of RMI 368-Xs hiding behind the camping gear in my mothers garage. Low until I decided I wanted a fully polyphonic basic piano to use. CS-60 wasn’t cutting it for simple composition and the Hammond was buried in the barn. The glowing Soundonsound review of the RMI added to my enthusiasm.

The worse off of the two is pictured above.

The 368-X is the same as the earlier 368 model, but has a plastic clam shell type enclosure where the earlier models had wood. Keys are basic Pratt-Reed and this example needs more than a few if I am going to replace the ones with cigarette burns. Someday I suppose I’ll fix this one up – especially if the asking price of $1000+ I see on Reverb is ever realized by anyone. One sits completed on eBay as I type here – sold for $875 with the foot controller and local pick up in Prescott Arizona. Maybe I’ll fix this up now so I can afford the Rhodes a friend is selling.

Lets move on to the wood one and fixing it up.

Starting work on the better of the two. Here my assistant tech / apprentice is removing keys to be cleaned. You can spend a lot of time trying to clean a the keys in situ, one of those short cuts that is really not a short cut, or you can suck it up and remove them for a proper cleaning.

There are a LOT of capacitors in this thing! Each note has an oscillator and what I suppose is a filter network to limit the passed frequency range in a few parallel paths to form a complex note and the electrolytic probably stores a little energy so the note doesn’t drop off after the initial sound. I bet more than a few of these electrolytic caps are out of tolerance. I may have to replace them someday. Also of interest to maybe one or two of you: each note has an AC122 Germanium PNP transistor. Can you say fuzz stomp box?

Key bed was entirely removed for cleaning and so I could attempt to get inside the power switch to remove R4, the dropping resistor that sends a manageable amount of voltage to the pilot light on 220V units. After the switch to 120V I am pretty sure this resistor will keep the pilot light from glowing at all. There is a lot of additional disassembly to do to get at that resistor so I decided to save it for when I replaced the capacitors I identified as troublesome after testing.

It’s important to be observant when working on something like this. Even if you have no technical knowledge I bet you can identify that one of these contacts is not like the others. This got bent back into place by some needle nose pliers

Many hands make the load light. These keys were really dirty. I like to soak them for an hour in a bucket of warm water + dish soap then give each a quick scrub then rinse. The midday sun is an effective drying agent!

While the keys were being cleaned I got into the electrical. Wire nuts, packing tape and frayed or disconnected wires are not very confidence inspiring. I guess it needs not only conversion to 120V but some clean up. Note the “D. F. 10 16 73” hand written note.

Not a very good scan, but all I could find on the Internet. The 40VAC means I need a center tapped +/- 20V transformer. The label on the keyboard says 5W, so 5W/40V=125mA. I’ll give it a safety factor of 2 and say I need a transformer with at least a 250mA rating. I found just such a beast on an electronics surplus site for $20 shipped. Raj who keeps Lazyelectrons helped me figure this out.

Here is the new transformer set into place for testing. The mounting holes don’t line up so I will have to make an adapter plate. Note that key contact has been bent back into place.

Sometimes you just have to go for it. I had this chunk of aluminum laying around from a reel to reel I parted out. I eyeballed a profile that would fit the existing mounting holes, made some marks through the mount holes and set about to make it. It’s easy to get hung up trying to make a part like this perfectly, when good will do.

Note there are two sets of mount holes. I was rushing to get this done before I had to do some chores and I oriented the rectangular pattern of holes incorrectly. Those two black metal clips act as nuts. They were also in the reel to reel parts pile – the new transformer will mount to those.

After its rectified and regulated it’s supposed to be 24V. 27V shouldn’t hurt since I’d bet every component in this thing is rated at 20% tolerance with probably a 50% or more safety factor on top of the tolerance. There is probably a pot on the power supply board that would reduce this – need to have a look. Wall voltage is about 10% higher now than it was in the early 70’s, so this reading makes sense. More electrolytic capacitors I will probably someday have to replace.

After a lot of work, liberal application of soapy water and some metal polish the wood one looks nice! How does it work/sound you ask? It sounds like an RMI!

Here is the list after testing. I’ll be fixing these someday I suppose…

  • Lowest note doesn’t sound
  • Volume slider is scratchy
  • Pilot light doesn’t glow (as expected)
  • Organ setting is extremely quiet compared to the other settings.

I’ll also be fixing the second piano to sell before too long – drop me a line if you might be interested. It will not look like the broken down hulk in the first picture.

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