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Bringing Back to life Maunette
 
 

In 2001 Bret Moreland bought the remains of an original RCA Theremin with the idea of restoring her and bring her back to life. Four years later, thanks to the exceptional work of Steven Hasten, that goal has been reached (as you can read in Bret's words in the Levnet post reported below) and Maunette (as Bret named her) is singing again. In this article, Steve tells us the story of Maunette's resurrection.

Date: Fri, 13 May 2005 11:50:30 -0700 (PDT)
From: Bret <........@yahoo.com>
To: levnet@korrnet.org
Subject: [Levnet] Maunette is Alive!

Brought back from her watery grave by Steven Hasten, Maunette, my RCA theremin, is singing again. Well, in my control she is moaning.

When I bought her 4 years ago, she was nothing more than a rusted main chassis and power supply. The theremin had been so water damaged that the only surviving piece of the original rca cabinet was the shelf mounted to the main chassis. The pitch coil was broken in several places, there was an extra audio transformer in the main chassis from a previous hack attempt by someone to repair her. The main tuning caps were stuck in one position, would not rotate. She was a mess.

The newly wound pitch coil is beautiful and evenly wound. Steven emptied the can holding the power supply caps (potted in wax), and replaced them with modern caps such that it looks original. He will have to list all the areas that needed repair/replacement. All the wiring is meticulously and cleanly done. Very professional work.

The crate arrived yesterday morning via DHL. It took great strength to remain at work, but I admit I went home early. The crate Steven made is built like a tank, designed to carry eggs. She arrived without incident. Steven gave me step by step instructions on how to open the crate (open this side, remove this piece of foam...). There were about 40 screws on the one crate panel. Thank goodness for cordless screwdrivers.

I carefully installed the tubes (again clearly marked with location), the solid brass antennae that Steven hand made, wired up a 15" jbl speaker, and plugged in the vintage looking cloth covered power cord. Nervously I turned on the power switch, the jewel glowed, in a few seconds she was screaming like a newborn - OOoooOOOoooOOooOOOOO. She powers up quite quickly and stabilizes without much wait. My dog was impressed.

The tuning caps for volume and pitch work smoothly. I thought I had read some comments about the main volume control not doing anything noticable, but this knob definately allows me to control the maximum loudness. The pitch control allows a wide range of zeroing, I just need to learn to set it to a usable range for play.

Since the original cabinet was destroyed by water, Steven had a new body made for my lady. She is gorgeous, made of oak, honey stained to show her lovely grain, and protected with many hand rubbed coats of tung oil. The cabinet is in the style of the rca, without legs. I will probably have a table made of oak, so that the legs look integral to the main cabinet. I need to figure out the optimal height for me.


This is the first full fledged theremin I have ever played (i have a pitch only theremaniacs device). I now understand how much of a challenge it is going to be to learn to play.

When I was beginning to plan the restoration, I asked Howie to recommend someone to rewind the pitch coil. I had planned to restore her myself. He suggested Mark Kepplinger. I wrote Steven Hasten to ask him about Mark, because I knew Steven had played Mark's theremin. Steven wrote back saying not only would they rewind they coil, but they wanted to do the entire restoration. I didn't know at the time that Steven builds Mark's theremins. I shipped the chassis parts off, and the work began. I believe Mark acted in an advisory mode, and Steven did the work. Thank you Mark and Steven, and Uncle Howie for sending me in their direction. Thanks to all the levneters for their great technical rca web pages, and advice to Steven during the restoration. Thanks to Carlton Smith for selling me the chassis'.

I feel like it is Christmas and my birthday rolled into one. best,

bret in boulder



Maunette "before" 001
Maunette front view before restoration.


I guess It was about two years ago when I was contacted by Bret Moreland.

He was referred to me by Howard Mossman because "Uncle Howie" knew that I was involved in working with Mark Keppinger here in Portland, Oregon. It seems that Bret knew right off that his RCA's pitch antenna coil would need serious repair, and Mark and I have some experience winding coils, since I help him build the tube theremins he designed.

I told Bret that Mark and I would love to handle the entire restoration.

I did not have much experience "restoring" old electronics but I am an avid electronics hobbyist and have been since I was young. Very young. I used to build "Linear Amplifiers" which used tubes to increase the output power of your citizens band radio to far above the legal limit of 5 watts. I was also familiar with tube electronics from the many jobs I had in my youth. I have repaired television and radio, and for most of my working life I repaired juke boxes and pinball machines.

Bret agreed to let us handle the project. I took on the entire job myself as Mark already had his hands full with his real job.


Maunette "before" 002
Maunette rear view before restoration.


Bret brought what he had of "Maunette", as she was named, to a local packing and shipping company near his home in Boulder, Colorado. They expertly packed and shipped her off to me to me here in Portland. Maunette's entire cabinet had been completely destroyed by water and the only surviving piece of wood was the slide-out shelf on which the main chassis is mounted.
The chassis were nothing more than a pile of rust, the pitch control coil was gouged out in many places. There were no antennas and we had only the rectifier tube.


Maunette Restoration 001
The three boxes Bret sent to Steve.


I recieved three boxes, one with the main chassis, one with the Power supply, and one with the antenna coils. Despite the fine job of packing there was shipping damage to the concentrated coil which is a small tightly wound spool of hair fine wire, located inside the base of the pitch control coil, along with a coupling capacitor. It had come off it's wooden core and partially unwound. only a few windings were disturbed and I was able to repair it and glue it back in place.



Maunette Restoration 002
Damaged concentrated coil.

Maunette Restoration 003
Repaired concentrated coil.


That was just the beginning and the rest took far too long to do because due to demands at my job, for about a year I had to work seven days a week with no days off. Then I got one day off and soon two.


Maunette Restoration 021
Volume coil bottom.


The volume antenna coil was scuffed in several places but there were no breaks.

[Click on images to enlarge]




Maunette Restoration 019
Volume coil back.

Maunette Restoration 020
Volume coil back closeup.

Maunette Restoration 022
Volume coil front.

Maunette Restoration 022
Volume coil front closeup.


The pitch antenna coil was another story. It was scuffed and windings were broken in many places.

[Click on images to enlarge]




Maunette Restoration 004
Damage.

Maunette Restoration 005
Damage.

Maunette Restoration 005
Damage.

Maunette Restoration 006
Damage.


I considered repairing rather than rewinding it but the damage was quite severe and with Mark's advice I rewound all 1440 turns with a hand cranked winder that Mark made to wind his theremin coils.

[Click on images to enlarge]




Maunette Restoration 007
Damage.

Maunette Restoration 008
Damage.

Maunette Restoration 011
Pitch coil back.

Maunette Restoration 012
Pitch coil front.


It has a counter and It really didnt't take long to do. I used a modern #33 wire which was recommended by Art Harrison who has done extensive study of the RCA theremin.


Maunette Restoration 017
Rewound coil.


Next I fired up the power supply and found that one of the plate voltage sources was gone. I traced it to a burned resistor. It was the large 2,250 ohm wire wound plate resistor mounted in the center of the SPU chassis underside. Mark found a similar resistor for me that fit the same mounting holes but the value was different so we piggybacked another resistor to trim the value to 2.25k.



Maunette Restoration 013
Power supply.

Maunette Restoration 014
Power supply filter ready to remove.


One component that can go bad over time is capacitors. When rebuilding tube amplifiers and the like it is common practice to "re-cap" it. We replace, without bothering to test, all the filter and bypass capacitors.

We do not test them because many will test good but introduce noise under power. Filter caps in the power supply can cause hum and bypass caps can provide less filtering, allowing for noise such as radio frequency whine into the audio. I chose to replace all the filter and bypass capacitors.
This is not for the timid.


Maunette Restoration 015
Power supply filter removed.


To preserve authenticity, since we are talking about a piece of history here, I chose to put the modern replacements inside the original cans that held the original huge paper capacitors. I removed the filter unit from the SPU chassis and by heating it in the oven I was able to remove the internal caps and replace them with new ones. Then I sealed the whole thing back up with wax and remounted it.



Maunette Restoration 010
Filter can caps open.

Maunette Restoration 009
Filter can caps removed.


Believe me, that sounds a lot easier than it was! The SPU was ready to go.


Maunette Restoration 018
Theremin power supply bottom view.


At this point I contracted a friend to make us a cabinet. Using the basic RCA style as a guide we created a beautifull solid oak cabinet. We did not try to copy the RCA exactly, we went for our own version. It cost less that way. The best estimate I could get for a replica was $700. Perhaps when Bret learns to actually play his RCA he will invest in a replica cabinet but for now the one we made is really gorgeous!


Maunette Restoration 018
Theremin chassis bottom view.


Next I recapped the main chassis and in testing the components I found another bad resistor. It was part of a multi section voltage dividing resistor centrally located in the chassis. It was the last section which is 10,000 ohms. Since it was part of a large multi-section unit, I could not remove it, and so I piggy-backed a 10k resistor onto the terminals.


Maunette Restoration 016
RCA Theremin detail.


Re-capping the main chassis was quite an undertaking and I am not sure I recommend it in hindsight. I might wait to see how the instrument plays first if I get the chance to do another RCA. I am currently working on setting that up!


Maunette Restoration 023
Maunette, rear view.


The tuning capacitors were frozen so I removed them and repaired them. I had to re-set the rivets holding the phenolic insulators to the frames and tighten the nuts that held the stators to the insulators because the plates were shorting together due to wobble. I oiled and remounted them.

There was a modern interstage audio transformer that was added to replace the one which couples the detector/modulator to the first audio stage. I found nothing wrong with the original transformer and so I removed this added one. The "supposed bad" transformer is in the plate circuit which is supplied by the plate resistor I found defective. I would assume that there was a short, likely a tube, and when the plate resistor burned open the transformer was thought to be burned open. Perhaps the person doing the work didn't test the resistor.

I was now ready to apply power to the main chassis and see if she made any sound. We bought a speaker transformer and the tubes and Bret had them shipped to me.

I wired up the transformer to the terminals on back of the SPU. I had to replace the original terminals because someone had soldered to them and they were unuseable.

I hooked up a small speaker and with the chassis and coils prawled out on my workbench and some stiff wire attached to the coil terminals for temporary antennas I put power to her for the first time in possibly seventy years.

With a loud screech Maunette came to life. I could see the control tube glowing and dimming as my hand approached the volume coil so I knew that it was working and only trimming the oscillator would be necessary to get correct response. The range was not playable but I did not expect it to be, Padding would definitely be needed. My newly wound pitch coil and new antenna would surely have a different resonance than the old arraingement.

The first cabinet My friend made came out wrong. The opening for accessing the front panel controls was too big and not positioned correctly. We also needed more room under the shelf for the SPU chassis. The cabinet was assembled with biscuits and glue and so there was no way we could just knock out the front panel and make a new one. I felt real bad for my friend, wasting all that material and time, but perhaps I will use it to build a custom theremin designed to fit it. If I sell it I will at least reimburse him for the materials. He made a second, slightly larger cabinet and this time I left the positioning of the shelf and cutting the control opening to myself. I painstakeingly measured, fit and ligned up everything then by putting ink on the tips of the tuning controls I pushed the chassis forward and let the control shafts mark thier own centers. I used those marks to cut holes with a hole saw and then a jigsaw to join those holes into a nice neat oblique. I filed, sanded, and sweated untill the hole was smooth and neat. Then I cut the hand hold and wire pass-thru holes in the shelf. With similar sweat I finished those holes as well. Solid oak is quite hard. I drilled the holes for the antennas and power and play/off switches. Then the cabinet went back to the builder for finishing.
A few days later I got it back, beautifully stained honey oak, and I put several coats of tung oil on it.


Maunette Restoration 026
Maunette, front orthogonal view.


Now to put her into her new cabinet and get her tuned properly. I needed to add small capacitors to the trimmer caps to bring the oscillators into the range specified in the RCA service notes.
This however caused the theremin to operate in a much higher musical register than intended.

It was way past the point where tone is clean and we hear lots of "birdies" as we approach the pitch rod. It was obvious that I had to experiment and find the frequency at which my new PCRC (Pitch Control Resonant Circuit) would best work. The frequency had to be high enough to play high notes, but not high enough to cause it to go to far out of resonance as you approach. That is the trick in designing a good linear theremin with good range. By adjusting the oscillators to the point where the PCRC has the most effect on swinging the frequency, but not too much where the resonant frequency of the coil is too far away from the oscillator frequency.

I also had to add some capacitance to the Volume control oscillator circuit to get the current swing correct. I did not have the reccommended adapter and so I used a terminal strip and jumper which I added to the back of the chassis to break the plate circuit of the first amplifier tube. The Service Notes say to adjust the frequency of the volime control oscillator so that 2 milliamps are flowing with the hand away from the loop. There is not any appreciable increace in volume if we allow more current, just cutoff becomes less effective. The Theremin still won't cutoff completely, there is still the faintest output unless you touch the loop, which is noisy to do. That is why Clara had a piece of tape on her loop. This is reported by many RCA owners and there is a fix involving the use of a small nine volt battery to provide bias. I did not try this, I felt "Maunette" had had enough for now. Bret can try that fix if he thinks he wants better cutoff. I also added a fuse to the line circuit. There was no provision for a replaceable fuse in the original design.

I learned something about about playing while the RCA was in my posession and that is that while modern theremins can easily provide complete cutoff, once one learns to play the theremin you seldom actually mute the instrument during a song. A beginning thereminist like my self will tend to rely on the cutoff to seperate the notes and hide the slides. As one progresses and developes a fingering technique less slide is heard and the volume hand drops less between notes.

As Charlie Lester helped me learn, that is what gives it a natural sound, not disconnected. No space between notes and no audible slide, unless you want to slide.

I kept the theremin here for many months more and experimented with different frequency ranges.

I left her on twenty-four hours at a time to see if she would overheat. She stays cool.

Drift after warm-up is minimal and she warms up quicker that my "new" tube theremin.

When I just could not think of another good reason to keep her, I packed her up and sent her home to Colorado.

[Click on images to enlarge]




Maunette Restoration 028
Maunette, rear orthogonal view.

Maunette Restoration 029
Maunette, rear view uncrated.

Maunette Restoration 025
Maunette being uncrated.

Maunette Restoration 027
Maunette, front view.


I charged Bret nothing for my labor since this was a learning experience. He did however send me a nice check as a gift of appreciation. I promptly spent part of it on a new Etherwave and I really like it. Thank you Bret.

I now am ready to hang out my shingle as an experienced RCA Theremin repairman and I invite anyone with an RCA, not working, to send it to me. There is not one part of the circuit that I have not studied in detail. I feel I could repair anything after this experience.

Listen to Maunette's voice in the Audio Library





 

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