RCA cabient from scratch and help needed on the next steps

Posted: 7/7/2006 5:37:56 PM
Dayfan

Joined: 10/8/2005

Mike,

I sketched what to do to make your Radiola 60 power supply exactly the same as the Theremin power supply. I pulled the power supply out of my Radiola 60 to see exactly what you were dealing with. Refer to the sketch I made at http://members.tde.com/mmckeown/Theremin/rad 60.jpg and figure 7 in the RCA manual. In effect, all that is being done is make the power supply as shown in figure 7. In answer to your questions:

I would cut the pilot light wires at the resistor connections. I would keep the power switch and add enough wire to reach the new switch location.

The Radiola 60 gets the audio from the plate circuit in a different way than the Theremin. Make the changes shown in my sketch and figure 7 and you will be ok. Remember you now have high voltage on one of the output terminals. The Theremin runs the high voltage to terminal 2 through the speaker and the Rad 60 doesn't.

The center tapped resistors were a common way to reduce AC hum in the old days. Remove the black wire between the center taps and the green-red wire going to the output jack. Add a wire from the left, unconnected center tap to terminal 6. This in reality just grounds it. The green-red wire to terminal 5 from the other tap goes to the main Theremin chassis where it is grounded through a 550 ohm resistor bypassed with a 2 mfd capacitor.

Remove the wire going from the other output jack to the lower right terminal (#8) of the choke/capacitor assembly. Add jumpers between #2, 7, and 8 terminals of the assembly. Cut the red wire going from terminal strip number 2 to terminal 1 of the assembly so that you can connect each side of the cut wire to each output jack. Add the capacitor across the output terminals.

Colors are always a problem with these old cloth covered wires. Be very careful not to bend sharply or damage the cloth insulation. I use an ohm meter to be sure of what wire goes where.

The "plastic" ring in the bottom hole of the Rad 60 cabinet is similar to the one in the bottom of the Theremin so you can use that. Also the audio transformer marked GW-42 is the same as the interstage audio transformers in the Theremin.

Sorry about the oversimplification of the power supply comparisons but the changes are minimal and if you make the Rad 60 power suppy look the same as figure 7 you should be ok. When you power the supply up do it without the tube at first and if no smoke, put in the tube and check for smoke again.

It may take me a few days to get to the coils.

Mark

Posted: 7/7/2006 6:06:03 PM
mikebuffington

From: AZ

Joined: 11/25/2005

Mark,

You are amazing. The sketch looks like plenty of info to go off of. I will let you know any progress or questions when I get to it.

Cheers,
Mike
Posted: 7/7/2006 9:22:08 PM
Jason

From: Sammamish, Washington

Joined: 2/13/2005

I love to see readers helping each other out with projects like this. It's so much fun following this project along!!
Posted: 7/8/2006 12:13:06 AM
mikebuffington

From: AZ

Joined: 11/25/2005

Mark,

Just to check, my power supply has a green-red wire going from the first audio output to terminal 2 of the assembly. I don't see it in fig.7, and though you don't mention removing it, you don't have it drawn in. I assume it should be removed. (also, you missed one terminal in your sketch which would throw off the numbering - not critical, but thought I'd mention it. I might redraw all this info when I actually go about making these changes so others can use it as a reference).

One of the things throwing me off was the green-black wire. In fig. 7 it's just called "black." Your drawing has clarified much and made me more confident about what I'm doing.

I figured the ring cap on the bottom of the radio would be similar/the same, and I'm glad to know that the transformer is the same! Now another question: are the two concentrated coils inside the radio's oscillator coils the same as the one under the theremin's pitch coil? They look remarkably similar to the photos on Art's page. I don't have the ability to measure them yet, but will sometime soon...

Jason,

This discussion wouldn't be happening without this forum. I'm glad to get the help and I am also glad it's getting out there in case other people need this info/want to do something similar!
Posted: 8/6/2006 12:15:26 AM
mikebuffington

From: AZ

Joined: 11/25/2005

A little update to the project (link to my photo blog):

http://bedsidestory.livejournal.com/32379.html

Nothing new really, just some photos of the wire cutting process. next steps (and there are many!):

cabinet:
fix the doors
drill holes
finish

chassis:
bondo 2 of the 5-pin tube sockets and metal holes
sand and paint (match paint color)
drill new tube holes (4-pin) and reposition metal catches for the tubes
mill slits for new transformer

brass:
make antenna
plate them.

coils:
get new plastic coil forms or cut down cardboard, paint black and lacquer
wind wire (make a wiring jig first?!)

solder wires and resistors
get a few more miscellaneous parts
play it!
Posted: 8/9/2006 4:27:24 PM
Dayfan

Joined: 10/8/2005

Mike,

I checked your photos. Looks good. Answers to your comments:
Para 1. The copy of the Radiola 60 schematic that I have is small and of poor quality. If you break the red as I have marked you should be ok. Take out the green-red. The manufacturers were always changing things without telling anybody so you can see lots of strange things. I'm not sure about the terminal I missed. Can you expand?
Para 2. OK
Par 3. The "concentration coils" you mention are very interesting. They might work or in series might work as the concentrating coil in the Theremin. Measure the wire sizes and the physical dimensions as well as the DC resistance and post as well as a picture. I will look at it and let you know what I think. Good find on the audio transformer. I still need one more.

Mark
Posted: 8/11/2006 4:11:07 PM
organ man

From: Kalamazoo, MI

Joined: 3/20/2006

Nice job on your cabinet. I don't know if you've put your finish on yet, but back in the day they would use grain filler on Mahogany. Then they would stain it and varnish it like Mark said. I just used some grain filler (paste) not too long ago and I put on a coat of linseed oil first to keep the wood from getting blotchy with the paste. Then I sanded off any excess paste and applied my finish. I didn’t use stain, but sanding off the thin layer of linseed will probably allow the wood to absorb the stain nicely. I suggest a test piece.

Ben
Posted: 9/12/2006 11:52:55 PM
mikebuffington

From: AZ

Joined: 11/25/2005

New update on the theremin construction!

Since the RCA Radiola 60's chassis is so similar (probably the same stock) to the theremin's, I figured I should keep most of it intact as possible. I thought about removing the plastic plate that holds the tubes, but six of the sockets are on one strip and it would just get messy. My best bet was to re-drill holes. The Radiola uses seven 5-pin tubes and one 4-pin tube while the theremin has four 5-pin and three 4-pin tubes. Fortunately, the 4-pin socket on the Radiola is the same for the theremin, but two tube sockets needed to be re-drilled.

I measured the holes and created a jig. I have linked to a pdf file so others can use this too (one is sized for legal paper — don't scale down the image when you print it.). I found that if I rotated the tube 45 degrees, I would get 3 of the metal catches to still touch the tube pins and would have the cleanest drilling possible.

http://mikebuffington.net/photos/cuttingjiglegal.pdf
http://mikebuffington.net/photos/cuttingjig.pdf

http://mikebuffington.net/photos/cuttingjig.jpg
http://mikebuffington.net/photos/4pinabove.jpg
http://mikebuffington.net/photos/4pinunder.jpg

I have all the RCA tubes and now they all fit in the chassis.
http://mikebuffington.net/photos/alltubes.jpg

I sanded, primed and painted the transformer. I color matched the paint at Lowes. If anyone wants the info to get the same color, I can give it.
http://mikebuffington.net/photos/transformerbefore.jpg
http://mikebuffington.net/photos/transformerafter.jpg

Mark, regarding the numbering, I counted 9 terminals, numbered below. Your sketch only had 8 listed.
http://mikebuffington.net/photos/powerterminals.jpg

The concentrated coil is much thinner than I thought. It was hard to judge depth looking down the small coil form. As soon as it slid out after unscrewing it, I knew it was not like the theremin's. The dimensions are 1+1/8" O.D., 1/2" I.D., 1/8" thick. Don't know the wire gauge yet (it's about 1/64" thick). The coils measure 46.8 Ohms. If I measured the resistance correctly, that would be about half of the theremin's. I wonder if putting the two in series would work.
http://mikebuffington.net/photos/concentratedcoil.jpg

Ben, thanks for the comments. I will look into wood grain filler now, since I haven't finished the cabinet yet.
Posted: 9/13/2006 7:56:32 PM
Dayfan

Joined: 10/8/2005

Mike,

Clever fix on the sockets. I checked the number of terminals on the power supply and you are right, there are nine, I apparently didn't see that my #1 was actually your #1 and #2. This shouldn't make any difference with what you do with the red wire. You should check the voltages and see if they are in the ball park with the voltages in the manual. I used two inductors in series on the RCA replica I made and they worked fine. I think you should use the two coils you have in series. Be sure that the windings go in the same direction with each other and with the large coil itself. Are you going to use the other coils in the instrument? It should be fairly easy to calculate the windings you need to use the forms. They don't have to be exactly the same frequency as the RCA because you are hetrodyning them anyway especially with the pitch coils. You might have to play with the volume coil or parallel capacitors to get resonance with the volume oscillator. I use the following link for a nice coil calculator:
http://www.oldtemecula.com/
theremin/parts/calculator.htmoil.

Mark
Posted: 9/24/2006 12:07:36 PM
Dayfan

Joined: 10/8/2005

Mike,

I measured the inductance of the flat coils in the Radiola 60 and they are both 4.5 mH. I think if you put them in series that that will work. As I said before, be sure the windings of the big coil and the concentrating coils are in the same direction. I looked at the three 1.5 in diameter coils mounted on the back of the chassis and the forms should work perfect for the pitch and volume oscillators. Be sure to mount them as shown in the RCA manual to minimize coupling. The trimmers are too low in capacitance 58,58 and 35 mmfd to use but the flatish tin mica caps may be usable as well as the caps in the two cans. I will check the cap values.

Mark

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