How to Wire an RCA Varacoustic Ribbon Microphone
In the 1930s and 1940s, RCA produced the Varacoustic line of microphones as a forerunner to its well-known 77 series. The recording community's renewed interest in ribbon microphones at the beginning of the 21st century has led some people to experiment with the SK50 and 6203/6204 Varacoustic microphones. Many of these microphones are still usable but might require wiring repair or impedance alteration in order to be placed into service today.
Other People Are Reading
Things You'll Need
- Soldering iron
- Rosin core solder
- Flat-blade screwdriver
- 1/16-inch heat shrinkable tubing
- Heat gun
Instructions
-
-
1
Remove the three screws on the back of the microphone and remove the front half of the microphone housing. Do not remove the two screws holding the variable pickup pattern plate in place. Locate the four terminals on the transformer terminal board. Find the ground terminal by tracing the black wire from the ribbon motor to the terminal board. If it is not connected already, solder the microphone cable shield wire to the microphone ground terminal.
-
2
Check the integrity of the ribbon motor yellow wire. This wire should be connected between the ribbon motor and the terminal adjacent to the microphone ground terminal. If the connections are loose or damaged, re-solder the yellow wire in the correct location.
-
-
3
Identify the transformer primary winding wires. Solder one of the wires to the microphone ground terminal and the other wire to the terminal adjacent to the ground terminal. Identify the three transformer secondary wires and decide which available microphone impedance is appropriate for your application. The SK50 model offers 200 Ohm or 15000 Ohm wiring while the 6203/6204 microphones offer 30, 150 or 250 Ohm impedance options. Solder one of the microphone cable center conductors to the third terminal board connection and the other microphone cable center conductor to the fourth terminal board connection.
-
4
If you are using an SK50 model, solder the black secondary wire to the third terminal board connection. For a 200 Ohm microphone impedance, solder the green wire to the fourth terminal board connection. For a 15000 Ohm microphone impedance, solder the blue wire to the fourth terminal board connection.
-
5
If you are using a 6203 or 6204 model and want a 30 Ohm impedance, solder the yellow and black secondary wires to the third and fourth terminal board connections. For a 150 Ohm impedance, solder the yellow and green secondary wires to the third and fourth terminal board connections. For a 250 Ohm impedance, solder the black and green secondary wires to the third and fourth terminal board connections.
-
6
Insulate the end of the unused transformer secondary wire by sliding a short piece of 1/16-inch heat-shrinkable tubing over the end of the wire and shrinking it with the heat gun. While the tubing is still warm, pinch the heat shrink tubing with your fingers to close the end of the tube. Reassemble the microphone case and reinstall the three screws on the back of the microphone.
-
1
Tips & Warnings
Signal polarity is affected by the ribbon motor to transformer primary wires and by the secondary wires' connections to the output cable. If you find that your microphone is out of phase with other microphones, switch either the output cable wires on the terminal board or the ribbon motor to transformer wires. Do not invert both connections, since this leaves the microphone in the same phase orientation.
Some 6203 and 6204 microphones might lack a screw at the bottom edge of the microphone and are disassembled by removing the two screws at the side edges of the casing.
Do not use a continuity tester to check the integrity of the ribbon motor or connections to the primary side of the transformer. The continuity meter power can cause damage to the ribbon.
Do not allow any metal shavings or metallic dust in your workspace, since these can easily become stuck to the powerful ribbon microphone magnet.
Do not allow strong air currents or exhalations to strike the ribbon motor, as this can damage the delicate ribbon material.
References
- Photo Credit soldering image by Bube from Fotolia.com
