Rebuilding an Astatic D104 Microphone

Introduction

At a recent Hamfest, I picked up an Astatic D104 microphone. As it turns out, there are two main models of these microphones - those with an internal amplifier and those without it. Internally the crystal microphone module has an impedance around 500k to some megaohms, which works well with older vacuum-tube radios, but for newer transistor radios a lower output impedance is desired. This is where the amplifier is useful. However, my crystal module appeared broken, so I needed a replacement and opted for a modern, low-impedance electret condenser.

Parts List

• Twinax cable to go from your radio to the microphone, plus a little extra for inside of it and stripping. Thinner twinax is better for this application.
• 2-pin control cable. Add two more pins if you modify the base to add frequency up/down buttons.
• Electret condenser module. I used a CMA-6542PF from CUI, but you should pick one with a lower bass response as this one is a little tinny.
• Connector to attach to the radio. Here's mine.
• Vibration-isolating padding. I used part of an old wool sock.

Tools List

• Several flathead screwdrivers.
• Small steel pick.
• Soldering iron. A variable power model is nice as you'll be soldering some small pins and some large solder tabs.
• 3.5mm audio plug pigtail. Useful for testing.
• Alligator clip wires. Useful for testing.
• Audio recorder. You can use a PC or a tape-recorder.
• Ohmmeter or beeper meter.

Disposables List

• solder
• solding flux

Component Testing

Before starting, verify each component works.

1. Test the audio recorder using the internal or a known good microphone.
2. Attach electret condenser to computer, referring to the manufacturer's datasheet for the wiring diagram. Use the alligator clips to connect it to the 3.5mm pigtail. Use your audio recorder to test it.
3. Double check twinax and PTT wire using the ohmmeter

Replace wiring on the head

The D104 comes in two main parts - the head and the stand. The head contains the microphone element.

1. Unscrew the head from the stand.
2. Remove 4 head plate screws to reveal the crystal module. You'll need two screwdrivers, one at each end, to do so. If you're a little OCD, remove each screw at a time, and reinsert it into the bolt so that the screws and bolts remain paired.
3. Desolder the two wires from the crystal module
4. (Optional)remove hidden screw from neck, and clean it. This screw will connect the twinax shield to the conductive body of the microphone.
5. Solder in new condenser. For now, make sure you connect the condenser ground to the common microphone ground.
6. Pack with sound-insulating material.
7. Replace screws.

Choose your own Adventure

At this point, you could trace out the wiring through the stand, add your microphone connector, and be done. Or, you could entirely rewire the stand yielding superior shielding and noise immunity over the original design. Up to you, but plenty of these microphones work without differential shielded audio feeds. Also, rewiring the base will give you the opportunity to clean the PTT switch, but I suspect it rarely gets dirty.

Rewiring the stand

Since my radio supports a separate audio ground than the digital one, and my existing base wiring only had one ground, I went ahead and rewired the base.

1. Open up microphone base by removing three screws.
2. Desolder all wires from wiring hub.
3. Remove existing wire going outside of the mic.
4. Remove flathead screw at top of tube
5. Desolder 3-pin connector at top of tube.
6. Remove two flathead screws from base of tube to remove lever.
7. Remove two smaller flatheads from tube. This disconnects the switch.
8. Remove last screw from base of tube.
9. Remove stand tube, may be a little sticky on the base. Pull the switch out the base.
10. Remove existing wire going up the tube.
11. Solder control cable to normally-open pins of switch.
12. Install twinax+switch into tube. Slide twinax and switch into base of tube. Install two small flathead screws.
13. Install connector on twinax.
1. Strip insulation from twinax.
2. Lay tube on side, so you don't drop any metal into it.
3. Use pick to unwrap shielding from twinax.
4. Solder twinax shield to head connector.
5. Solder two audio lines. Take note of which go to which pins noted in head rework.
14. Pull slack twinax back through tube, screw head connector in place, double-check PTT.
15. Route wires into base, place tube on base, install lever and opposite screw.

Assembly Testing

Testing PTT

1. Connect PTT and PTT ground to an ohm/beeper meter.
2. Press and hold button. Beeper should beep or meter should read close to zero.
3. Hold button down while moving wires, reading should not change.
4. Give them a bit of a tug, reading should not change.
5. Release button. Beeper should stop or meter should read open circuit.

Testing Audio

1. Connect Ohmmeter from mic pins. Mic should measure part of an ohm to a few ohms.
2. Connect mic pins to 3.5mm pigtail with alligator jumpers, test with computer or walkman.

Testing Crosstalk

1. Use ohmmeter to verify mic ground and PTT ground are disconnected.
2. Use ohmmeter to verify the only two non-open pins are the mic pins.
3. Using audio recorder, start recording. Press and release PTT several times. You should only hear the mechanical sound of the switch closing and not any pops or crackles.

Attach a radio connector

Connect up the connector corresponding to your radio. You may also need a transistor or resistor added to your PTT lines, but this is specific to your radio, not to the mic.