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A Station Boom Microphone

A few years back I found an interesting webpage by amateur G4IZH on converting a commonly available BM-800 microphone for amateur radio use. His page provided detailed info on converting these into a decent station microphone.​
The BM-800 came in a variety of color choices and I chose blue and silver. On the inside it's an electret condenser microphone requiring a separate power source (phantom power) to operate.

The modification converts the output to unbalanced and tailors the frequency response for better performance with an amateur transceiver.

It might not beat out an expensive, high-end amateur microphone, but then I wasn't willing to pay the high-end price either. And I prefer DIY projects whenever possible - for me that's what the hobby is about.
Picture

BM800 Microphone Conversion

Although these mics are inexpensive, they are still reasonably well constructed. The shell and frame are all metal and only the internal electret mount is made of plastic. It's easy to disassemble the BM-800 - just unscrew the end and the metal shell slips right off. Then there are two screws inside that hold the screen in place. And there's plenty of room inside for the conversion.
Picture
There is one circuit board in this mic. Some of these had a second unused blank PCB mounted on the opposite side but not this one.

Picture
The screen removed to expose the microphone element/holder. The screen now has a foam insert unlike older BM-800's I saw online.

Picture
Thanks to the foam insert, the internal microphone element is not visible through the screen even when backlit, unlike older versions where the element was easily seen. Hopefully there shouldn't be any need to add the external wind cap. Also, the front of the mic element faces the lettering on the body.

Picture
Besides the foam cap, the accessories include a metal shock mount and 2-wire microphone cable. The shock mount works fine as is, but the original conversion circuit required a 3-wire mic cable because it used a separate lead for power. I altered the mic circuit to use phantom power on the audio line, eliminating the need for a third wire. Now I can use the supplied 2-wire mic cable along with my windcamp adapter to feed "phantom power" to the mic from the FT-817, instead of adding a battery inside the mic's case. I was able to match the hand mic's gain using the 5-volts available from the 817's microphone jack

Picture
This mic has a different electret module than the one shown on G4IZH's site. This one is smaller and has the FET built into the module hence there is no FET on the PCB. 

Picture
The only components on the back of the PCB are two 47uF electrolytic capacitors. The caps stick up about level with the frame, so I added washers under the add-in board's screw holes to space it up a little just to ensure nothing shorted to the tops of the caps.

Picture
A closeup of the PCB gives a better look at the traces and components. The red and black wires for the mic element connect on the left and the XLR wires on the right.

Picture
The mods to the PCB and connections to the add-in board. The blue cap is the 2.2uF across R4. The violet wire picks off the audio from Q5, sending it to the input of the Lo and Hi audio filters. The PCB traces from C5 to the collectors of both Q1 and Q2 were cut to remove them completely from the circuit. The white wire from the power filter connects at C5. Also, C3 and C4 were removed.

Picture
The add-in board has the capacitors and resistors for the power and audio filters, and the VR1 audio out level adjustment. Using an o'scope I adjusted VR-1 to provide the same level out as my hand mic.

I modified the original mod, adding phantom power for the mic on the audio lead, by removing the yellow power wire from pin 3 and connecting it to the red audio wire on pin 2. Then a 4.7uF capacitor was added between the red wire and the wiper on VR1, to block DC voltage from getting to the potentiometer.

Picture
Schematic with the component values in my converted BM-800.  A few of the part values in my mic circuit vary from the G4IZH schematic.

Picture
The converted microphone mounted on a scissor boom arm using the included metal shock mount. Not bad for less than thirty dollars including the scissor arm.

Last update 5/11/2017
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  • Home
    • About
    • Contact
  • Astrophotography
  • The Observatory
    • The New Observatory
    • - About the ExploraDome
    • - About the Lancaster Dome
    • - Building the Lancaster Dome
  • Equipment
    • CCD Cameras >
      • Orion Starshoot Pro
    • Mounts >
      • Orion HDX110 (EQ-8)
    • Telescopes >
      • Astro-Tech AT6RC
      • Celestron C-11 XLT
      • Zhumell 152mm Refractor
  • Projects
    • C11 XLT Mods
    • Orion SSPRO Camera >
      • Disassembled Views
      • Noise Reduction Mod
      • Temp Controller Mod
    • Ultima 8 Mods
    • Zhumell Refractor Mods
  • Non-Astronomy
    • Amateur Radio >
      • Boat Anchor Radios >
        • BC-348N Receiver
        • Collins 51J4 HF Receiver
        • R-392 HF Receiver
      • DIY >
        • Station Boom Microphone
      • Ameco >
        • Ameco AC-1
        • Ameco OCM
        • Ameco PT-2
        • Ameco R5A
      • Heathkit >
        • Heathkit SB-102 Station
      • Knight-Kit >
        • Knight Kit Span Master II
      • Yaesu FT-817 >
        • Yaesu FT-817ND
        • LDG Z-817 Autotuner
        • Windcamp LiPo Battery
    • Antique Radios >
      • Building A Radio Workbench
      • Radio Dial Neon Clock
      • Radios >
        • 1938 Zenith 6-S-254
        • Echophone EC-1
        • Eltz Radione 2
        • Panasonic RF-5000A
        • Silvertone Model 4465
    • Geiger Counters >
      • AN/PDR-27G Radiac Set
      • Lionel CD V-700 Geiger Counter
      • RCA WF-12A Geiger Counter
    • Test Equipment >
      • DIY Test Equipment >
        • DIY Curve Tracer
        • DIY Signature Tracer
      • Military Test Equipment >
        • BC-221-AF Frequency Meter
        • LM-21 Frequency Meter
        • TS-323/UR Frequency Meter
        • TS-465 C/U Sweep Generator
        • TS-505D/U VTVM
        • TV-3C/U Tube Tester
      • Tube Testers >
        • Alternating Current Sink
        • Drug Store Tester
        • EMC Model 200 Tube Tester
        • Hickok Model 6000A Tube Tester
        • Hickok OZ-1 (550X)
        • Precise Model 111 Tube Tester
        • Precision Model 10-12
        • Supreme Model 385
        • TV-3C/U Tube Tester
      • VTVMs and Multimeters >
        • B&K Model 290 Hi-Low FET Multimeter
        • Hickok 225K VTVM
        • McMurdo Silver VOMAX 900
        • Micronta 22-220 FET Multimeter
        • Simpson 360-2 Digital FET DMM
        • TS-505D/U VTVM
      • Hickok >
        • Hickok 19X RF Generator
        • Hickok 225K VTVM
        • Hickok 288X RF Generator
        • Hickok OZ-1 (550X)
        • Hickok 6000A Tube Tester
      • Simpson >
        • Simpson 360 DMM
        • Simpson 360-2 DMM
        • Simpson 480 Genescope
      • Supreme Instruments >
        • Supreme 385 Automatic
        • Supreme 560 Vedolyzer
        • Supreme 560A Vedolyzer
        • Supreme Model 561 AF/RF Generator
        • Supreme's Model 562 Audolyzer
        • Supreme Model 562 Audolyzer circa 1941
        • Supreme Model 562 Audolyzer circa 1946
        • Supreme Model 563 Audio Generator
        • Supreme Model 576 RF Generator
      • Other Test Equipment >
        • Rider 162 Chanalyst
        • Tektronix 326 O'scope
        • VU-Data Model 941B Oscilloscope
    • ---------------
    • Fossils
    • Rocks and Minerals
    • Star Trek >
      • Star Trek Rock Mood Lamp
      • Star Trek Communicator