Uploaded January 2020 | Updated September 2026, 2 weeks ago
I needed a microphone for my Heathkit HW-104 which is identical
to the SB-104 but without digital readout . No one makes those ceramic microphones or crystal microphones anymore the Heathkit rigs came from Heathkit with a Turner 350 Microphone but the ceramic cartridge in those microphones is many years old now and few of those microphones still work . So I tried using a condenser microphone also called an electret microphone .I did not have success doing this I tried all sorts of things but the audio was very distorted . I was about ready to give up when 2 days ago I looked at the back of the microphone I was trying to use and saw there 1000 ohms impedance . Then it occured to me these radios were designed for a ceramic microphone the Turner 350 to be exact a high output higher impedance microphone . The problem was an impedance mis-match ? Could it really be that simple I thought . Yes it was . I took a 1000 ohm 1/2 watt resistor and soldered it from pin 16 of the B board the audio/regulator board to ground and got clean clear very good audio during transmitting . Sometimes the answer to a problem is very simple and I had
overlooked impedance matching the microphone itself . Note in the video you see two capacitors in parallel this was to get .1 UF a single .1UF capacitor is all that is needed also there are 2 15 K resistors in series or 30 K a single 27 k 1/4 watt or 1/2 watt is all that is needed actually anything close to that should work ok . on other manufacturers rigs you may have to use a resistor of higher value than the 1 K resistor to ground try some different values until you get good audio 4.7K 10K 2.2K etc two of those little small alligator clip leads are handy for doing this . One last comment I listened to this Heathkit HW104 on a dummy load with a Icom 746 there was HUM in the signal . I tried listening then with a Yaesu FT857 and the audio sounded great . On the air tests then It sounded great to other stations I got very good reports on the audio so be careful in testing I don't know why the Icom 746 had hum in it but testing with another rig and on the air showed everythin to be working FB 73 Bob
I needed a microphone for my Heathkit HW-104 which is identical
to the SB-104 but without digital readout . No one makes those ceramic microphones or crystal microphones anymore the Heathkit rigs came from Heathkit with a Turner 350 Microphone but the ceramic cartridge in those microphones is many years old now and few of those microphones still work . So I tried using a condenser microphone also called an electret microphone .I did not have success doing this I tried all sorts of things but the audio was very distorted . I was about ready to give up when 2 days ago I looked at the back of the microphone I was trying to use and saw there 1000 ohms impedance . Then it occured to me these radios were designed for a ceramic microphone the Turner 350 to be exact a high output higher impedance microphone . The problem was an impedance mis-match ? Could it really be that simple I thought . Yes it was . I took a 1000 ohm 1/2 watt resistor and soldered it from pin 16 of the B board the audio/regulator board to ground and got clean clear very good audio during transmitting . Sometimes the answer to a problem is very simple and I had
overlooked impedance matching the microphone itself . Note in the video you see two capacitors in parallel this was to get .1 UF a single .1UF capacitor is all that is needed also there are 2 15 K resistors in series or 30 K a single 27 k 1/4 watt or 1/2 watt is all that is needed actually anything close to that should work ok . on other manufacturers rigs you may have to use a resistor of higher value than the 1 K resistor to ground try some different values until you get good audio 4.7K 10K 2.2K etc two of those little small alligator clip leads are handy for doing this . One last comment I listened to this Heathkit HW104 on a dummy load with a Icom 746 there was HUM in the signal . I tried listening then with a Yaesu FT857 and the audio sounded great . On the air tests then It sounded great to other stations I got very good reports on the audio so be careful in testing I don't know why the Icom 746 had hum in it but testing with another rig and on the air showed everythin to be working FB 73 Bob
