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AT-6666/SS 9900 mic wiring finally Demystified and Accurate

Dr_DX

Well-Known Member
Jan 29, 2006
236
280
73
I have seen so much misinformation about this topic here and elsewhere on the web, that I felt compelled to make this post. I hope AT-6666/SS 9900 owners will be able to use this information to successfully wire ANY microphone to their radio. Whether it will sound good or not, is another matter.

To make any microphone work, including the UP/Down channels, you only need 3 wires. That's correct. Pin 1 = Audio, Pin 3 = TX/Channels, Pin 5 = Shield/ground. Even though the mic plug is a 6 pin, the stock mic that comes with the radio only has a 4 wire cord. There are no wires attached to pins 4 & 6 in the mic plug. They are not needed.

In a nutshell:

At least 2 different mic PCB's:
PCB CBM01 ver 20150820
PCB CBM01 ver 20180813 - has additional 102 from audio (pin1) to ground

mic side pin out
1 - white - audio - 5v goes to high side of electret element
2 - blue - nc
3 - red - ptt - goes to channel resistors and PTT switch
4 - nc
5 - black shrink tube - shield/ground - goes to circuit ground
6 - nc

Channel Up = pin 3>3.9k resistor>momentary switch>ground
Channel Down = pin 3>2.2k resistor>momentary switch>ground
TX = pin 3 switched to ground via PTT switch
Stock mic's electret element impedance = ~1000ohms

Radio side pin out
1 - audio - 5v phantom DC (blocking cap C900 in radio)
2 - ground - tied to pin 5 and goes to circuit ground
3 - ptt - TX control and channel up/down
4 - Mic Det*
5 - ground - tied to pin 2 and goes to circuit ground
6 - 8v DC- through a buffer can be used to eliminate microphone battery - 7v or 9v

To use a mic with a dynamic element, just put a 104 blocking cap in audio line to keep the 5v off of the dynamic element.

For power mics, you may also want to run a 104 blocking cap in the audio line to keep the 5v out of the mic's amplifier circuit.

Mic's output impedance of 1000 ohms will pretty well match the radio's input impedance.

To run a power mic with a 7v mercury or 9v battery, you can use the 8v on Pin 6 and eliminate the battery (for 7v, use a 1N400x diode to drop the voltage down to ~7.3v).

For mics (hand held or base) that don't have up/down channel buttons, a SPDT MOM-OFF-MOM switch can be mounted and wired to easily provide that function. Here is an example:
https://www.jameco.com/z/1MS4T6B11M...20VAC-28VDC-Panel-Mount_317316.html?CID=MERCH

Pin 3 to switch's center pin, and the 2 resistors for up/down channels to the switch's outer pins and ground.

Radio side mic circuit:

upload_2021-4-15_21-38-0.png

upload_2021-4-15_21-42-56.png

Older mic PCB:

upload_2021-4-15_21-46-44.png

*The only thing I don't know is what "MIC DET" means on pin 4 inside the radio. One of you sharp tech's can probably figure it out. Whatever it is, it isn't used on the stock mic.

I hope this helps someone that wants to get the channel up/down working or experiment with different mic elements to see what sounds best.

Good Luck!

Dr_DX
 

Eldorado828

8-2-8 in the Lonestar state
Feb 21, 2016
2,003
3,494
273
The Lonestar State
Thank you for taking the time to bring so much clarification. I had figured out the wiring and have done a little experimentation with different mics. You've illustrated it really well so we can take it a step further now. May try adding that capacitance and trying mics another round to see if that brings a different outcome.
 

Eldorado828

8-2-8 in the Lonestar state
Feb 21, 2016
2,003
3,494
273
The Lonestar State
Bump.

Lots of good stuff here, just can't get better sounding audio from anything other than the stock mic.

I'm thinking there's some changes that need to be made inside the radio audio circuit in order to get results from an aftermarket mic.
 

Eldorado828

8-2-8 in the Lonestar state
Feb 21, 2016
2,003
3,494
273
The Lonestar State
I have seen so much misinformation about this topic here and elsewhere on the web, that I felt compelled to make this post. I hope AT-6666/SS 9900 owners will be able to use this information to successfully wire ANY microphone to their radio. Whether it will sound good or not, is another matter.

To make any microphone work, including the UP/Down channels, you only need 3 wires. That's correct. Pin 1 = Audio, Pin 3 = TX/Channels, Pin 5 = Shield/ground. Even though the mic plug is a 6 pin, the stock mic that comes with the radio only has a 4 wire cord. There are no wires attached to pins 4 & 6 in the mic plug. They are not needed.

In a nutshell:

At least 2 different mic PCB's:
PCB CBM01 ver 20150820
PCB CBM01 ver 20180813 - has additional 102 from audio (pin1) to ground

mic side pin out
1 - white - audio - 5v goes to high side of electret element
2 - blue - nc
3 - red - ptt - goes to channel resistors and PTT switch
4 - nc
5 - black shrink tube - shield/ground - goes to circuit ground
6 - nc

Channel Up = pin 3>3.9k resistor>momentary switch>ground
Channel Down = pin 3>2.2k resistor>momentary switch>ground
TX = pin 3 switched to ground via PTT switch
Stock mic's electret element impedance = ~1000ohms

Radio side pin out
1 - audio - 5v phantom DC (blocking cap C900 in radio)
2 - ground - tied to pin 5 and goes to circuit ground
3 - ptt - TX control and channel up/down
4 - Mic Det*
5 - ground - tied to pin 2 and goes to circuit ground
6 - 8v DC- through a buffer can be used to eliminate microphone battery - 7v or 9v

To use a mic with a dynamic element, just put a 104 blocking cap in audio line to keep the 5v off of the dynamic element.

For power mics, you may also want to run a 104 blocking cap in the audio line to keep the 5v out of the mic's amplifier circuit.

Mic's output impedance of 1000 ohms will pretty well match the radio's input impedance.

To run a power mic with a 7v mercury or 9v battery, you can use the 8v on Pin 6 and eliminate the battery (for 7v, use a 1N400x diode to drop the voltage down to ~7.3v).

For mics (hand held or base) that don't have up/down channel buttons, a SPDT MOM-OFF-MOM switch can be mounted and wired to easily provide that function. Here is an example:
https://www.jameco.com/z/1MS4T6B11M...20VAC-28VDC-Panel-Mount_317316.html?CID=MERCH

Pin 3 to switch's center pin, and the 2 resistors for up/down channels to the switch's outer pins and ground.

Radio side mic circuit:

View attachment 44115

View attachment 44116

Older mic PCB:

View attachment 44117

*The only thing I don't know is what "MIC DET" means on pin 4 inside the radio. One of you sharp tech's can probably figure it out. Whatever it is, it isn't used on the stock mic.

I hope this helps someone that wants to get the channel up/down working or experiment with different mic elements to see what sounds best.

Good Luck!

Dr_DX
@Dr_DX

Should we use the same audio line capacitor on the president radio cousin's such as the Lincoln 2 , Richard and others?

I know a common complaint is some of the president radios jump channels with a power mic, might it be that phantom voltage getting into the mic circuit that causes it?
 
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Dr_DX

Well-Known Member
Jan 29, 2006
236
280
73
@Dr_DX

Should we use the same audio line capacitor on the president radio cousin's such as the Lincoln 2 , Richard and others?

I know a common complaint is some of the president radios jump channels with a power mic, might it be that phantom voltage getting into the mic circuit that causes it?

Here is the 6 pin mic wiring for a bunch of President radios including the Lincoln II+:

1644378507468.png

Note: On my stock Lincoln II+ mic, the .001 (102) cap to bleed off treble is missing inside the mic.

On the Radio side:

1644379297289.png

You will see that there is a 102 before and after L30 on the audio line inside the radio.

The Mic is "always" on - not switched. This is so the VOX function will work. There is no "POP" in the audio line when the mic is keyed.

Power mics switch the battery, which is why there is a POP in the Audio line as Shockwave mentioned (amp circuit turning on and off).

If the power mic was wired correctly, there would be no audio POP. Simply bypass/jump the switch for the battery (amp circuit is always on). Make sure the audio output isn't switched either (connects directly to Pin 1 through a 104 cap).

Then use the 13.2v from pin 6 to eliminate the battery. Go through a dropping resistor and use a Zener diode to create whatever voltage the battery supplied - 9v, 7.5v, 3v, 1.5v.

BTW, pin1 (audio) has 8v phantom DC on it. This powers the stock mic's electret element. If using a Dynamic mic or a power mic, you will want to use a 104 blocking cap to keep the 8v out of the mic element/amp circuit.

Also, the 13.2v on pin 6 is needed for the channel UP to work.

You can make the stock mic perform better by relocating the electret element. On the Lincoln II+ mic, it isn't located in the element cavity inside the mic. It can be relocated there (you will see the slot that is on the side of the cavity that the 2 small red and black wires will fit into). Use some foam/dirty sock to immobilize it in the cavity. Also make sure it doesn't short out against the screw head. This should make the stock mic more "sensitive".

TX and Channels are on pin 3. Anytime you do that, there is a chance of the freq. changing when keying/unkeying.

Good Luck.
 
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