Slick Tricks On Microphone Wiring
First we’ll cover incompatibility. Some power mics leave the amplifier circuit connected to the radio audio pin. This is all right for most radios, the more expensive type in particular.
Older power mic like the Turner +2, the old Cobra dynamics, old Radio Shack desk mic, and others didn’t switch the preamp out of the audio path.
Most CB and Ham rigs share amplifier circuits. It doesn’t make sense to duplicate preamplifier and amplifier circuits for the transmitter that are already there in the receiver, so signals are switched to these circuits.
This saves space and reduces cost. Many older and new inexpensive radios don’t switch the radio mic amp off. That’s what causes a squeal or loud buzzing through the speaker in the receive mode.
This can be eliminated by adding a large value resistor say 47K Ohm to 100KOhm but the audio will be about the level of a stock mic.
The only way to fix this problem is to put another set of contacts in to switch the audio lead. If you have a scrap mic you might rig something up. Generally it’s not worth the trouble. Have you ever had trouble with wiring a stock mic? What could go wrong?
Many new radios have the same connectors and wiring but some mics will work on all radios and others only work on some. For example you can take a Uniden Pro 510XL and a Uniden Pro 76XL then swap mics.
The 510 will still work (with slightly lower modulation) but the 76 won’t have any modulation. The mics are wired the same, but the mic elements are different. The mic from the 76 has a dynamic cartridge, the type used in stock mics for many years.
The mic from the 510 has a electret condenser cartridge, less expensive and has a small preamp built in, thus eliminating one preamp stage in the radio. Usually the hand held mics are imprinted on the back cover, if they are electret condenser.
If you are not sure, just remove the back cover and if the cartridge is approximately 1/2″ in diameter, it’s a electret condenser.
This type mic requires a DC supply voltage to work and these radios are designed for them. They are usually the cheapest cartridge the manufacturer can get their hands on and the sound quality is usually muffled.
Uniden’s seem to sound the best. If you have a radio that has an electret condenser mic, the best replacement is a good quality power mic such as an Astatic D104, D106M6 or a 575M6.
If echo is what you want, the Sadelta ME-3 is very good, but the Road Noise EC-2018 is the best. Another type of mic is the non-amplified crystal or ceramic type.
These are Hi impedance mics and will only work on the older tube type radios like the Browning Eagles, Tram D201 and others. Hi-Z mics are 10,000 Ohms and up which won’t match up to the newer solid state radios.
These radios are Lo-Z and are around 500 Ohms. Power mics with Hi-Z cartridges have a preamp that will match both types of radios. Next I’ll cover the defective mic. There are many things that can go wrong with a mic.
Most common is a break in the coiled mic cord. This can be at either end and usually intermittent, causing transmit or receive breakup.
First you will need some way to monitor your results, a mic test box or a radio with another to monitor with, an Ohm meter, or a test light. The mic test box or the radio with a monitor radio nearby would be the best.
To determine which end it is, hold one end secure while wiggling and bending the cord at the other end.
Then do the same to the opposite end. If this test doesn’t determine which end it is, the shield might be corroded A corroded shield will cause very scratchy audio as the cord bounces around. The solution is to replace the cord.
If the cord is broken at the base of the mic, the cord can be cut back and repaired. Just cut the cord about an inch below the strain relief, strip it back the same amount as is left in the mic and remove and replace one wire at a time.
If it’s broken at the mic connector, write the wiring code down and cut the cord about an inch back from the strain relief, remove all wires, add fresh solder to the pins of the connector and reconnect the wires.
PTT switch connections can go bad and usually cause intermittent problems. You can test for this by holding the cord steady and wiggle the PTT key back and forth.
Contact cleaner may solve the problem, but I’ve found it to be a temporary fix in most cases.
Power mics with no modulation, the battery & connections should be checked and cleaned if needed. On stock mics the cartridge can be checked with an ohm meter.
Put one lead on each terminal of the dynamic cartridges you should get a reading of about 500 Ohms and hear a click every time you touch the leads of the meter to the cartridge. A suitable meter is available at Radio Shack, Part #22-221 @ $24.99.
Another reason you may have difficulty wiring a mic is that someone has modified it. The most common modification is the color code change when they replaced the mic cord. Mic cords aren’t standard, they have all types of color combinations, and the shielded wire color also varies.
Keep in mind that the shielded wire is the audio wire regardless of its color. I’ve seen mics from the manufacturer use the white or red wire for the audio connection while the blue wire was shielded.
They work most of the time, but put them in a mobile and they squeal like a pig. The global market has made consumer products less standard than ever.
The best way to determine the color code of a mic is to use and Ohm meter. The one listed above will work fine. When dealing with a mic of unknown color coding, I start with the shielded wire and check that it’s wired to the cartridge.
Attaching one lead of an ohm meter to the shield and the meter lead to the shielded wire, press the PTT and watch for a reading of approx. 500n or listen for a click when you key it.
On a 4 wire mic: Assuming the audio is right, next with the mic un-keyed and the meter lead is still attached to the shield, check for continuity (zero Ohms) to the remaining 2 wires. The one that reads zero Ohms is the receive wire.
Make sure you lose continuity when you key the mic. Finally check the remaining wire for continuity to the shield while the mic is keyed. If it reads zero Ohms this is your transmit wire, you’re all done.
The only difference in metering a 5 wire mic is that the shield is only connected to the cartridge and the 5th wire is the PTT common connection. See the diagram below.
Typical Microphone Wiring Diagram.
On a 5 wire mic: Confirm the audio leads as in the 4 wire instructions. Finding the common wire is a 2 step process.
Step 1: With the mic un-keyed, meter between 2 wires at a time on the three remaining wires until you get continuity (note which wires read zero ohms).
Step 2: Then with the mic keyed and one meter lead on the remaining wire connect the other meter lead one at a time to the 2 wires found in step 1. The wire that reads continuity again is your common wire (usually black or blue is common). The remaining wire from step 1 is receive and the remaining wire from step 2 is transmit. The following are some common wiring codes:
Road Noise & Copies Astatic Sadelta
4 Pin Cobra/Uniden
1 Shield & Black
1 Shield & Blue
4 Pin Midland
2 Shield & Black
2 Shield & Blue
3 Black4 Red
5 Pin Din Cobra/Uniden/Midland
3 Shield & Black
4 N/C5 Blue
3 Shield & Blue
4 N/C5 Black
4 N/C5 Green
5 Pin Din Cobra Plus
4 Shield & Black
4 Shield & Blue
5 Pin Din Sears/Radio Shack
1 Shield & Black
1 Shield & Blue
5 Pin FemaleCobra/Uniden SSB
4 Jump To Pin 2
Notice the 5 pin female Cobra/Uniden code under the Sadelta column. It’s a 4 wire mic, but it will work by jumping pins 2 & 4 together. This will work on most 4 wire mics. Din Connectors are the toughest to solder. Sometimes the wires are too large to fit into the holes.
In this case you can solder the wire to the side of the pin. On some connectors it’s possible to flatten the pins making it easier to make a secure connection. I’ve wired thousands of mics in my store using these methods. I hope you find this information as valuable as I have. Also the MT Plus mic tester is available through us @ $49.95.