A significant Hum occurs (when the pre- amplifier is switched on) and when touching case . This causes RFI (particularly when operating a power amplifier).
The Hum is eliminated by this fix.
The Fix is: The Hum completely disappears by: cutting through a single copper trace on the amplifier switch board. The copper trace is from the a battery negative to the metal brackets that contact touch the metal bottom plate.
This cut:
• isolates the metal case completely from the MIC-GND (microphone ground wire) which is connected to pin 7.
• disconnects the negative lead of the batteries of the amplifier circuit from the metal base plate and the upper metal case
body.
Reason this fix works?: The circuit diagram in the Kenwood MC-60 manual is badly drawn. It is not possible to follow the circuit logic in the broken format the circuit is drawn inside the Owners manual. When you actually draw out the entire circuit you find Pin 7 the MIC GROUND connects to negative of the battery in the pre-amplifier circuit and then directly into the amplifier circuit itself. This negative microphone wire is connected directly to the metal bottom plate which is also screwed onto the metal body of the microphone. When the pre-amp is switched on, touching the outside body of the case is the same as touching the microphone ground with your fingers and introducing more capacitance. As this MIC-GND circuit floats and is not grounded onto the main chassis ground pin 8, this has the effect of significantly changing the capacitance in the pre-amplifier circuit. The change in capacitance causes the hum, which then causes the RFI.
RFI can be further reduced by:
1. On the microphone curly cord lead soldering a 0.01uF ceramic capacitor (50V) in 2 places inside at both plugs at both ends of the lead. That is 4 capacitors in total. The capacitors are located between:
• 'pin 7 MIC-GND MIC -' and 'pin 8 CHASSIS-GND', and
• between 'pin 1 MIC MIC + ' and 'pin 8 CHASSIS-GND'.
Once you make these adjustments. NO MORE RFI.
from ZL4AI, Jeff King
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A FIX FOR ELIMINATING KENWOOD MC-60 MICROPHONE HUM AND RFI Version 2.0Version 2 completely replaces version 1, eliminates 1 more minor hum and an early wiring defect.
Terminology used in this document:
MIC Plug: Plug which screws into the base of the microphone connecting to the wire base unit.
BASE Plug: Plug which screws into the base of connecting to the curly wire to the transceiver.
TRANSCEIVER Plug: Plug on the curly wire which screws into the transceiver.
BASE UNIT CASE: The metal case of the base stand.
STEP 0: Conduct testing described below in Step 4 to establish audio hum.
STEP 1:Some early MC-60s were wired in the factory with a fault which is the wires inside the microphone with 'MIC-GND MIC -' reversed with 'MIC MIC + ' The circuit diagram in the Kenwood Owner's manual has this error (drawn out).
Use an ohm meter at the MIC Plug to verify if Pin 7 is connected to the Mic case; Connected shows the Mic is wired correctly.
The Fix 1.0 is: Inside the 'MIC Plug' unsolder wires at pin 1 and pin 7: Reverse these wires by soldering the wire originally connected to pin 7 to pin 1 and the wire originally connected to pin 1 to pin 7.
STEP 2:A significant Hum (when the pre- amplifier is switched on) and when touching the BASE UNIT CASE occurs (when in transmit mode by pushing the lock button). This causes RFI (particularly when operating a power amplifier).
The major Hum #1 is eliminated by this fix.
The Fix 2.1 is: The major Hum #1 completely disappears by: cutting through a single copper trace on the amplifier switch board. The copper trace is from the a battery negative to the metal brackets that contact touch the BASE UNIT CASE metal bottom plate.
This cut:
• isolates the BASE UNIT CASE metal completely from the MIC-GND (microphone ground wire) which is connected to pin 7.
• disconnects the negative lead of the batteries of the amplifier circuit from the BASE UNIT CASE metal base plate and the upper metal case body.
Reason this fix works? : The circuit diagram in the Kenwood MC-60 manual is badly drawn: It is not possible to follow the circuit logic in the broken format the circuit is drawn inside the Owner's manual. When you actually draw out the entire circuit you find Pin 7 the MIC GROUND connects to negative of the battery in the pre-amplifier circuit and then directly into the amplifier circuit itself. This negative microphone wire is connected directly to the BASE UNIT CASE metal bottom plate which is also screwed onto BASE UNIT CASE upper metal body stand. When the pre-amp is switched on, touching the outside body of the BASE UNIT CASE is the same as touching the microphone ground wire with your fingers and introducing more capacitance. As this MIC-GND circuit floats above ground and is not grounded onto the main chassis ground pin 8, this has the effect of significantly changing the capacitance in the pre-amplifier circuit. The change in capacitance causes the hum, which then causes the RFI.
The Fix 2.2 is: Fix 2.1 above leaves 'BASE UNIT CASE' not connected to ground: A slight Hum #2 still occurs when the case is touched.
The Fix 2.2 is: Inside the 'BASE Plug' and the 'TRANSCEIVER Plug' solder an additional wire (solid copper 30 mm long) to pin 8. Run this wire out through sleeve hole opening for the cord to the screw on metal clamp and around 1 screw of the clamp. This grounds the BASE UNIT CASE to Chassis of the Transceiver and eliminates slight Hum #2.
Slight Hum #3 occurs when touching the outside metal shell of the Mic when transmitting. This fix occurs because the MIC- GND wire is directly connected inside Mic to the microphone outer shell. A Fix is to take the Mic apart and break this connection. This fix is difficult to undertake and is not necessary as you never have to touch the microphone outer shell when operating.
STEP 3:RFI can be further reduced by:
1. On the microphone curly cord lead soldering a 0.01uF ceramic capacitor (50V) in 2 places inside at both plugs (MIC Plug / BASE Plug [or on wire inside BASE UNIT CASE) at both ends of the lead. That is 4 capacitors in total. The capacitors are located between:
• 'pin 7 MIC-GND MIC -' and 'pin 8 CHASSIS-GND', and
• between 'pin 1 MIC MIC + ' and 'pin 8 CHASSIS-GND'.
STEP 4: Testing all hum has been eliminated
Initial testing is best not carried out using a transmitter because it is difficult to simulate Hum conditions while transmitting.
Instead connect the TRANSCEIVER PLUG pins 1, 7 and 8 to a microphone an audio pre-amplifier from the music industry * which has not less than 55dB of gain to pins 2, 3 &1 as laid out below
MIC +: MC-60 TRANSCEIVER PLUG pin 1 --> XLR Pin 2
MIC -: MC-60 TRANSCEIVER PLUG pin 7 --> XLR Pin 3
CHASSIS GND -: MC-60 TRANSCEIVER PLUG pin 8 --> XLR Pin 1
Then listen to output of the audio pre-amplifier on headphones or a speaker
This is much easier to do than trying to transmit on a transceiver and listen at the same time.
Test Setup for No Audio Hum:Turn on the 50K switch on the MIC.
Turn on the MIC in switch (on the bottom of the MIC base) which adds 5.5dB of gain.
Turn the pre-amp on with greater than 55dB gain.
This provides the worst testing condition.
This takes the MC-60 signal from -50.5dB (5millivolts) to about 0dB (about 1 Volt).
You may hear Hum now.
Put your fingers on the Mic Base and you will hear Hum.
Note you do not need to push PTT because the MIC is continuously connected.
Undertake the Fixes to Fix #2.2 above until you hear no Hum.
Test Setup for No RFI:
Then set MC-60 with MIC amp switched on and
- impendence switch set on (500 ohm for a TS-940, TS-590S, TS590SG, or TS-950SDX) or
- 50K for Condenser Mic into an audio preamplifier and then through an attenuator of not less than 40dB
to provide an output microphone signal into the transceiver so the ALC meter does not exceed the peak limit value except for on an occasional voice peak and the Compression Meter does not exceed the 10dB peak limit value except for on an occasional voice peak,
feeding into a power amplifier set at 1,000 watts output PEP in SSB while speaking with your mouth 50 mm from the MIC mesh and your fingers holding onto the base of the MC-60 to verify no RFI occurs. [If some very slight RFI still occurs only when with fingers touch the case you should undertake STEP 3.]
* Music Industry pre-amplifier with more than 55dB gain: I used a Behringer Ultra Gain Pro MC2200. This has very low noise audio amplifiers.
Once you have made Fix adjustments 1 to 2.2 above expect to find: NO MORE RFI.
from ZL4AI, Jeff King