Modifications for the Kenwood MC-85 and RC-1A phone
I am a Ham Radio Operator (call signed 4Z4MJ). My station is exclusively
Kenwood (TS-940s, TL-922, TR-2400, TR-7950, MC- 85, and the PC-1A phone patch)
and I'm very content of having the honor to belong to the Kenwood family.
In Israel, I work as an electronic design engineer and on my free time, I'm
always anxious to solve design problems and add my own modifications to the
shack. I would like to present you the modifications I have made and hope they
might be helpful. (The mods deal with the MC-85 microphone and the PC-1A phone
patch; I enclose them with this letter and emphasize the changes with the
yellow marker, So let's start........
When I purchased my TS-940S with the MC-85 microphone, I was very happy with
the 940 but sad about the 85. The problem was R.F. feedback when I was
touching the microphone's goose- neck with my hand, (or when the linear was
on, it happened without even having to touch the mic).
Not only that, I was not able to function the S-201 (mic selector of the
MC-85) if I was connecting more than one rig to the MC-85. At first I thought
my MC-85 is malfunctioning, but from talking with five other hams in Israel
who own the MC-85, I heard that they all suffer the same problems!!
The main problem was caused because of a ground loop. There were two (2)
grounds to ECM unit (you know it's illegal).
The ground on the p.c. near the Q1 transistor is absolutely fine, but what
about the one that makes a ground connection between the ECM unit and the top
of the goose neck? The solution is described on the enclosed paper marked (1).
The addition of (2) and (3) were needed to give more R.F.
attenuation to the common mode and to the differential mode rejection
In summary, (2) and (3) are necessary but (1) is a MUST. These mods cured
completely all the RF feedback problems that one can have with the MC-85.
Before closing the microphone's base cover, I added a few more mods. I hope
they are clear from the modified scheme and the notes. (Notice that all these
mods can be done without having to remove the printed circuit board.) A
similar thing happened with the PC1-A phone patch...started because of a
"serious" problem and ended with extra "very helpful" mods.
The "serious" problem was that no matter how loud I put the RX gain control,
there was never enough audio to the telephone line, and not only that, the
audio was distorted! (See IRI of April, 1987, Issue No. 64, Page K64, where
Luchessa WN6O, suffers a similar problem and asks: "Any remedial information
will be appreciated".) The problem was solved by changing D5 and D6
(originally they were clipping the audio at 0.7 volts). I made the change by
putting 3 silicon diodes on each leg, so now the clipping is done above 2
volts, and not at 0.7 volts, R11 was changed from 27 to 10 ohms. Again, before
closing the patch's cover, I added a few more mods.
A. I couldn't work vox properly with the PC-1A because I couldn't null deep
enough the bridge. VR1 was partly mulling at the far end of it's position.
Adding a 1.5k ohm in parallel with R5 (or alternatively just changing R5 to a
single 470 ohm resistor) brought the null to the mid-position of VR-1.
B. Although the null is now centered, it was not deep enough. Adding on 0.1 uf
disk capacitor in parallel with RF balanced the reactive component of the
bridge, the null became extremely sharp!
C. An addition of a high pass filter to the patch that gives the same effect
as the "low cut" position on the MC-85 microphone was "very helpful" when the
PC-1A was connected to the SSB HF ring. The filter is simply made of a 10K ohm
resistor and an 0.02 uf capacitor. (see it on the modified scheme (next
With all the changes I made to both the PC-1A and the MC-85, I'm most
satisfied and I would very much like to hear your remarks (maybe published in
the I.R.I. too!)
Mod for MC85 Desk
Peter - PB2DD
Mod for MC85
I have bought my
Kenwood MC85 desk mike second hand, just for its shape. It
would look nice on the desk beside my MC80, which is currently in use
for a UHF radio. After creating the connector Icom compatible, this mike
seems to be very sensitive for HF fields. You should know that my wire
antennas are very close by the radio shack. After the modifications with
the extra earth wires, as described in an earlier article on this site,
it was better, but on some specific parts of the HF bands, still
Measurements showed that the small electret element appeared to be
connected with one side to its housing. This housing was in contact with
the metal of the goose neck, so an earth gap / leak was created! No
wonder this goose neck was so sensitive for HF fields. I’ve solved the
problem by isolating the electret’s element, by wrapping this in thin
Teflon (plumbers) tape, making sure it is isolated from the goose necks
ground. All other modifications were removed and all problems with
distortion were instant history!
now operates like you would expect.
I hope that this
modification solves your possible problems also.
I have an Kenwood MC-85 but it was very RF-sensitive when i touch the
microphone neck especially when the micgain was in the upper level.
I removed the bottom plate of the MC85 and:
1. Connected a wire from the bottom of the mic Goose neck and directly to
the microphone cable shield that comes from the Transceiver and enters the
2. Connect the shield that comes from the transceiver to the bottom plate of
3.I dismounted the screw that connects the metal housing of the MC85 to the
PCB and connected instead the Shield that comes from the Transceiver and
enters the MC85 to the shield from the microphone wire the comes out of the
I have not any problems with my MC85 anymore and is a very happy owner.
Connect Kenwood MC-85 to
Connect other microphone to Yaesu FT-100.
Here´s a description on how connect Kenwood MC-85 to Yaesu FT-100
It was problems was PTT switch shall connect.
Here below does I a description how microphone shall connect to
radio and which components that be needed.
A:= 82 Kohm
B:= 39 Kohm
C:= 27 Kohm
82K be needed to Up switch
39K be needed to Down switch
27K be needed to PTT switch
Kenwood MC-85 5 4 6 4 3 <->Connect to digits in modular
Connection. (2 and 6 connect along
in the mic Before the connect to pin 4
in modular connection.)
6-pins modular jack
Mic-jack Yaesu FT-100 / 90 / 8100.
Pin 1: SW 2 multi-function switching
Pin 2: N.C. (9600bps packet data output FT-90)
Pin 3: + 9 v
Pin 4: GND
Pin 5: Microphone Input
Pin 6: SW 1 PTT, UP och DOWN
Manual and schematics download
The KB2LJJ takes no responsibility for any damage during the modification or for
any wrong information made on this modification.