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 respectively.

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 C.M.
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 month)).

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 mike

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 present.
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!

 The microphone now operates like you would expect.

I hope that this modification solves your possible problems also.

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.

   Components
   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

MC-85 instructions Manual
 

MC-85 Schematic
 

ATTENTION
The KB2LJJ takes no responsibility for any damage during the modification or for any wrong information made on this modification.