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Mod for MC-85 MIC 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 nulling 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!).

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 73's
   John Ribeiro
             KB2LJJ