The club (Highfields Amateur Radio Club) has a Trio MC-60A base microphone (the name plate says MC-60 but it is the MC-60A because it has the switchable amplifier built in) wired to fit their Kenwood TS-570 but, on Shack Warming night, this microphone was giving a few problems. It seemed that RF was entering the mic somewhere (the clamps on the plugs were loose and one plug was hanging off, therefore loose wires seemed a likely suspect).
There were some other problems mentioned / noted, though they were, mainly, cosmetic; The plastic yoke (holder) for the actual microphone was broken and had been taped up many moons ago (the tape was sticky to the touch, as is usual!), the name badge had fallen off (but was still available to be replaced) and a couple of feet were missing from the bottom. Being the fool that I am I volunteered to take the MC-60A home and repair it.
On checking the wires at both ends of the cable were firmly attached. I checked on the Kenwood TS-570 mic wiring page and cross-referencing to the MC-60 / MC-60A page I confirmed that the wiring was, indeed, correct for the radio (it should be, it was being used on that radio previously!). I also noted on the MC-60 / MC-60A page an observation that I'd added from Walt, N9WB, about some of these mics being incorrectly wired at the factory, giving RFI problems and generally making them sound "bad".
I opened up the casing to ascertain if this was the problem and a none too pretty sight met my eyes (see the photo right).
The battery tray that holds the 2 AA cells had its' end broken off and this end was loose inside the casing, on closer inspection I noticed that it had had an attempted repair at one time (someone had tried to melt it back together). Could this have been the cause of the problems with the loose connections touching something they shouldn't? Probably not, as you will see later!
I know from the schematic that the MC-60A uses the batteries only if the radio does not supply 5 volts (to pin 5) and the "Mic Amp" switch is in the "IN" position, but (unless there is a problem with the radio) the TS-570 should be supplying that voltage. Anyway a new battery tray was put on the list of required parts along with the rubber feet.
Back to the cable and the socket connections. I ascertained that the socket was wired correctly, so this was not the issue for this microphone. After double checking all the connections for any stray whiskers, dry joints etc. I replaced the plug covers, replacing the missing clamp screws with a couple from my junk box. I added some "anti loosening" fluid to (hopefully) stop the clamp scerws shaking loose while being moved around from storage to shack and back again.
The plastic on this yoke is quite thin where the most stress happens, in fact just 2 mm (0.08 of an inch) on the sholders as the yoke curves out to take the microphone.
There is some thickening at the very front and rear, but not enough to make a great deal of difference. Interestingly, just where the break has occurred, the designers decided to have an empty void. I'm sure that if the moulding had been solid there, or even some webs moulded in, then the chances of breakage from stress would be lessened. This can't be the only one that has broken at the sholder can it? A quick search on the 'net shows that is a fairly common problem across the range that use this type of yoke, in fact Steve, GW6CUR, tells me that his broke in exactly the same place too! I wonder why.
Replacement should be easy enough though, just undo the clamp screw, remove the yoke and replace with a new one which are readily available for a few pounds from most Hi-Fi etc. shops yes? No!. The Kenwood / Trio microphone yoke fits to the stand the opposite way to the Hi-Fi & stage microphones, having the single tongue in the centre on the stand and the yoke tongues fitting each side, rather than the other way round. There is also another problem, the microphone itself is smaller in diameter than a standard stage or Hi-Fi mic so the yoke is, correspondingly, smaller too. A search on the 'net turned up a Kenwood parts supplier in the US who has this part listed but the shipping to the UK would probably make it prohibitively expensive. I could find no UK sources for this part listed after half an hour or so of searching so I looked at the standard microphone holders that are readily, and cheaply, available locally. I purchased one that was designed to screw onto the top of a microphone stand, with a view to make a repair that could easily be repaired again should the worst happen.
With a new yoke for a stage microphone to hand I set to work on the modifications that I wanted to make to the yoke for its' fitting. Firstly I got some black felt and placed inside the yoke and slid in the microphone. The fit was still a bit sloppy with a single layer of felt so I doubled it over and tried again. This time the fit was excellent so I removed the microphone and felt (after marking the felt for trimming) and took the base (the screw collar that fits to the stand) off of the yoke (top piece). I trimmed the felt to size, glued one face of it and doubled it over again. I applied more glue to the outside of the felt, where it would sit in contact with the yoke, and placed the glued felt carefully in the yoke and slid the microphone into it to hold the felt in place until the glue set. I set this aside and went to work on the stand fixing. The photo shows the felt lining after the glue had set.
Because of the centre tongue on the MC-60A shaft the fit of the screw-on collar of the holder base was very sloppy side to side but, I noted, the centre tongue would fit exactly in between the 2 outer tongues of the new holders' base. I set to work with a small drill and file to remove enough plastic from between the 2 plastic tongues so that the MC-60A centre tongue would just come up between them. With a little fine fettling the fit was perfect and the screw-on collar was now seated well and no sloppiness was evident, just a "slightly loose all round" fit.
I decided that I would affix the holder base (semi) permanently with some epoxy putty, this would fill the gaps and could be left to set without having to tape, clamp etc. it in place. If the yoke breaks again then a standard one can easily replace it, with the addition of some felt of course! If, on the other hand, the base of the holder should become damaged, the plastic and epoxy putty can be cut off and replaced too. I mixed the putty, placed some through the original screw hole in the MC-60As' centre tongue and around the tongue to fill the voids either side. Working quickly (the putty sets in 5 mins) I placed the holder base over the MC-60As' shaft and pressed down. Excess putty squeezed out and was removed before it set. While I had the base to hand I applied some glue to the location for the name plate and pressed that into place. I then put the base aside for a while to let the putty and glue cure.
I had separated the base plate from the body before I started so I now replaced the battery tray with a new one purchased locally for less than a pound. This was a simple job and I'm not going to detail it here. While I had been purchasing the parts for this repair I had drawn a blank on the rubber feet that I wanted. Helpful staff at a couple of shops offered to order them in (they carry them but were out of stock) but I declined, sure that I had some in my junk box. I found 4, 2 were slightly taller than the ones fitted, the other 2 were twice as tall as the original. I fitted the original feet at the front and the slightly taller ones at the rear when I re-fitted the base (the rear feet hold the base plate on, the front ones do not).
Because I was skeptical about the broken battery tray being the cause of the original RFI problem with this microphone. I researched about the MC-60A and RFI problems and I discovered that the MC-60A is a terrible beast for RFI! It seems the cause is the amplifier circuitry. There are many articles and fixes for the problem on the 'net, from simply switching the Mic Amp to "OUT" to total Mic Amp re-builds using better quality / tolerance components!
I've decided to try one of the easier RFI remedy methods, as seen on mods.dk entitled "Final easy fix for the MC-60 mike". This entailed fitting a .01 µf 50 v mylar capacitor across Mic Audio & Mic Ground and tying Mic Ground to the casing, both done at the rear of the socket on the body. From the factory, Mic Ground is tied to the body via the base plate and then, hopefully, through the screws that hold the base plate on to the body, so a good, soldered, connection can't hurt. I couldn't source a 50 v mylar locally but I did obtain a 400 v one, the higher voltage should not be an issue. I'll see if it cures the problem when the TS-570 is fired up again.
Whilst the microphone was stripped down I gave it a good clean. Re-assembly was simply putting everything back together and tightening a few screws. A quick wipe over with a cotton cloth took off my paw prints afterwards. While not looking quite as good as new this microphone should now give a few years more trouble free service, with no more sticky fingers from the tape around the top! A new "Property of Highfields ARC" sticker to replace the one that was looking a bit worse for wear and all was ready for return to the Club and testing at the next "On-Air" night.
