Microphone troubleshooting/wiring question

Trying to trouble-shoot a xmit problem I'm having in my ASW-19 and I think we're narrowing the issue to the 30+ year old Peiker TM169 mic I'm using. The radio itself is no beauty being the same age Avionic Dittel ATR720, but according to the radio shop it puts out a full 5 watts.

The complaint is when I xmit, my voice is so garbled as to be unintelligible...others flying with me say it sounds as if I'm overmodulated. The mic gain on this old radio is (according to the manual) a 2 position pot, either fully "carbon" or fully "dynamic" with no mid setting. When set to carbon, nothing comes out of the set at all..when on dynamic the above mentioned complaint occurs.

Now the wiring schematic says the shield wire is run separately to its own ground, but as it sits now it's wired into the radio ground. A call to one of the soaring merchants revealed that wiring it that way (radio ground)is his recommendation as well.

So....comments on what to really do with the shield wire as well as any experiences with mic failures would be appreciated. I hate to just start throwing parts at something without finding out what's broken first. What goes bad on a mic..the internal diaphragm or is it something else?

Also, I have a buddy with an oscilloscope, if someone knows a way of testing the mic as it sits in the aircraft (we've already ohm'ed it out and it's fine) using his scope then that info would be appreciated as well.

TIA for any info,

Rob S.
ZAP

On Jul 28, 3:30*pm, RAS56 wrote:
Trying to trouble-shoot a xmit problem I'm having in my ASW-19 and I think we're narrowing the issue to the 30+ year old Peiker TM169 mic I'm using. The radio itself is no beauty being the same age Avionic Dittel ATR720, but according to the radio shop it puts out a full 5 watts.

The complaint is when I xmit, my voice is so garbled as to be unintelligible...others flying with me say it sounds as if I'm overmodulated. The mic gain on this old radio is (according to the manual) a 2 position pot, either fully "carbon" or fully "dynamic" with no mid setting. When set to carbon, nothing comes out of the set at all..when on dynamic the above mentioned complaint occurs.

Now the wiring schematic says the shield wire is run separately to its own ground, but as it sits now it's wired into the radio ground. A call to one of the soaring merchants revealed that wiring it that way (radio ground)is his recommendation as well.

So....comments on what to really do with the shield wire as well as any experiences with mic failures would be appreciated. I hate to just start throwing parts at something without finding out what's broken first. What goes bad on a mic..the internal diaphragm or is it something else?

Also, I have a buddy with an oscilloscope, if someone knows a way of testing the mic as it sits in the aircraft (we've already ohm'ed it out and it's fine) using his scope then that info would be appreciated as well.

TIA for any info,

Rob S.
ZAP

Rob, A dynamic mike is essentially like a speaker in reverse. In fact
you can use a speaker as a low frequency mike. It is a tiny magnet
attached to a diaphragm suspended in a coil of wire. When the
diaphragm vibrates, the magnet moves up and down and generates a weak
voltage. If you connect the oscilloscope to the
mike and place it on the lowest voltage range (millivolts) you will
see an AC voltage when you talk into the mike. Dynamic mike cartridges
are cheap and available.
Try Radio Shack. However, because you are getting voice through I
doubt it's the mike. More likely that something is generating too much
gain. Some sailplane mikes are "amplified". I don't know how but they
must have a small simple circuit in them. I would first try direct
hook-up of a different dynamic mike or (mike cartridge) and if that
sounds better, just replace. If it works but is very weak then you
need to find and fix the little amplifier circuit. If it is neither of
those two
then the audio gain and modulation stage of your transmitter needs to
be looked at. Some have internal gain pots and that may have drifted
or changed somehow.
Did this problem come on slowly or all at once?

Thank you Soartech, for the response.

I bought this plane at the end of 2010, didn't fly it till the 2011 season and it has had this issue (to my knowledge) since day 1. The radio has been opened up in the past, an avionics shops sticker is on there from some undocumented repair from 25 years ago.

Part of the problem is trying to find a tech/repair manual for a 30+ year old radio of foreign extract...there's a mic gain externally, but only the aforementioned carbon/dynamic pot. To actually do something other than change the either/or of that would require cracking open the case as well as having a shop manual to help guide the tech and tell him what's actually in front of him. Another nagging factor is I've already thrown 300 bucks at this problem, w/o a resolution. How much more $$$ am I willing to spend or is it time to replace the old set with something newer? Don't even get me started on the radio shops I've dealt with and their reluctance to take on a small, low profit snipe hunt like I'm offering them. They'd much prefer to be having their techs installing new panels/autopilots, etc, vs this project!

Hence I'm going the cheapest way I think I can...I've gone ahead and ordered a replacement Peiker mic for ~$100. If that doesn't solve the problem, then I think it'll be new radio time, unless someone can recommend a decent radio shop (in the USA) that might have enough experience with old foreign sailplane radios to take a look at it at a reasonable cost and give me a thumbs up/down on if pursuing a repair is worth the effort.

Thanks,

Rob