Box 2 problem

[Victorian]

Reading the "flying 2000' over an ATZ" thread reminded me of a deeply irritating problem that I seem to have got stuck with. I can't operate 'Box 2' at all because of the following problem:

I have a 430 (Box 1) and an MX-170C (Box2) with a (Original fit) KMA-20 audio panel and Sigtronics intercom. The problem is that I can't select the box2 audio while transmitting on Box1, or vice versa, without loud and distracting audio breakthrough. This is so bad that I can't effectively speak because of the audio racket in the headset.

My avionics installer has effecively washed his hands of this saying that the two radios don't have a mute function and there's no way to shut the audio up. But surely the audio panel is supposed to cut the audio from both radios during transmission? He's convinced it's RF breakthrough and I've spent some money moving the antennas further apart to no effect. Now his idea is to put one of them under the fuselage.

However the audio breakthrough is so strong and consistent in either direction that I just don't believe that it's an RF issue.

Ideas anybody? Thanks, hopefully.

[A and C]

It is sounding like an RF breakthrough, most modern radios have a TX interlock function so that a pin on box 2 is conected to the MIC Key on box 1 (and vice versa).

When the MIC key is activated the pin on the opposite box being pulled to earth turns up the squelch on that box preventing audio breakthrough.

All that having been said the MX170 is not a box with a good reputation, the audio panel is getting on a bit and the sigtronics intercom is very subject to breakthrough if all the screened lines are not properly installed so due to the age of the equpment and wiring you may have to live with the problem or spend some money on new equipment.

[Victorian]

Thanks very much for that prompt reply A&C. I'm still really puzzled by this. The King KMA-20 operating guide says this:

"When you press the mic button, the output of all aircraft recievers is electronically muted so there is no cockpit feedback in your transmission". Note their use of the word 'all' suggesting that crosstalk between radios could not reach the headset. Their specification further states: "Input Muting: Audio Muting is 40db or better when a transmitter is keyed"

The original installation of 2x KX170 worked fine with no interference during box 2 operation. I've swapped the KMA20 for an identical unit with no difference in results.

So my questions are:

Is it true that the KMA20 mutes 'all' receivers? If so, where does the sidetone come from? Is that the 40db referred to in the KMA20 specification?

If it's true about the muting, how could RF breakthrough possibly produce deafening interference in the headset? I take your point about shielding of the leads to the Sigtronics but I reasonably sure my installer has looked at this and in any case I don't see how a shielding problem could produce the deafening breakthrough that I experience when using either radio.

I note that the SPA-400 wiring diagram here (http://www.sigtronics.com/pdf/air_pdf/SPA-400_KMA20_Inst.pdf) shows only the aircraft mic key and mic audio signals are passed back to the KMA20, not the individual radio mic key and audio. Could this be anything to do with it? (Noting of course that it worked perfectly well with the original KX170 radios).

Finally, I understand that the MX170 may not have the best reputation (the company are certainly very hard to contact) but is there any specific way in which it could cause my breakthrough problem? I've had no problems with it at all and it's a clearer and more reliable radio than the 430.

[A and C]

Without the KMA20 circuit drawings in front of me I can't comment on the 40db input muting both radios however I would assume that it should be working as you have tried another KMA20 box.

With greater detail that you have given in your latest post I think it is time for a back to basics check of the wiring, this has to be a full check on each pin of the whole audio system (including the NAV audio). the system needs to be checked for each pin going to the correct place and more criticly going to the wrong places.

Some pins are not connected in aircraft instalations but have bench test functions within the box so all the pins that are installed in the conector block in the back of the rack will need to be checked even if they have apparently got no function. It is a long and time consuming task but at this point I think it is the best option for finding your problem

[wigglyamp]

The 40dB attenuation of inputs on a KMA20 only applies to audio loads going to the speaker isolation amplifier. Phone audio is routed just through the selector switch directly to the headphone output so multiple loads can end up being parallelled which isn't good. This is why it's necessary when using mixed-manufacture radios and an SPA400 intercom with a KMA20, to put external loading resistors into the output of some radios (220 Ohm in series will do the job) to prevent them loading the intercom with a resultant very low intercom level.

Yiour best solution is probably to ditch the KMA20 and SPA400 and fit a Garmin GMA340 or similar with a decent phone isolation amplifier and also get the benefit of a much better quality intercom. It still may not solve the issue of compatibility between units where one box breaks into spontaneous oscillation due to picking up RF from the other box on transmit - proximity of the boxes to each other can be enough to set thos off, in which case change the TKM to something with a Tx mute facility which will work with the GNS430 perhaps an SL30 or a late model second-hand KX155.

[A and C]

I would listen to Wigglyamp, he knows far more about these things than I do!

[Victorian]

Thanks very much for these detailed responses chaps.

The idea that one of the boxes might take off when the other transmits is intruiging, but remember this happens both ways round, ie 430 breaking into MX170 or vice versa. Very interesting point about the headphone not being muted though. I wonder if a lack of equalising resistor is a factor. However you can listen to both radios simultaneously with similar audio levels - it's only when you tx on one of them that the breakthrough occurs.

I'm also wondering about the Sigtronics box. They publish a sidetone mod for radios that don't supply sidetone. I'll check if this is fitted. Sidetone on the MX170 is much less than the 430.

I'm still clinging to the hope that this is a simple wiring error and that I won't have to buy yet more new boxes!

Thanks again