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By Flying_john
I have an SPA-400 intercom wired via a CP136 audio panel and had two KX155 Coms.

When I measure the Tx Key line (PTT) at the Com it is not switched to ground but varies between +5ve (not keyed) and +1.2Volts Keyed.

Whilst this satisfies the TX Key line input to the KX155, the radio I want to fit is a Funke ATR833 and this requires 0V or GND on the TX Key line and wont transmit with 1.2Volts on the PTT line.

Have any other Avionics engineers found this problem and found a solution, apart from putting a relay in, or whether infact the SPA-400 is faulty; but fault has been masked by tolerance of the KX155 to just a LOW voltage on the TX key line.


its a problem post changeover from KX155 to ATR833. Everthing else has remained the same. But on the TX Key line from SPA (via CP136) is not ground when keyed and I wasnt sure whether the SPA400 provides a true ground on the Tx Key line, or a "low" ,
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By wigglyamp
The SPA does provide a PTT ground but it's via a diode so won't be measurable as zero ohms from the radio key-line. There is no keying relay inside the SPA and pilot/co-pilot mic isolation is achieved by shorting the non-transmitting mic to ground via another diode and resistor. Normally they're pretty bullet-proof but it could cause your problem.
Hmm a diode would give me 0.6volts but I see 1.25 volts on the Tx Key line from the SPA-400 via the CP136. Only way to get the ATR833 to Tx is to ground the line.

If I switch to COM 1 on the CP136 then the KX155 Com 1 keys up perfectly as it always has.

I'm beginning to think its a ground loop issue perhaps and I need to re-visit the unused connections and the ground connections on the ATR.

It maybe they do not expect the mic GND to be connected to Power GND at the ATR833.

My quest continues....
Just had a reply from Funke with a mod sheet from Sigtronics. It seems its a recognised problem, particularly with KMA 20 audio panels and their solution is... yes you guessed it ... a relay.

So will fit one tomorow and hopefully all will be well, although I did notice audio intercom distortion when Rx squelch is broken and someone speaks in the aircraft.

Ah well.....
By gbdxo
Flying_john wrote:All solved with the addition of a relay and a 220 ohm resistor.


I've just installed anATR833 via a KMA20 and a SPA-400 and guess what. It won't transmit.

I'm guessing the relay is installed in the keyline from the KMA20 to the mic key of the 833.
Could you possible message me the diagram please. I've been pulling my hair out all day, upside down in a PA28.

Many thanks.

Hi Steve - this diagram shows the 220 ohm resistor:-

The relay diagram I no longer have, but it is fairly simple.

The TX key line from the audio panel is connected to one side of the coil of a 12volt relay. The other side of the coil has to be connected to +12v via a fuse or cutout.

The Normally open contacts of the relay: one is connected to ground and the other one is connected to the TX key line of the radio.

So what happens is when you press the PTT, the relay coil has 12v applied across its coil and the contacts pull in, grounding the TX key line of the radio.