Want to adapt PC headset to aviation radio

[MikeMl]

That's 450 hours of transmit time... (I dont think you will be replacing them very soon)

 

[Allan Broadribb]

The capacitors arrived today and I finished the project. However it is not working for me. When I key the mic it emits a high pitched wine to the receiver I have in the garage and also through the headset. I removed the mic and headset keyed the mic and it still emits the same wine.

I have checked it over three times and can not find any soldering problems or accidental groundings. I checked all of the connections for continuity to ground and they all appear to be good.

When I key the mic there is 6.15 +/- volts on mic hi and 3.0 volts on the output line to the mic.

I believe the battery switch is OK - when the mic is not keyed here is zero voltage on the emitter line from Q1 - when I key the mic there is 3.6+/- volts on the emitter line.

Hoping you can help me troubleshoot this. Thanks, Allan.

 

[MikeMl]

The capacitors arrived today and I finished the project. However it is not working for me. When I key the mic it emits a high pitched wine to the receiver I have in the garage and also through the headset. I removed the mic and headset keyed the mic and it still emits the same wine.

I have checked it over three times and can not find any soldering problems or accidental groundings. I checked all of the connections for continuity to ground and they all appear to be good.

When I key the mic there is 6.15 +/- volts on mic hi and 3.0 volts on the output line to the mic.

By "output line to the mic" do you mean where C1, C3 and R4 come together?
Are you measuring the voltages with respect to "MicLo" (which is the same as the radio ground)?

If so, those sound correct.

While you are transmitting, what are you using as an antenna, or are you using a dummy load?

I suspect that RF from the antenna is getting into the circuit, is being rectified and is causing the audio feedback (the howl).

To prove the point, do you have a shielded 50Ω dummy load you could transmit into? Or perhaps an outside antenna?

I believe the battery switch is OK - when the mic is not keyed here is zero voltage on the emitter line from Q1 - when I key the mic there is 3.6+/- volts on the emitter line.

You mean the wire labeled PP in the schematic. That would be the collector of Q1 (it is drawn upside down).

If it is RF getting into it, you could try adding a 1 nF (1000pF or 0.001uF or "102") capacitor in parallel with R5 to bypass any stray RF to ground from the base of Q2.

When you built it, where is the circuitry box relative to the electret mic?

Is the mic in the headset and the circuitry in a small box close to the radio?

How long are the wires between electret and box?

Between the box and the transceiver?

Is the wire between the mic and the box shielded?

Is the wire between the box and the radio shielded?

Are these shields through connected to the radio ground (MicLo)?

Just to check;
C1=100nF or 0.1uF or "104"?
C2=47nF or 0.047uF or "473"?
C3=10nF or 10000pF or 0.01uF or "103", right?

Give me some feed back. Try the cap and I have some other ideas if you are still having problems.

 

[Allan Broadribb]

By "output line to the mic" do you mean where C1, C3 and R4 come together?

Yes.

Are you measuring the voltages with respect to "MicLo" (which is the same as the radio ground)?

I connect my black tester lead to the ground plane of the circuit board and the other to the line to be tested.

While you are transmitting, what are you using as an antenna, or are you using a dummy load?

I am using the antenna attached to the hand held radio. There is a connector wire about 6" long to the circuit board. The connector wires to the headset are about 4' long.

You mean the wire labeled PP in the schematic. That would be the collector of Q1 (it is drawn upside down).

Yes - it is the collector of Q1 with 3.6volts.

If it is RF getting into it, you could try adding a 1 nF (1000pF or 0.001uF or "102") capacitor in parallel with R5 to bypass any stray RF to ground from the base of Q2.

I'll see if I can find one at Radio Shack today.

I have the following caps:
1109PHCT-ND CAP 50V .1UF AXIAL CERAMIC
1107PHCT-ND CAP 50V .047UF AXIAL CERAMIC
1103PHCT-ND CAP 50V .01UF AXIAL CERAMIC

Allan

 

[MikeMl]

You didn't answer the question about wire shielding. Attached is what I would recommend for shielding. Note that the return line for the earphone is isolated all the way to the earphone jack barrel to the radio. Does your radio produce sidetone? If so, then the audio howl could be caused by the headphone signal coupling into the mic preamp. The only way to fix this is if the earphone does not share a common ground with the mic...

In the glider, do you transmit on the rubber duck, or do you have an outside antenna?

The rf beads may be required if the bypass cap, nor wire routing fixes it.

 

[Allan Broadribb]

In the glider I use the rubber duck antenna. I do not know if the radio produces sidetone. It is basically wired exactly as your sketch although the mic and earpiece wires are separate. I am not sure about wiring the PC mic plug which is 3.5mm and I understand it is wired as follows: tip=mic input; ring=5volts power; sleeve=ground. If I can not use the standard PC headset then there is no point to the project.

I'll try the additional capacitor. When you say in parallel with R5 I assume you mean it will go beside R5 and connected to ground and the line out of Q2 the same as R5.

 

[MikeMl]

... When you say in parallel with R5 I assume you mean it will go beside R5 and connected to ground and the line out of Q2 the same as R5.

Yes. Let me know what happens...