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Folks - I have two different QMX rigs - one low band (rev 1), one high band (rev 2). In the configuration I have enabled TX ground on both on each band. I've soldered in a short jumper to correct the trace error on these early boards. Both rigs are at firmware version xxx.17. I've plugged in a 3.5mm cable into the PTT jack on the right side. The other end is bare wires (for testing). I've attached my VOM across the tip and ground, press the key, and.... nada. No ground happening. I've tested continuity with the cable unplugged and also verified that the wires I think are tip and ground are in fact tip and ground.

?So, is there some other magic stuff I have to do to get PTT ground working?
--
Jim / K7TXA
Eagle, ID

SKCC 10447T
BUG 301

?To test with a a VOM look for current with 12V on one lead and the other lead on the PPT tip or ring I would put a resistor in series with the 12V lead so that if/when the other lead shorts to the PPT ground ring the current is limited.
Note: some amplifiers such as the MX-50 require the PPT to sink more current (~39ma) than R205 will allow.? I built a transistor buffer into my MX50 to fix the issue.? On my QDXs I paralleled the R206 equivalent to allow it to sink more current.? For the QMX I decided I should fix the amplifier.?
I built a buffer circuit for transceiver with either grounded or high PPT control, Circuit below Note I added a diode across the amplifier's relay to suppress switching spike,? If you only plan to use it with the QMX you don't need both buffers:

Hi Chris - thanks for the reply and the schematic.

?However, I'm not understanding why, if I want to verify that QMX is actually grounding the tip of the plug when I transmit, can't I simply put my VOM across the two wires at the end of the cable plugged into the PTT jack of QMX? It should go from an open circuit to a closed circuit, yes? Is there something strange about that jack / connection that I can't see?
--
Jim / K7TXA
Eagle, ID

SKCC 10447T
BUG 301

Hi Jim.?

I had made the same assumption as you when I first tested my QMX PTT out after firmware 017 was released and I installed the jumper as you have done. But Hans explained that the circuit in the QMX sinks an external current, and a multi-meter can't be used to simply test tip to ground. It doesn't work that way.?

The device you want to trigger needs to supply some (low) voltage and current, and the QMX in the tip to ground configuration, will sink that current, looking like a short to the external device (but not to an ohm meter).?

In my case I need to trigger an external TX RX switch which allows my SDR Play receiver to share the same antenna that QMX uses. That device supplies 4v and a few mA of current (I forgot the amount) to that lead coming from the QMX, and the QMX effectively shorts that line (tip to barrel of PTT out) and triggers the device.

So, the QMX PTT out does in fact, work properly (once the board has the jumper installed, and the configuration settings are enabled for each band, as you've already done). It can't be tested with only an ohm meter, however.?

It can be tested using an external power source (3.2v for a typical LED) and an LED with current flowing through (in series with) PTT out tip and barrel. In QMX RX mode, the LED will be bright, when it goes to TX the LED will significantly dim or go off. (when configured as grounded on TX)?

So it does work as advertised, just not as both you and I assumed it would work.?

73

Roy - KI0ER?

Hi,
you can also test it with the Diode function if your AVO has it which operate at a higher voltage.
But the simplest test if a GND PTT line at the tip of connector is working is if you
- enable both the +5V and the GND PTT in the configuration menu and
- then measure the voltage between the tip and ring.
It should be around +5V.
--
73 Bojan S53DZ

Bojan, Roy, and Chris - thanks for the replies and tips on how to test that the PTT is in fact working. And surely enough, IT IS!! I metered between Tip and Ring and also between Ring and shield - 4.93 volts when I press the key. Most certainly not what most folks would have expected! But, now that I know PTT does function I'll feel safe connecting my SDR++ to the QMX through my MFJ-1708B-SDR box. Now I just need to get CAT-relay working so the QMX and SDR++ talk to each other!

?72/73 & many thanks guys.
--
Jim / K7TXA
Eagle, ID

SKCC 10447T
BUG 301