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QMX output is always 2W on all bands

 

Hi everyone,

I built the QMX kit configured for 12V operation (the kit was ordered on 13/04/2024), and while everything appears to be functioning correctly, I'm only seeing about 2W of output power across all bands.

Here are some data points and measurements I’ve taken:

  • Input voltage to QMX: 11.2V (measured with a Fluke DMM), very stable with minimal drop during TX.

  • Antenna/dummy load: SWR monitor seems to be functional and correspond to nanoVNA measurments.

  • Power display: Only 1 bar (4–5 dots) shows on the PWR display while connected to QRPLabs' 20W dummy load.

  • Measured voltage: The DMM reads ~11V across the dummy load during TX.

  • QMX Console app: Shows 9.5V and between 1.8W to 2W output during TX across all bands.

I’m not sure where to start troubleshooting this issue. What would be the best way to begin isolating the cause?

For context, I have the following test equipment available: oscilloscope, multimeter, bench power supply and another working QRP radio for reference/testing.

Please let me know if there’s any other information I can provide to help identify the problem. I’d be very grateful for any suggestions or guidance you can offer.

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Thanks in advance for your help!

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Best regards,
Igor — W6ETA

73
 

A few more data points w/ power supply connected:
Supply Voltage | TX Voltage | TX Power | SWR |
10.8 V ?| 9.4 V | 1.9 W | 1.18 |
11.8 V ?| 10.2 V | 2.1 W | 1.18 |
12.8 V | 11.0 V | 2.3 W | 1.17 |?
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Hello Igor,
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some of your screens show input voltage of 12.8v. ?Others show 11.8. ?Is that what you see there? ?Is the supply voltage being reported inconsistently or are you varying it?
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also the bias should always be steady at 30ma. ?The QMX is actually reading a voltage which (if everything else is right) should correspond to bias current. ?So this is an indication that something downstream of the bias circuit is not right.
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whenever I have a repair where the output is consistently too low, I get suspicious of T501 - the output transformer. ?

I think I would track down each of these anomalies thoroughly and separately.
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let us know
--
73
Jeff Moore
W1NC
 

On Fri, Apr 4, 2025 at 06:16 PM, Jeffrey W Moore wrote:
some of your screens show input voltage of 12.8v. ?Others show 11.8. ?Is that what you see there? ?Is the supply voltage being reported inconsistently or are you varying it?

Hi Jeff,

First of all, thank you for your reply — I really appreciate you taking the time to help!

In my previous post, I was experimenting with different input voltages using a bench power supply to see if the transmitter’s voltage and power would change accordingly. My theory was that the output power should increase at least somewhat linearly with the input voltage. However, regardless of the input voltage (ranging from 10.8V up to 12.8V), the output power changed only 0.4W.

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