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QMX Receiving FT8 but No Change When Antenna Disconnected

 

Hi all,

I¡¯ve completed assembly of my 60¨C15m QMX. It powers on and passes the RF and SWR sweep tests. Test ADC I/Q shows sine waves, particularly on 60m.

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I can hear what sounds like FT8 on 7.074 MHz in DIGI mode, and the tones appear to cycle every 15 seconds as expected. However, I¡¯ve noticed that disconnecting the antenna makes little or no audible difference ¡ª there¡¯s no significant drop in the noise floor or change in the signal.

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Additional observations:

  • The BNC center pin shows ~0 ¦¸ to ground (I understand this is expected due to the SWR bridge).

  • BNC center has continuity to one side of C510 and C526.

  • Test ADC I/Q and scope work as expected.

  • Reflowed L506, L508, L510, L511, L512, and L513 for better LPF continuity.

  • I¡¯ve confirmed IC402 pin 13 is not grounded, but I don¡¯t have continuity from the BNC center to pin 13.

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At this point, I¡¯m hearing what I believe is live FT8, but the lack of change when removing the antenna is making me question whether the antenna signal is actually routed through the intended receive path.

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Would appreciate any guidance or suggestions for what else to check.

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Thanks!

-Tanner

Diagnostics.zip
Diagnostics.zip

 

On Tue, May 27, 2025 at 06:44 AM, <tanner@...> wrote:

I can hear what sounds like FT8 on 7.074 MHz in DIGI mode, and the tones appear to cycle every 15 seconds as expected. However, I¡¯ve noticed that disconnecting the antenna makes little or no audible difference ¡ª there¡¯s no significant drop in the noise floor or change in the signal.

So something is definitely wrong.

Additional observations:

  • The BNC center pin shows ~0 ¦¸ to ground (I understand this is expected due to the SWR bridge).

  • BNC center has continuity to one side of C510 and C526.

C526 should show direct continuity.? And since the center pin has continuity to ground, it will also have continuity to anything else connected either directly or indirectly to ground, like C510, which connects to ground through T501.? So it is not very useful to try to trace the input RF path using DC continuity; you need an RF signal generator to do that.? The 'sweeps' are one way to do it with internal test capabilities.
  • Test ADC I/Q and scope work as expected.
  • Reflowed L506, L508, L510, L511, L512, and L513 for better LPF continuity.
  • I¡¯ve confirmed IC402 pin 13 is not grounded, but I don¡¯t have continuity from the BNC center to pin 13.
There should not be connectivity between the BNC center pin and any pin of IC402.? The received RF signal goes through C526, LPFs, and a T/R switch before getting to the receive bandpass filters.
Can you see this on the schematics?
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Your I/Q sweeps look the correct shape on 40m, but look at the vertical axis values - they are much too small, indicating not much internal test signal getting through your RF path.
Your RF sweep on 40m looks maybe OK, but what about the LPF sweep?
It is best on QMX to diagnose the Low Pass Filters first, because it is through them that the receiver works, and they interact with the bandpass RF filters.
So please attach a dummy load, and show screenshots of the LPF and SWR sweeps, which will allow us to help you further diagnose what is going on.
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Your 20m and 30m sweeps also show problems, but before diagnosing them it is best to see how your LPFs are working.
Stan KC7XE
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Attached the SWR and LPF sweeps.
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> There should not be connectivity between the BNC center pin and any pin of IC402.? The received RF signal goes through C526, LPFs, and a T/R switch before getting to the receive bandpass filters.

Hm, I have connectivity from the center pin to IC402 pin 1, 3, 4, 5, 6, 8, and 15.
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I'll upload the sweeps here while I try to follow where that could be coming from.

> Can you see this on the schematics?

Just found the ; I've got the rev 4 board.

LPF Sweeps.zip
LPF Sweeps.zip
SWR Sweeps.zip
SWR Sweeps.zip

 

I tested C526 with my BM235. In diode mode, it shows 1.9?V one way and 0.18?V the other. In capacitance mode, it reads around 1000??F, which is far off from the expected 0.1??F. I also confirmed that one side of C526 connects to the BNC center pin, and the other side has continuity to multiple pins on IC402 (1, 3, 4, 5, 6, 8, and 15).

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Just wanted to share those measurements as I continue debugging.
 

Please find your correct schematic here, linked from the qrp-labs qmx page (the one you got is from the qmx+ page):
This is for both rev3 and rev4 qmx.
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And sorry, I misspoke before.? My first statement is correct:
"Since the BNC center pin has continuity to ground, it will also have continuity to anything else connected either directly or indirectly to ground, like C510, which connects to ground through T501. ?So it is not very useful to try to trace the input RF path using DC continuity; you need an RF signal generator to do that."
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But my second statement was incorrect:
"There should not be connectivity between the BNC center pin and any pin of IC402." - this would only be true for RF connectivity.? From an ohm-meter perspective, as noted above, any node on IC402 that is grounded will show connectivity to the BNC center pin -- which are all of the pins you just noted, so your measurements prove they are correctly grounded.
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All of your LPF sweeps show that you are not getting a signal properly through them.? Note the vertical axis on all of them is all near 0dB top and bottom.? There should be a span of about 0dB to -20dB or more, showing the LPF filter shape.??And the SWR sweeps should be near the 1.0 line, not 2.0 and above.
Since this is on all bands, there may be a single common cause.
Causes that would do this on all bands:
1) faulty solder connections in T501 or L502 - this is the most common, due to enamel not getting burned off the wires during soldering; best to scrape them first.
2) faulty solder connections in T507 - because for LPF and SWR sweeps, the processor reads the forward and reverse power data via T507.
3) diodes installed backwards (D508-D513)
4) L504 or L514 not soldered well, or maybe cracked (they are a bit fragile, can be checked with an ohmmeter for continuity)
So those are the first places I would check.
And no, these do not directly address your Receive problem, but you have to do enough TX testing to make sure your LPFs are working before you can really get good data on the rest of the receive path (unless you have an RF signal generator and an oscilloscope, then you can test receive independently).
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I hope this is helpful
Stan KC7XE
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