开云体育

I recently finished?building?my QMX. However, when I plug in my headphones to the audio port, all I get is static that stays the same regardless of whether or not I have an antenna?attached or not.

Kai, please read QMX QMX_plus Audio PCM1804 from QRP Labs Wiki.

Could you send please screen shots from Terminal - Hardware tests - Rf filter sweep and Test ADC I/Q for a higher band and a lower band?

?

73 Ludwig

Ludwig,

I've attached the screenshots of the 60m and 15m bands for the RF sweep and the ADC I/Q. I'm going to go read the PCM 1804 and try to see if that's the problem

Thank you,

73 Kai

?

Kai,

?

because the level is very low (-85dB) I see two possible reason:

To decide you should do the 2nd test from QMX QMX_plus Audio PCM1804 section Diagnostics.

?

Do you have good output from the TX? How are looking the LPF sweeps? If this is ok, you may exclude the LPF section as a culprit.

?

73 Ludwig

Kai, those sweeps show that you may not be getting any RF at all through your receive path.?

Before trying the PCM1804 tests, I would first examine these four things:
1) make double-sure you installed the LPF switching diodes in the correct direction (D508-D513).

2) carefully inspect the soldering at the BNC jack, and make sure there isn't a bridge between the center and ground pins - noting that there is an extra hole for the optional SMA center pin that is easy accidentally bridge to one of the corner ground pins of the BNC; note that you must inspect for such a short visually, because a continuity check with a meter will always show a DC short between the center pin and ground, due to the T507 transformer winding.? While you're at it, check that none of the terminals of T507 have solder blobs bridging them.

3) inspect L513 and L514, making sure both ends are soldered, and the inductors are not cracked - they are a bit fragile, and some users have accidentally cracked them during handling after soldering them in;? you can check for a crack doing a continuity check with a meter, it should show continuity across the inductor terminals.

4) check carefully the soldering of all of the leads of the trifilar transformer, but particularly the one closest to the edge of the card - re-heat it and hold the soldering iron on it for a few seconds.

?

Any of those four things would not allow RF from the internal test generator to get through the receive path, and would affect all bands.? There are other possibilities, but these are the more common ones related to assembly.

?

And if you have a signal generator and sensitive oscilloscope, and use them as outlined in the PCM1804 test, you may be able to trace the signal from the BNC through the receive path and see where it is missing, at one of the 4 above locations, or elsewhere.

?

Stan KC7XE

My apologies, Ludwig and Kai - I was composing this while Ludwig responded.
Maybe there is something helpful in the test/inspection detail I added.
Stan

Hello Stan, indeed you told much more to check. Yes these are possible failures with a higher probability and easy to check.

And if no signal generator / oszilloscope is available there are some more DC levels easy to check:

73 Ludwig

Stan and Ludwig,

?

Stan, Ive checked everything you said to and all of those things seem to be as expected, I unforunity do not have very easy access to a oscilloscope or signal generator.

?

Ludwig, where are IC 402 and Q508 located? I can't find any mention of them in the assembly manual.

?

Thank you,

Kai 73

See them on pg 17 of the assembly guide.? They are pre-installed surface mount parts.

Both are on the bottom of the card, IC402 a larger part near the edge, Q508 directly up from it a ways.

Ludwig,

I found where the board diagrams are located ill check those voltages right now.

Kai?

On Fri, Apr 18, 2025 at 11:38 PM, Kai Bouwman wrote:

where are IC 402 and Q508 located? I can't find any mention of them in the assembly manual.

Kai, both are SMT elements preassembled. You will find both checking section 2.2 of the assembly manual. Please take care not to slip with the probe tips and connect pins.

For IC402: please check schematics page 4. Depending on the selected band one of the pins Y0 to Y3 is connected to pin w. Pin w is connected via T401 to 2.5V from R401 and R402. So these 2.5V are connected to one of the pins Y0...Y3. Please check if all the connections to T401 are at 2.5V and also the selected Y-pin.

For Q508: This transistor is blocking the signal path for RX when TXing. Pleas tell the voltages at D, S and G for TX and RX. We will check the function. I saw two broken Q508 when debugging some QMX.

?

73 Ludwig

Hello Stan, and now our reactions in a vice versa order ;-).

Ludwig,

I'm getting the following voltages:

?

Thank you,

Kai?

?

Ludwig,

I may have been testing the wrong pin. I tested only the pin that the assembly instructions asked for a continuity test on I tested the rest of the pins on IC402 and all of the ones closer to the edge of the board are reading about 2.5V from T401

?

73 Kai

Kai, sorry. I should have mention to check all the voltages in respect of GND!!!

No worries, I rechecked and all of T401 is around 2.5V and one pin on IC402 is also at 2.5V however on Q508 D and S are both at 0V and G is 3.29V

Thank you,

Kai??

On Sat, Apr 19, 2025 at 12:41 AM, Kai Bouwman wrote:

Q508 D and S are both at 0V and G is 3.29V

?

for RX OK. So no break of the RX signal path here.

More tomorrow (today). It's one past mitnight and my body is asking for sleep.?? 73

Kai, you may simulate a signal generator for a special test. Use some meter isolated wire as an antenna. Take care the wire doesn't connect anything electrical. Dial your QMX to a FT8 frequency e.g. 14.074 MHz and set it to digital mode. Listen to the audio and connect the wire to the connection between T401 and IC403 pin 9 (the wire from T401 next to the edge of the PCB and a screw hole).

It would be fine to check first by using a different radio if loud FT8 signals are present at the time. If not use a different band or an other time.

?

If you hear a difference between wire connected and not connected and some FT8 sound when wire is connected, the signal path from here up to IC407 is working in some way. If not maybe no FT8 signal is in the air or the signal path from the testpoint up to IC407 is broken.

?

Sorry it's not so specific but a simple way to gain some more information without special equipment.

?

73 Ludwig

Ludwig,

My apologies for not responding today, I was busy all day with a search and rescue exercise.

Anyways, I tried what you suggested and made sure that there was FT8 activity on the frequency and did not hear any difference.

Thank you,

Kai Bouwman

Kai, please don't apologise. No duty for a fast reaction, it's our hobby.

?

Your result points more to a broken PCM1804. It's the highest probability. But to exclude something more you may test this:

If you don't want to replace the PCM1804 by yourselves you may contact Jeff Moore (please check th forum for his name).

?

I wish you success. 73 Ludwig