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Hi gals and guys!

It's my fist message on this forum, so please let me that Hans is doing a great job with QRP Labs with his fantastic products (QCX-mini, QDX and QMX to name just a few).

Recently I've purchased high-band QMX rev. 3 and found some time to build it. Except for a few small stumbles (eg. non working buttons) I would say it was a successful build. Just a few days ago I had pretty decent sweeps (audio, RF, LPF and SWR) for all the bands except for 15m and 17m. I thought it may be some continuity problem with the T508 and T512 so I reworked the solder joints, put the radio together and... I lost TX and RX capabilities. This means no power output (on the "Diagnostics" screen after pressing T) and totally random test sweeps for all the bands: audio charts show no LSB/USB separation, RF filter shows random values around the middle part of the chart, LPF chart goes steeply to 0db on the lowest frequency and stays there until the end of the range. When I connect an antenna and power meter, I get 0W reading on transmit and barely can hear other stations (the signal is very faint). "Input Analysis" screen shows "NO SIGNAL".

I didn't touch anything else - the LPF toroids for 15m and 17m were the only thing I reworked. So I inspected all the solder joints and connections on the PCB multiple times, fixing the ones that weren't looking good enough, but it didn't help.

When transmitting, the power consumption goes from around 0.1A to 1.1-1.2A and the BS170 get warm. That's a good sign, isn't it? ;)

Any ideas what I should do next, keeping in mind I don't have any oscilloscope (just a regular multimeter and nanoVNA)? Do you have any ideas what may be the cause?
I really hope you can help me...

Have a great day,
Damian SP9LEE

Hi Marek,

Thanks for the hints, I'll check up if there's RF signal in the places that you suggested.

Answering your questions: I've built 12V version (but testing at 8-12V), did factory reset multiple times (so I have the default config for 20-10m), and of course a dummy load is connected. I've also checked if I can hear anything on the bands so I had an antenna attached for a while during the tests :)

When it comes to the SWR protection, I've read somewhere that it doesn't kick in on the "Diagnostics" screen when pressing T. But I'll disable it entirely, just in case, good hint.

Hi guys,

I'm sorry for a late update in the topic, but I didn't have any RF probe so I had to make one before I could gather the measurements that you've suggested.

My initial checks showed around 6V on TX (probed on T501 input) and something non-zero on C526, but after a short while I started getting 0V with no obvious reason. I assumed it's my home-made RF probe's fault (that was my first attempt, so yeah, possible) so I ordered something more legitimate - an oscilloscope. And then all the crazy part happened. I started with no RF on T501, IC503 and down to IC501/IC502. Then I got some square-wave starting from IC501/IC502 up and an RF on T501 only for a while, because it disappeared again out of the sudden.

My further investigation led me to IC204 (3-channel output clock generator) and IC202 (NAND gate). The TXCO gives exact 25MHz signal which is processed and outputted by the IC202. However the pins 6, 9 and 10 of the IC204 (generating CLK2, CLK1 and CLK0 signals respectively) behave strange. CLK0 and CLK1 are stable, with RF around the currently chosen band/frequency, but CLK2 was outputting RF only for a short while, showing 0V for most of the time.

Assuming my measurements were taken properly, no CLK2 signal would explain everything:

• transmitting 0W output and heating a pair of BS170 MOSFETs (IC501 XOR gate produces unmodulated constant +VDD to Q503 and Q505)
• super random audio/RF/SWR sweeps (CLK2 signal is required to activate the return path thru LPFs and Q508 to RX_IN)
• working RX mode (only CLK0 and CLK1 signals are required for the receive path to work, if I interpret the schematics correctly)

• I checked the continuity and resistance between the pin 6 of IC204 and R518 (100kOhm). There is a continuity to the resistor, but the resistance reading of the R518 is usually around 20kOhm and only sometimes I can get 100kOhm (by trying with different angles etc.). The resistance between pin 6 of IC204 and GND is about the same: 20kOhm. Isn't that weird?
• Maybe that's my oscilloscope's fault (I've got a handheld FNIRSI 2C23T), but the TXCO seems to show a pulsating amplitude of the 25MHz signal (so Vpp may vary from 0.6V to 3.3V, according to what I've seen so far). Generally I had this "feeling" the entire GND would be oscillating. Maybe it's just me and my poor engineering skills?
• I've inspected the entire board and haven't noticed any single short. All the joints are maybe far from being perfect (hey, I'm just a programmer), but they seem to be solid enough.