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I finished assembling my QMX+ (including a battery mod) and finally got the chance to test it.? In the diagnostics everything appears to check out, until I press P.? At that point the SMPS bias current for every band is ~4.6mA (where the PIN forward bias is 30mA).? If I press T (while connected to a dummy load), the RF output and the Transmitter Voltage turn RED, with the Voltage reporting ~9.5-10.0V.? Power is consistently listed as 0.0W, and the SWR (into the 50 Ohm dummy load) is all over the map.

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Any ideas what I did wrong?? I've looked at all of my soldering connections multiple times and don't see any bad joints or bridges.? I've checked everywhere I can think of for continuity, and everything has checked out so far.

Did you remember the loop between the bnc and the output from the swr bridge ?

I did, but on another readthrough of the directions, I missed that it said to put it in from the underside of the board.? Let me try changing that.

No good. :( I redid the loop connecting the SWR bridge to the BNC connector and the results are still the same.

I just discovered a slight change.? On 6m, the Bias SMPS current is about double at ~8.2mA.

I just finished my build sounds similar to your issue?

0 watts

9.xx volts?

swr jumping a little mostly around 1.2-1.5

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I am going to start form the bnc and work my way back through the circuit unless I find any other advise?

let me know if you discover anything!!

On Apr 6, 2025, 15:43 -0700, mitchellrlawrence via groups.io <mitchellrlawrence@...>, wrote:

On Sun, Apr 6, 2025 at 06:43 PM, <mitchellrlawrence@...> wrote:

I am going to start form the bnc and work my way back through the circuit unless I find any other advise?

Be aware that the VSWR transformer winding puts a DC (not RF) short across the output BNC connector.

73, Don N2VGU

On Sun, Apr 6, 2025 at 04:02 PM, Paul - AI7JR wrote:

Hard to understand measuring SWR if you truely have 0 watts out... :-)

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Paul -- AI7JR

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That's why I mentioned it in my original post.? The SWR is reported in the Diagnostics page on the terminal when I hit T for Transmit.? For me, it's all over the map, sometimes reporting in the 6-7 range, sometimes so high it just reports ####, and other times dropping down to the 1-2 range.? All of that while it claims 0W output (and a wattmeter on the dummy load shows no power).

SWR is calculated by measuring the forward and reverse voltages obtained from T507 and calculating the SWR from those.? It there's no power out then those values can still be measured, they will both be close to zero, with noise.? A 'SWR' value can be calculated but it's just done from two noise measurements which is why the value varies widely.? The power is derived from the forward value only.

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Chris. G5CTH

I picked up an oscilloscope multimeter, (50MHz bandwidth) and checked the signal coming out of the QMX+ and got some interesting results.? When I tried to transmit through the Diagnostics in the Terminal, I got nothing, but when I used the Tune SWR function, I got clear sine waves on every band except 6m (to be expected as it's outside the bandwidth of the scope).? On 160m and 80m, for example, it was a roughly 1V signal.? On 60m it was closer to 5V.

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It's been decades since I did any EE in college, and signals processing/RF was never my strong suit.? What else should I be looking at?

In a spirit of double and triple checking everything, I rechecked continuity for my connections on T507 and T501.? T501 had intermittent continuity between two of the primary windings.? After reflowing the connections, I am now getting a signal out of the BNC connector when I hit Transmit and the SWR has stabilized going into the dummy load at close to 1.0 (except on 6m, which is fluctuating between 2 and 4).? However, I am still getting the extremely low SMPS bias current, and Diagnostics still reports 0W output.? The power meter on the dummy load is showing a little power output (possibly as high as 5W for 160m and 80m, but going down for successive bands).

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First a couple of basic checks on your measurement setup -

- make sure your scope and probes are set up correctly: most scope probes have a? 1x and 10x option, and it needs to match that same option on your scope, otherwise the readings will be incorrect by 10x

- when you measured the output, did you have a dummy load attached?? hopefully so, unless your scope has a 50-ohm termination option turned on (but most inexpensive scopes don't have an internal 50-ohm termination).

- when you measure, are you measuring peak-to-peak voltage of the sine wave, or an RMS value?? Most scopes report both; peak-to-peak is probably best for debugging

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If the above are done correctly, you are seeing very low power out of the QMX, but real power.? 4W out should be about 40Vp-p, but Tune SWR default is 50% voltage, so about 20Vp-p.?? The 5Vp-p you measured would be 62 milliwatts (5Vrms would be 100mW).?? So please first confirm your measurements.

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If they are correct, it means your BS170s are not amplifying the signal much, if at all - or maybe all your LPFs are turned off due to no bias current, so the signal isn't coming out.

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Most often we don't have a bias current problem, but Jeff has noted that sometimes the L501 gets broken (one of those 330uH coils), or C504 (the big tantalum capacitor) fails.? So you could check those two items.? Also check that L504 and L514 are soldered in properly and not cracked (since I understand they are a bit fragile) - and that your diodes are not soldered in backwards.? Any of these could cause havoc with the bias current.

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Stan KC7XE

Looks like the L501 is broken.? Guess I'm ordering parts, and trying again.

I believe it said to 'solder' it under the board, not 'put' it under the board, which would mean soldering on top.?

On Tue, Apr 8, 2025 at 06:51 AM, Stan Dye wrote:

- when you measured the output, did you have a dummy load attached?? hopefully so, unless your scope has a 50-ohm termination option turned on (but most inexpensive scopes don't have an internal 50-ohm termination).

- when you measure, are you measuring ms would be 100mW).?? So please first confirm your measurements.

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Please do not measure the output of a QMX using the internal terminator of an oscilloscope, you have a good chance of magic smoke coming out of the oscilloscope.

I received the inductors late yesterday and just finished replacing L501.? I'm now getting ~30mA for the bias current.

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It's still reporting 0W output in the Diagnostics when I hit Transmit into the dummy load.? SWR is back to being all over the place, too.

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Guess I'll have to break out the oscilloscope again...

I'm glad the inductor solved the bias problem.? One issue out of the way.

What transmit voltage does it show in the diagnostics above the 0W value?? And what power draw does it show from your supply, and what does input voltage go to?? With the 250mA limit on your , you won't be able to get much power out.

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Note that SWR jumping all over when the power is near zero is not likely an indication of any problem.? It is a mathematical divide-by-almost-zero issue.? If you can measure real power output with an external meter, more than about 100mW, when the diagnostic shows zero power, it could mean there is a problem with a T507 connection.?