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Looking for some troubleshooting steps.? I can measure V,A,R,C,L.? No scope.?
9V build, after build and testing, I had power output on all 4 bands.? I put it in the case and lost 20M transmit, may not be the cause.
I have made contacts on 40M, now 40M has no power output.? I think the cause is tuning my mag loop was too much high SWR.? my loop doesn't do 80M, so that is why it survived?? I do have a solution to the tuning issue in the future.

What are the first steps to troubleshooting this?

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Marc
N8LO

Marc, have you tried putting it on a dummy load and measuring the output from the dummy load using a DVM? The best way to do this is using the terminal applications to transmit, so you can eliminate the computer as the problem.
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73, Dan - W2DLC

Hi Marc,

As Dan stated, the first step is to connect to a dummy load and verify that the current in receive mode is less than 140ma.? If not, you will need to remove the 4 BS170 MOSFETS and try again.?

Once the receiver works, you can test the low-pass filter selection by measuring the voltage on each of the 6 switching diodes D1,2,3,4,5, and 6.? The banded side of the diode (cathode) should read zero volts for the two diodes that select the band chosen.? The other four should be close to or above 5volts.? This should be done whit a dummy load connected.? You change bands using the RF Sweep function in the terminal program.

You can now go on to test the transmitter into a dummy load.? Do not install the 4 MOSFETs yet.? First, verify that there is 2.5volts DC on each of the 4 BS170 gate pins while transmitting using the terminal transmitter test program.? If you do not have 2.5volts on all 4 drain pads, you will need to replace IC5.

After you have verified IC5, measure the resistance between the source and drain on each of the BS170 MOSFETs.? It should be high.? I would guess over 1kohms, but I have not measured it.? If that checks out, reinstall the 4 BS170s and test the power output into the dummy load for each band.? It is also a good idea to measure current while transmitting (it might be hard to do when using your only DDM to measure power out).

Let us know if you find and fix the problem or the results of your tests if the problem is not solved.

The above are my recommendations.? I do not warrant that it is the best way to test.? It is what I have done in the past.
73
Evan
AC9TU

Evan, you are getting this to a very concise set of steps. It is really helpful. What is the transmit current draw expected at the power supply on the QDX?
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Colin Kaminski - K6JTH
https://www.qsl.net/k6jth/

Question: ?The current in receive mode where? ?Where am I measuring current. ?The draw from the power supply? ?
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Marc
N8LO

I have been testing with a dummy load using a standard SWR/Power meter to determine if I am getting power to the dummy load. ?Is that what you mean? ?I can tell you software is not the issue. ?I have run tests using WJTX to trigger and the terminal app via putty.
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Marc
N8LO

Normal receive current at the power supply is 140-150mw.?
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Colin Kaminski - K6JTH
https://www.qsl.net/k6jth/

Thanks for the clarification, measured the current at the output of the buck converter. ?.19 mA, which at 9V gives me 170mW on receive. ?I hope that means I’m not pulling transitors?
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Marc
N8LO

On Sun, Aug 14, 2022, at 04:28 PM, Colin Kaminski wrote:

Evan, you are getting this to a very concise set of steps. It is really helpful. What is the transmit current draw expected at the power supply on the QDX?
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Colin Kaminski - K6JTH
https://www.qsl.net/k6jth/

The current draw is the total from the power supply.? The amount depends on the voltage.? Most rev 3 builds the same receive current: 140 to 160ma.? Rev 1 builds are lower, around 110 to 120ma.? ?Anything under 160ma seems to be OK.

For Transmit, 12volts is around 800ma.? 9volts is around 1ma.? These will vary based on the power output that depends on the antenna loading.? It is best to verify with a 50ohm dummy load.? I have read as high as 1.2amps for the 9volt build with 5watts out.? My 9volt rev3 QDX measures 1.0amps with 4.8watts out.? For 12volts, it can be as high as 900ma with 5watts out.? My 12volt rev 1 QDX measures 725ma with 4.6watts out.

The above is based on what I have seen with 6 different QDX hands-on measurements and multiple online remote support.? Again, these are my observations.? Others may have differing experiences.
73
Evan
AC9TU

Marc,

The current you are reading on receive is too high.? I would recommend pulling and checking the BS170 MOSFETS.? I suspect that one or more have turned into resistors instead of being MOSFETs.

73
Evan
AC9TU

Thank you Evan. I only have one to measure and wasn’t sure.?
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Colin Kaminski - K6JTH
https://www.qsl.net/k6jth/

Thanks Evan, I will pull the mosfets and test them. ?Hope that fixes my issues. ?I will let you know how it goes.
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Marc
N8LO

I think Colin meant?

?	Re: QDX v3 Lost transmit on 20/30/40 From:?Colin Kaminski Date: Sun, 14 Aug 2022 17:30:06 CDT Normal receive current at the power supply is 140-150ma. NOT MW

OK, so I pulled the BS170’s and made a simple circuit to test them. ?The LED lights up on all four. ?Is there a better test?
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Marc
N8LO

Without the BS170’s my receive current is the same. ?I am measuring the current from center pin of power connector with + probe and touching the - probe to the ground of coax. ??
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Marc
N8LO

Marc,

You do not describe the test circuit that you say verifies the BS170s.? You should have very high resistance between the gate and the source, as well as the gate and the drain.? Also a high resistance from the source to the drain without a connection to the gate and the positive lead of the DMM on the drain.? Can you describe the test circuit you are using?? What is the value of the voltage being used for the test?? How are you turning on and off the gate voltage to verify that the MOSFET does act as a switch and not just a resistor?? Here is a DIY test circuit for a MOSFET:

Note that the BS170 is an N-channel MOSFET.

If you use something like the above and the LED turns off and on with the switch on the gate, then your BS170s are good.?

You have stated that the receiver current is the same as before removing the BS170s.? The next step is to verify that IC2 can still provide 5volts and that it is not very hot to the touch.

Can you tell us if the LED comes on flashing, then goes solid after a few seconds?? If so, and if IC2 is not overly hot and the output of IC2 is 5volts, then I suspect your current measurement may be off.? I would test the RF and Audio Sweeps in the terminal program and then see if FT8 signals can be decoded with WSTJ-X software.? You will need to connect an antenna for the FT8 test.

If all that checks out, you are ready to test the transmitter.? I start with using the Transmitter Test function in the terminal program.? You should have the QDX back on the dummy load for these tests, even though the BS170s are still not in the QDX.? Verify that you are getting 2.5volts DC on the gate pads with the BS170s not installed.? This is done while pressing the PC t to key the transmitter.? The LED should flash the 3 pulses and pause pattern, showing that it is going into transmit.? When you release the t on the PC, the gates should all read 0volts.? You need to test all 4 BS170 gate pads.? If any fail this test, you will need to replace IC5, the 74ACT08 transmitter driver.

I think that is enough for now.? Let us know what you find and we can go from there.
73
Evan
AC9TU

Marc,

The best test that I know of is to use one of the Transistor Testers sold on Amazon or eBay:


Others are OK, though not as good for inductor or capacitor tests:


73
Evan
AC9TU

“?If any fail this test, you will need to replace IC5, the 74ACT08 transmitter driver.”
It is notable that this chip has 4 and gates and they are used in parallel. Two gates in parallel are linked to two BS170s in parallel. (I am sure you?know that Evan, but I thought it was relevant for people following along.)?
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Colin Kaminski - K6JTH
https://www.qsl.net/k6jth/

Hi Colin,

You are correct.? To get the drive needed to overcome the capacitance in the gates of the BS170, you need the two gates of the 74ACT08 in parallel.? ?If one of the two gates has failed, it will load down the other gate most of the time, and you will not get the squarewave output required by the BS170s.? This will show up as a lower voltage when measuring using a DMM on DC.

The DMM DC voltage test is not the most accurate, as some failure modes will not show up.? The best test is with a dual trace oscilloscope with one probe on one set of the parallel gates and the other probe on the other gates.? You should see two 5volt square waves 180degrees out of phase.

I am trying to keep it simple.
73
Evan
AC9TU