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

Wait a minute!
I do have a pbs, an LED and a resistor.
So, it's WSJT-X 'Tune' followed by my pressing the n.c. pbs.

Hi Gary.
All I have to do is punch the 'Tune' button in WSJT-X and the AH-4 does the rest :0)
72,
Bill, N4QA

Bill
How do you tune the AH-4 with the QDX?
Gary
W9TD

?

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. ??
--
Marc
N8LO

I would put getting the RF out of your power supply high on the priority list.? First rule of troubleshooting is to fix what you know is wrong.
I would not be comfortable with my power supply voltage (or DMM) swinging around like that.??
I have a nice old Simpson analog meter which seems to be immune to RF; it is good for a check.
73, Don N2VGU

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?
--
Marc
N8LO

This will be an informative article for you.

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

Willie,
This first step is to do a very close and?thorough inspection of the board for shorts or bridges between traces.?

What happens when you use the Terminal Transmitter Test program?? Does it still drop out?

I think you might have a short occurring when you go into transmit mode.? That could be across the 5volt regulator output as it supplies all of the QDX logic ICs.? Can you monitor the output of IC2 when you go into transmit to see if there is a sudden drop, and then it comes back?? This could be an IC5 failure.? Does the LED continue with the 3 pattern flash?

If you have a hot air SMD rework station, I would remove IC5 to see if the transmitter drops out after the 1/4 second.? If not,? then replace IC5 and remove the BS170s.? Try again,? If you can, test the BS170s with a transistor tester.? Replace all 4 if one is bad.

The above are just suggestions.? I could be misinterpreting the symptoms you are describing.? Please proceed at your own risk.

73
Evan
AC9TU

Thank you Hans,
It took me a few days to check everything. I am only using the built in tools. They have changed since the manual and I didn't notice the scale changes till today.

I am using a Rev 3 board. I ran through T2 with a fine tooth comb. It had one extra winding at the beginning so I removed it. The solders have continuity. I am attaching 10 screenshots of the built in diagnostic tools. I played with the 40M RF filter for some time. The peak is still too high at 20 turns on both inductors. I am wanting to fine tune everything as much as possible before I close up the case. This is an exercise in understanding as much as anything. I am interested in what you see?
--
Colin Kaminski - K6JTH
https://www.qsl.net/k6jth/

OK, so it's only one way, but it adds evidence that this little QDX - no bigger than a pack of cigarettes - has a good transmitter indeed !
Over the past 24 hours, I've made probably 50 or so 2-way contacts on the 80, 40, 30 and 20 meter bands.
But, 20 is providing a bit of Delta Xray !
My antenna, fed by an Icom AH-4 remote autotuner, is an end-fed wire, approximately 80 meters in length, averaging 5 meters AGL - not so good, especially on the lower bands.
Anyway, I'm having a blast.
Thank you, Hans, all the gang at QRP Labs and, of course, the gentleman who sold the QDX to me and all list members/owners who lent tremendously helpful hands in getting the QDX going !
73,
Bill, N4QA

Just some final thoughts after rethinking and calculating the levels.
When using a BS170 buffer you come up with something like 150mW output RF power on 50 Ohm.? This is the RMS voltage around 2V.? To get to the required 1V RMS you need to add a simple 6 dB resistor attenuator.? With the attenuator you gain three things:
- protection of a SiLabs CLK0 output
- proper level signal for a TS
- good return loss to a TS 50 Ohm input (RL = 12 dB)

73 Bojan S53DZ

Please have in mind also that the Si5351A CLK0 output is a square wave 5V signal with a 2.5V DC component.? TS VFO input must be a sinus AC signal.? So don't forget C1 to decouple.

73 Bojan S53DZ

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

But, no matter what that test might show to you I would suggest you do the buffer amplifier with BS170 to protect the Si5351A output.

73 Bojan S53DZ

开云体育

Hans,

Hi Al,

Good news! Just want to mention, the diode will drop 0.7V across it in this application. This is the reason the FET is used since it has a lower voltage drop across it.
This is will contribute to the lower current draw in Transmit (0.69 A). It's lower than what's usually seen.

Thanks for pointing that out. I'm aware of the voltage drop and this is a price I'm willing to pay at the moment, I compensate with a bit higher voltage if needed. :)

Do you have a way to measure transmit power out in Watts into a 50 Ohm (dummy) load? I'm just wondering if maybe one of the PA FETs (BS170's) is open and giving you the lower current reading. I don't want to be an alarmist, but it's something to consider.

I can estimated by measuring the peak voltage on the dummy, which is 13,89 V at input of 9 V (-0,7 V) and 7 A, so in the 3 W region I would guess.

When connecting an antenna the input voltage and current are different than with the dummy load.
I observed that my PSU changes voltage depending on the antenna, band and load ... maybe RF getting back in it.
For example when I set 8 V on 30 m while receiving, while sending the PSU goes to 9 V and the QDX draws 1,1A. On 80 m it goes from 8 V to 11,1 V and draws 0,77 A...
So now for every band I dial it in to have 9,7-10 V with load.
This seams to work reasonable well, WSPR goes over the Atlantic on multiple bands and I had some QSOs on 40, 30 and 20 m around Europe.
Maybe I need to decide on one band and build the 50 W amp. :D

Hi Wayde,

I use the following SWR bridge and tuning indicator with mine so that the max SWR 'shown' to the QDX will be 1:2... A fun and easy kit build and seems to work well as insurance for me!

That looks really nice!
I'm thinking about going the extra mile and building a full tuner like this ZM-4 ATU .


73 Alexander DM8TE

Bertel,
The Elecraft W1 power/SWR meter is the most RELIABLE.


Charles Brown
N4SO