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QDX rev 5 Troubleshooting
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Hi all, trying to workout why my QDX isn't transmitting on 20m & 30m. Built the QDX a few weeks ago, and have many QSOs on 40m and some on 80m. I can receive on all band but transmit on 20 shows no power out, with the LED doing the slow flash transmit as per 40m and 80m. I’ve checked the continuity on all of the coils to the next component, so the enamel is cleaned of and the wire soldered to the trace, although I had to add a bodge wire for one of the T1 connections! And I’ve generally inspected and re-flowed anything looking suspect, everything ? Trawled the group and came across the diode test, below are my outputs using an oscilloscope. The outputs marked with “Vpp” show the voltage peak to peak, whereas the “V” are DC. ? Any ideas on what to check next? ? ?
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On Mon, Aug 21, 2023 at 1:03?PM <laycockc+GIO@...> wrote:
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开云体育Suspect L9 but really need to know DC levels on diode cathodes when in receive 20m. They change with band switch but not with TR switch.If correctly near 0 on D3 and D6 cathodes then L9 47uH may have invisibly failed to short with lost inductance/shorted turns which eats your RF. On other hand if stay at 5.13v then suspect open Q5. I managed to kill both with a single solder bridge so can be both. If static DC levels are switching sensibly then simple diagnostic is swap L9 in from a working filter channel. Good luck, -Nick VK2ZTY On 22 Aug 2023, at 03:03, laycockc+GIO@... wrote:
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The inductor L9 is pretty fragile and has been known to fail to open. If that happens, the PIN diodes in that LPF will fail to forward bias and transmit power will be nil. The link I posted earlier was for a QMX? that had an inductor failure in the analogous position on that radio. JZ KJ4A? On Mon, Aug 21, 2023, 3:44 PM Nick via <nrepin=[email protected]> wrote:
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Chris ,
I took another look at your voltage readings. The 5.13 volts PIN reverse bias voltage looks very low! It should be higher than that in receive mode, and much higher in transmit mode as the TX RF derived boost kicks in. Check over the voltage doubling bias circuit very carefully. What PIN? reverse bias voltage do you see in RX mode? It should be close to your power supply voltage .inus a diode drop. JZ KJ4A? |
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Hi Chris Make sure the input diode has been tested or replaced. May be open on forward bias. Assuming RF is getting into the LPF and choking is good at L9, RF must be being either shunted or attenuated in the LPF. Remove and inspect and/or maybe jumper the toroids to see effect on RF levels? ?Using low power supply and dummy load should be not much risk to finals. -Nick |
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Thanks again John and Nick, ? Managed to get some more PD time. I disconnected the cat on D3 and D6 to see if that told me anything but I don’t think it it did, results below, look like the only delta is D3 and D6 stay at 5v on TX, but I guess that’s because LPF3 is not disconnected. ? ?
? Still the big delta on TX remains. TBH I don’t really understanbd how the circute works, bit, sure but the whole soup to nuts is stilkl beyond me so I’m taking your advice and interpreting it as best I can. I’ve started looking at T1 to see what that tells me: ? ?
? Now I’m really confused!! ? |
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First, it is considered good form always to post your name and call at the end of a post.? Then, others can direct their posts to the correct person.? Also, when posting in response, direct the post to the person you are responding to.
To the original poster: The QDX has gone through 5 revisions of the circuit board.? The differences can impact the usefulness of the help from the forum.? Please post the board revision and the model to help us give you the best information. To answer your question on how the LPF selection works, there is a detailed description in the construction manual on page 73 for the Rev 5 board.? Others will be around the same page with the same heading.? You can get the correct manual from the QRP-Labs QDX web page: Basing the selection diodes forward in a simplified description connects the two leads like a switch.? Reverse bias opens the switch.? The caveat is that the RF peak voltage must not exceed the reverse bias voltage, or the diode will conduct.? ? For receive, that is in the millivolts, and the 5-volt supply is enough.? When transmitting, the voltage can get over 45 volts peak to peak or 23 volts peak.? That means the reverse voltage on the diode must be above 24 volts for safety.? There is no 24-volt DC supply in the QDX, so Hans developed a voltage doubler circuit for transmitting.? That doubler is D8 and D9 with capacitor C27.? This happens only when transmitting and does require a good RF output.? If there is no RF, The doubler does not work, and D7 provides the input voltage supplied to the QDX until the doubler kicks in. The supply to forward bias the LPF diodes is different for different boards.? The original board (Rev 1) used the input voltage through a MOSFET switch and then a resistor to bias the selected diodes forward.? The same circuit was used up to Rev 4 when Hans came up with using the processor to act as the controller for a switching buck regulator to reduce the power dissipation required by the series resistors.? There are other aspects of the new design that benefited the QDX operation.? This is an example of why it is important to list the board revision when asking for help. To address how to verify that the LPF selection is working, first, confirm in receive mode that only two of the diodes (the two that select the band) have the band (cathode) close to zero volts.? The other four should be at 5 volts.? That would verify that the selection process is working.? At that point, you should be able to receive and decode digital signals with the QDX. To test in transmit, first verify that there is DC over 15 volts (this would depend on the voltage supplied) on C27.? As in the receive selection test, verify that only two diodes are at zero volts DC and the others are close to the voltage measured on C27. There is a diagnosing help page on the QRP-Labs web pages: I? hope that the above helps.? Good luck in finding the issue. 73 Evan AC9TU |
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开云体育? Hi Chris, Whoa! Sounds far fetched but reported DC behaviour on D3 and D6 is inverted logicwise to what I would expect and is definitely the odd one out. Now I notice the same on your original post so not related to you now disconnecting cathodes.? Can you document/verify please the signals on X6 as you change band selection? Am traveling far from home in Ireland so can’t verify what should be but I think active low for each band? Could you also verify please that Q3, Q4 and Q5 have identical part markings? Kind regards de Nick VK2ZTY On 29 Aug 2023, at 4:18 pm, Evan Hand <elhandjr@...> wrote:
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Others have posted to Chris as the original poster so I will direct my apology there.
Chris, I needed to look at the posting message title to see that the board in question is a Rev 5.? That is an error on my part.? I apologize for my oversight.
Vs = Vinput to QDX less some component voltage drops When transmitting, the forward bias supply voltage is a pulse-width modulated voltage that maintains the configured bias current in the band configuration screen.? If the receive selection works, the transmit selection should work.? The selection is not changed during transmission.? The voltage to keep the unused filter diodes reversed biased increases as the transmitter's power increases.? Verify that the voltage doubler provides greater than the input voltage to the QDX while transmitting to keep the unused filter diodes reversed-biased.? The best way to verify is to measure the peak voltage at the input to the LPF input diode anode with an oscilloscope and verify that the voltage on C27 is greater than that voltage by at least a diode drop. The most common problem with failing to select the filter is a cold solder joint on one or both LPF toroids.? The next most likely is that the diodes are reversed.? I am an early adopter of the QDX, so I do not have the Rev 5 board to verify voltage.? The above is based on my Rev 1 QDX and schematic analysis. 73 Evan AC9TU |
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Hi Evan, no prob’s we are all human :-) ? Looking at C27 on transmit I noticed that my oscilloscope was reporting 5v whereas my multimeter was showing 50v! All the readings below are with the multimeter including a re-done D1-6 cat. Q3, Q4 and Q5 all look identical with SA printed on them X6, I did notice that it’s labelled LPF _0 1 2, which I assume correspond to LPF 1 2 3, that tickled my OCD :-) I’ve used the PCB labelling below. ? ?
? ? Chris 73 de M0IDE |
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