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Hi Everyone,
New problem om my QDX.? Keep in mind, my QDX has been trouble free for the past year. The RF sweep on 15m is suddenly down to -50db.? All the other bands are at -1 db (10,11,17 ), bar 20m that is on -10 db but has always been there.? I checked and reflowed L12 - same results.? I then check for 2.5v on IC3 from pin 3 to 6 - no issues, all are correct. I checked continuity between 3-6 with pin 9 on IC3? -no problems.? Reflowed L3 and L8 - no difference.?
As far as I know 15m and 17m share the same LPF and 17m has no problems.? ?How is it even possible that 15m can be so deaf while it shares the same filters and bandpass as 17m.? ?
So at this point I am now lost.? Any help will be much appreciated.
73 de ZR6TM
Timo
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Hi Timo,
I would try a factory reset.? It may be that during transmission, the EEPROM was corrupted.
What version of firmware are you running?
73
Evan
AC9TU
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Thanks Evan
Did a factory reset, but no difference on the RF Sweep.? See the attached sweeps.? Really a mind-blowing problem.
73 de ZR6TM
Timo??
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Apologies Evan, Audio filter sweeps, not RF Sweep.
Timo
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Evan, my thinking is that it could be at this point where the CPU chooses with capacitance to use with which band, but I am really just spitballing here.? So perhaps a problem with c29 to c31.??
?
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Timo,
I believe you have a bandpass filter issue, not an LPF.? 15 meters shares an LPF with 17 meters but uses its own BPF.??Resolder L12, especially the tap that connects to pin 5 of IC3.? Also, check that you have 2.5 volts on pin 11 of IC5 when 15 meters is selected.??
73
Evan
AC9TU
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Timo,
I read your post after typing my reply to the prior post.? You have identified the correct area to search for the connection issue.? IC3 is a mux used to select the tap on L12 and the corresponding capacitor to make the BPF.? The filter number is the same as the X or Y number, where
X and Y 0 select filter 0
X and Y 1 select filter 1
X and Y 2 select filter 2
X and Y 3 select filter 3
As configured in the Band Configuration screen.
15 meters uses filter 1.
73
Evan
AC9TU
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Evan, I check pin 11 on IC5 and it only outputs 0.852v on 15 meters.? So, that is the problem.? Rest of the pins per band outputs 2.450v as expected.? So does this tell you that the bandpass filter for 15 meters has a problem, or IC5 has a problem.
I checked C29 to C31 and they seem to be ok.? L12 also seem to be ok if I do the continuity tests to IC 3 pin 3-6. What is your view.
73 de ZR6TM
Timo
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Timo,
I assume that you meant IC3 and not IC5.? I believe that IC3 has a problem.? The input for 15 meters is shorted, which is why you do not have a signal on 15 meters.? I suggest you replace IC3.? If there were a short on L12 then the other bands should be affected as well as 15 meters.
Note, IC5 is the driver for the transmitter.
73
Evan
AC9TU
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before replacing anything try turning off display background.
I have found 15 meter lpf display to give good result when display off and screwed up when display on.
If i had to guess 15 meter is subject to the frequency of the power boards depending on load?
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Has pin 11 been checked for a path to ground like a leaky C28?
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bearhawkpatrol,
Note that this is a QDX, not a QMX.? There is no display.
73
Evan
AC9TU
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Alan,
I would have expected an impact on all bands if there was an external short.? Since only 15 meters has the issue, it points me to the mux IC3.? The QDX has over a year of operation without issue, which is another pointer to an internal failure of IC3.? Still, it does not hurt to do a close inspection for a short on C28.
73
Evan
AC9TU
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Apologies Evan, I my head I was thinking IC4, but I wrote IC5.? So, IC4 pin 11 is at 0.854v if the QDX is on 15m.? As you are aware IC4 manages the IQ degree (Tayloe mix) splitting.? Therefore I would expect? Pin 11 to be at 2.450v if the QDX is in RX mode.? I checked pin 10 - 13 and they are at 2.450v apart from Pin 11.? The path is that the signal from L12 to pin 5 on IC3, is providing the signal to IC4 through the 180 degree split of T2.? Pin 11 on IC4 provides the 90 degree split which seem to be at 0.854v now .? With the 90 degree signal mostly missing between the 180, 270 and 0 degree signal, my QDX has that terrible dB loss on 15m.
Take note: this is just my theory on the problem.? ?
I think IC4 is the problem, but I could be wrong.?
73 de ZR6TM
Timo??
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Timo,
If there were a problem with IC4, then none of the bands should work.? Here is the model of the FST3253:
The input selected connects one of the X or Y pins with the corresponding output:
Xn => Z
Yn => W
A short on Y1 (pin 11) of IC3 will be connected through the other section of the mux to the Z pin 7 and then through T2 to IC4.
You can disconnect T2 from pin 7 of IC3 and see if you still have a short on pin 11 of IC3 when the 15-meter band is selected.? This would be a good test before you try to remove IC3.
73
Evan
AC9TU
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Evan,
You have a valid point here that IC4 is for every band and not just 15m.? Ok, checking pin 11 on IC3 and if that is a problem I will replace IC3 (I think it must be the third time I replace IC3 on this QDX).
73 ZR6TM
Timo??
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Timo,
What board version is it?? What firmware?? Have you done any mods to the QDX???
You should not have to be replacing IC3 that many times.? I would check that Q6 is switching correctly.? You may also want to make the QMX modifications that add a diode from the gate to the source and change C33 to .001uf and R33 to 470k.? Here are the instructions for the QMX:
This was not required for the QDX series, but it should not hurt.? The fix was for the low-band version of the QMX.
What is the power output for each of the bands?? I noted that one of my QDX had a higher output on 15 meters.
73
Evan
AC9TU
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Hi Evan
Board = V4
Firmware? = 1.10
mod - 22k resistor on the antenna socket.
I did the diode mod on my QMX and will do the same now on the QDX.
Just to check something
I presume you are talking about R13 and not R33 on the QDX, that needs to be changed to 470K ??
I run at 7v for a 9v QDX - output is around 1.8w on 15m.? The antenna SWR is 1.1.
73 ZR6TM
Timo
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Timo,
You are correct.? It should have been R13, not R33.? My eyes are getting old.
I have not seen that many reports of IC3 failures on the QDX.? There may be something else going on here.? As I stated in the prior post, making the changes to the QDX should not hurt the reception and could prevent future failures.
The other recommended mod is a 1n4148 diode across L14 to prevent the end of transmission spike.??
73
Evan
AC9TU
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Added the diode across Q6.? Finicky to miss all other possible shorts but can be done.
How would you add a diode across L14 - From the start of the winding to the end of the winding in total Evans ?? Will the cathode of the diode be on the end of the winding of L14, in other words, winding number 20 on high band ?
73 ZR6TM
Timo
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