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Let the smoke out of the QDX.? I had a problem with intermittent transmit on 20 & 30, "touched up" some solder joints, ended up unknowingly bridging gate to either source or drain with a solder blob on one of the final transistors, and poof.??

All finals are testing short (source to drain) with DMM.? Q5 is visibly toasty.? Before I put in a parts order, what else can / should I test?? Just a DMM, no scope here.

Thanks,
Jason KB1ZOK

On 29/07/2022 15:38, jasongilroy@... wrote:

Q5 is visibly toasty.

Jason

Q5 or IC5?

If you do mean IC5 it would be an idea to confirm that IC11 is operating. We can give ideas on how to do that.

73 Alan G4ZFQ

Jason

Q5 or IC5?

If you do mean IC5 it would be an idea to confirm that IC11 is operating. We can give ideas on how to do that.

Q5.? Note that I found the solder blob after the smoke, and it's just my assumption that it's the root of these issues.? So if Q5 smoking is inconsistent with that, it's certainly possible I've got another issue somewhere.

Hi Jason,

To build on Alan's question, do you mean IC5 and Q7?? When I worked on a QDX rev 3 with finals blown, Q7 was shorted.? IC5 was not always destroyed.? When IC5 was damaged, you could find a minor blemish on the surface of the IC caused by the internals heating up when it failed.?

My practice is to remove the BS170s, inspect IC5, test Q7, and get the receiver working before installing the new BS170s.? Then verify that the gate pads are getting the drive from IC5 when testing using the terminal program.? This is best done with an oscilloscope rated for 20MHz or better.? A DC voltage check of 2.5volts on the BS170 gate pads (all 4) indicates that iC5 is working if you do not have an oscilloscope.? The last step is to install the new BS170 finals.

FWIW
73
Evan
AC9TU

To build on Alan's question, do you mean IC5 and Q7?? When I worked on a QDX rev 3 with finals blown, Q7 was shorted.? IC5 was not always destroyed.? When IC5 was damaged, you could find a minor blemish on the surface of the IC caused by the internals heating up when it failed.?

I tested Q7 for short from source to drain, it is not testing short.? If there is another test I should run for Q7 let me know.

Q5 is visibly scorched / blemished at the drain pin.? It is also not testing short from source to drain.??

I have not run any test on IC5, but it is not visibly blemished.

Thanks,
Jason KB1ZOK

Jason,

You should also test Q7 from drain to gate.? This shows up as a short between the power input and ground.

This is the first instance of Q5 failure that I have seen or read about on this board.? I would inspect R2, R4, and R9 for damage.

Here are 2 pictures of failed IC5.


73
Evan
AC9TU

Jason,

Further thoughts:

I would verify that the QDX will receive using around 150ma of current.? Then use the terminal program in RF or Audio sweep mode to select the band to confirm that the diode LPF filter selection is working.? The two diodes that select the LPF filter banded side (cathode) should be close to zero, and the other four not selected are at 5volts or so.? This should verify that Q5 is working correctly.

I would do the above before I install the 4 BS170s.

73
Evan
AC9TU

You should also test Q7 from drain to gate.? This shows up as a short between the power input and ground.

Some additional info: R2, R4, R9 all test reasonably close to their marked values (in circuit, of course.)

Q7 drain - gate = 2M ohm
Q7 source - gate = 10.5K ohm
Q5 drain - gate = 1K ohm
Q5 source - gate = 125 ohm

Inspected IC5 under loupe and nothing visibly concerning there.? Will run tests w/terminal program later.

Appreciate the help!

Jason KB1ZOK

Further info:

BS170s removed.? QDX only drawing 1-2 mA (NOT 100-200!) in receive.??

I measure ~8.8VDC on the banded side of the diode at each filter regardless of which band I have selected.

Thanks,
Jason KB1ZOK

If the current is that low in receive, then IC2 has most likely failed open.? Did you test to see if there are?5volts on the output of IC2?

Yes, I see 5V on the output of IC2.

Jason KB1ZOK

Jason,

The next step would be to verify IC1.? It should have 5volts on the input and 3.3volts on the output.? A correct input and no output would mean a bad IC1.? I am using the fact that the current is low, so it would not be a shorted 3.3volt rail.? The 3.3volt trace could have a break before getting to the microcontroller IC8.

Beyond IC1 or an open trace, I would then suspect something is wrong with the microcontroller.?

The above are suggestions.
73
Evan
AC9TU

The next step would be to verify IC1.? It should have 5volts on the input and 3.3volts on the output.? A correct input and no output would mean a bad IC1.? I am using the fact that the current is low, so it would not be a shorted 3.3volt rail.? The 3.3volt trace could have a break before getting to the microcontroller IC8.

Beyond IC1 or an open trace, I would then suspect something is wrong with the microcontroller.?

Thanks for the suggestions.? It sounds like I am hosed.? IC1 is showing 5V input and 3.3V output.? I didn't dare to try to measure voltage on IC8, but I do have continuity from the 3.3V rail to a handful of pins on IC8.? I also checked for 3.3V at IC11 and it is present.

Jason KB1ZOK

Jason,

Have you tried to connect to the terminal program on a PC?? It would be worth a quick test to see if the QDX is responding to the USB poll at startup.

Sorry to read the data so far.? On the surface, it does look hosed.? Maybe Hans or someone from QRP-Labs may have other suggestions.

73
Evan
AC9TU

Have you tried to connect to the terminal program on a PC?? It would be worth a quick test to see if the QDX is responding to the USB poll at startup.

The QDX responds to CAT control via both the terminal program and WSJTX.? LED lights as expected (including the "firmware update mode" when disconnect / apply power quickly.)

Audio filter sweeps are extremely bad on all bands.??

20m USB -41db, LSB between -60 and -80
30m USB -48db, LSB between -60 and -80
40m USB -56db, LSB between -56 and -90
60m USB -54db, LSB between -60 and -80
80m USB and LSB basically on top of one another between -60 and -80

Thanks
Jason KB1ZOK

BS170s removed.? QDX only drawing 1-2 mA (NOT 100-200!) in receive.??

I previously tested this using the 10A setting on my multimeter as the uA/mA setting wasn't working.? I found that the low-current fuse was blown, replaced it, and retested using the lower current measurement setting.? I am now seeing about 200mA drawn on receive.? Does this change the calculus at all?? Any hope where once there was none?

J