On Thu, Sep 14, 2023, at 07:12 AM, Jerry wrote:

Which Final FET Fails?

My low band QDX has just failed for the fourth time. ?Each time only Q9 has failed (shorted D-S). ?No other parts have been replaced. ??

My high band QDX also failed once, also Q9. ??

Both are built as 9 volt version, power is via a linear LM317 regulator with 1 amp current limiting.

Any ideas why this one part is more stressed than others?

I have just added 1N4148 diodes as in the QMX schematic.?

Jerry,

The diodes in the QMX schematic are Zeners, NOT 1n4148.? Also, the single 1n4148 for the QDX protection is across L14, the RF choke that feeds power to the finals.? My simulations have shown that the Zeners OR the commutating diode across L14 are good choices.? You do not need both.? The simpler solution is John's commutating diode.? The commutating diode could be a 1n4148.? I have only bench-tested the commutating diode.

73
Evan
AC9TU

John Z
Thanks for your response. Looks like steady state SWR of 3 will heat the BS170s to their PD limit, so I assume?SWR <2 is OK.

Transients from an auto tuner that can be as high as SWR of 10 will cause transient power dissipation significantly higher than the allowable dissipation.? Unfortunately we don't have sufficient thermal data on the BS170 to understand transient thermal response at the silicon chip in order to determine allowable SWR Transient time product.??

Bottom line is?Don't?Use an Auto Tuner without reducing the transient at the QMX.? Reduced power or attenuation between QMX and tuner.? ?

Matched antenna sounds good, but I (and a lot of others) use temporary antennas in portable settings. I just activated in a national park yesterday and there was no way to put up a matched antenna. Even if you have a 'matched' antenna in one place, who knows what it'll be like at another. And even if it's perfectly matched in a fixed location, what about the day one end falls down the night before?

So I still think a manual tuner with an absorptive bridge you can switch in/out is the way to go unless the radio has SWR protection built in. (QMX down the road?) That will take care or whatever the mechanism is for finals failure in QxX radios. ATUs just seem to be a good way to fry the finals.?
--
73, Dan? NM3A

On Thu, Sep 14, 2023, at 02:12 AM, John Farmer VK7JB wrote:

Hi Kirk and Evan,

I’ve ordered some replacement IC5’s and am waiting for them to arrive down under. ?This weekend, I’ll put the oscilloscope onto the gates of the BS120s while the old IC5 is still in place and see what I find. ?I’ll also see if the high current state persists with the (probably) damaged IC5 in place. ?I didn’t notice IC2 getting hot. ?I’d better check that’s still doing its job.

73,
John
VK7JB

John,

If you decide to test IC5, I would strongly suggest that you remove the BS170s and reduce the supplied voltage to 7 volts.? While trying to do that diagnostic, I destroyed the processor on a failed QDX that I got from another Ham.? I did not remove the BS170s when I should have.?

The safe route is to remove all 4 BS170s and replace IC5, then do the IC5 testing.? After removing the BS170s, you can verify that IC5 works as expected (0 volts on the BS170 gates when receiving and 5 volts square wave-2.5 volts measured with DMM).? Also, limit the current supplied to 200ma if you have a current limiting supply.? Without the BS170s, the QDX will not need a lot of current.

Suggestions from experience ;-)
73
Evan
AC9TU

It is very tempting to use RD-16 transistors, but how to create the necessary bias on the gates without complicating the circuit? With a direct replacement as for IRF-510, the RD-16 transistors will not open.
--
73, Oleh, UY2ZA

"I have just added 1N4148 diodes as in the QMX schematic."

Wut?

JZ

Which Final FET Fails?

My low band QDX has just failed for the fourth time. ?Each time only Q9 has failed (shorted D-S). ?No other parts have been replaced. ??

My high band QDX also failed once, also Q9. ??

Both are built as 9 volt version, power is via a linear LM317 regulator with 1 amp current limiting.

Any ideas why this one part is more stressed than others?

I have just added 1N4148 diodes as in the QMX schematic.?

Albert, good on you for using the QRP Labs rigs for this. I suggested as much a couple of days ago in a LICW post where the OP was asking for ideas for a code oscillator. Why bother when you can build an entire rig for $55 and get a great practice oscillator in the bargain? I saw some weirdness early on with my QMX keyer, but not since, and I don't recall specifically what it was.

Regarding speed, though, I'd recommend trying to practice at 20 WPM. If you get good at 12, you will eventually hit the wall trying to get past it since slower speeds allow you to count dits and dahs, whereas 20WPM forces you to learn the rhythm of the characters, and eventually words. Just leave lots of spacing early on, and you'll start locking in the characters. It might seem impossible, but stepping from slower to faster speeds is actually far more difficult.

Bad usb lead. Now working.

Hello, When I plug the board in to my win10 pc and go to device manager I can't see it in the list of com ports, also the board gets very hot. Any ideas gratefully received!

Regards,

Roger.??

And one more to change the subject line and start a whole new thread.?


Andre

Dude,? I'm simply answering your concerns and addressing what you said in black and white.

If you can't support what you, yourself, said - just say so.? Actually, I think you just did.



?

To answer my own question I bought another QMX kit and built it with the yellow L513 toroid wound as shown below. Of course this time around all the capacitors are in the right spots too. Result: with 12.0 volts applied an 5 honest watts is produced on 80, 60, 40, and 20 meters. Oddly only 30 meters is the outlier at 4.2 watts. What did you say? Am I going buy another QMX and wind the transformer twisty style? Uh, no :) 73, Sandy KB3EOF

Do you also have a scale that measures in milligrams?? I don't know whether it's a true need, but it might be interesting to know how much material was removed by weight.

Funny you should ask.? My lab used to weight filters after sampling for total suspended particulates in the air, years ago before contracting that stuff out.

That's a U.S. penny and the Mint says that should be 2.50 grams....? The scale hasn't been calibrated in years though we still have a weight set around here someplace. It'd do OK for before & after.

开云体育

That's great to hear Howard. Enjoy.

On 14/09/2023 07:12, John Farmer VK7JB via groups.io wrote:

I’ll put the oscilloscope onto the gates of the BS120s while the old IC5 is still in place and see what I find

John,

Just be aware that in at least one case, I think a QCX, the Si5351 and processor were damaged by 12 volts flowing back through the driver IC from a BS170 with, presumably a drain/gate short.
As Evan says testing with a likely damaged part still present is somewhat risky even if the 12V is not there.

73 Alan G4ZFQ

I think you need to learn how to not get angry about words written by strangers on the internet.

I'm open to having a technical discussion, and I apologize for not noticing the details of your construction. I will not be engaging in whatever pissing match you seem to be wanting to start.

If you really need research on this, I have access to a vertical mill and a serviceable micrometer...