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Hello, I recently just fished putting my QMX Rev 4 kit together with the windings necessary for 12v input. As I was going along I verified I all continuity checks as instructed and verified no pads were bridged. Upon attempting to power on the device, I connected my 12v power supply to the barrel connector. The screen immediately lit up and I had black rectangles across the top half of the screen. A second or two later I started seeing smoke, heard a pop, and the screen went dark. Upon further inspection, I was able to discern that the Q108 MOSFET on Plug-in PCB #1 blistered and exploded. The L101 inductor nearby also looks a little singed but the enamel coating does not appear to be compromised. I do not see any other areas of the device that appear to have been compromised or damaged.

I was wondering if anyone may be able to assist with what actually went wrong here. I have ordered a new power supply board as well as a replacement MOSFET to try my hand and replacing the bad component.

My current theories on what went wrong are:

Austin,

If your screen lighted up and Q108 blew without you pressing the power-on button, it means you have one or more shorts going on.? That display tab can cause shorts in the power supply, but I'm not sure it would do all of what you described.? Other likely places for power supply shorts to happen on QMX is solder bridging of some sort on the power connectors, either on the main board or on the small SMPS board.? If you didn't trim the connector legs that get soldered to the top of the SMPS, they are dangerously close to some of the SMPS components where solder bridging can occur.? There are also other close-proximity areas that could possibly short when you assemble the display board to the main board, such as around the encoder bodies, and at the back of the power connector.

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From a circuit point of view, to cause what happened to you, you had a short (or less likely, a failed component) that bypassed the power switch, and another that either held the PWM_5V line high or somehow shorted Vin or 12V to the 5V bus.? Perhaps there is a single fault that could cause both of these.

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One possible scenario is this:? A solder bridge or solder flake shorted the Vin pin to the PWM_5V pin, which are adjacent pins in connector JP101 (and on the SMPS card).? This combined with either a) a short to another adjacent pin, PWR_HOLD, or b) something holding PWR_ON low, or c) a Vin short to 12V.?? Any one of those combinations would put the Vin voltage on the +12V rail, and turn on Q108 full time, such that it tries to pull the 5V rail to 12V.?? Then zener diode D108 starts conducting strongly to try and hold the voltage to 5V, burning Q108 and the coil (and probably also shorting D108).? Meanwhile the display turns on from the 5V supply.

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It shouldn't be too hard to trace down the culprit(s).? Before putting in new SMPS, do measure the resistance between Vcc and Gnd on the main board - it should be a few k-ohms.? If it measures in the low ohms, you have some sort of failure on the main board that you need to fix.

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And if possible, use a 250mA current limit on your power supply until you get a full successful power-on and startup.? This is highly recommended, along with using a lower voltage, like 7V for initial power-on testing.?? That would have saved you from burning the parts, and will save you in future tests from burning additional parts if you don't have the problem(s) fully fixed.

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Stan KC7XE

On Fri, Mar 28, 2025 at 11:00 AM, Austin K4HYD wrote:

It's still unclear why the MOSFET would blow in this case as it's rated for 20V

The MOSFET would blow from over-current, not over-voltage.? And high current will happen if +5V is somewhere shorted to ground, or Q108 is turned on all the time, which causes zener D108, which is trying to keep the voltage down to 5.6V to conduct very strongly and perhaps fuse shorted in doing so.? A 12V short to the 5V bus would also cause a lot of current to go through Q108, at least until D106 fails shorted.

From your photos, your soldering looks fine.?

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The fried transistor could be causing that continuity now.? Since you have ordered a new one, you could just remove the current one and see if the short goes away.? (It may be easier to cut the legs off first to remove the device, then un-solder the remaining parts of the legs.)

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The coil looks like it also took a lot of heat; it may be best to replace it.? You can test its continuity by probing the top of D106 and C107.? But even if it still has continuity, if there are internal wire shorts due to burned insulation, it would have a lower inductance and be less effective in filtering the power.? But it may be ok - you could also measure the inductance using the same probe points, if you have an LCR meter.

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It would be good to test D108 and see if it looks like it shorted internally, since it would also have been sinking lots of current.

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The real question, though, is "what caused this?".? I don't know.? As noted before,? it seems like one or more shorts had to be there.? I would still verify on the connector to the main board that none of the connector pins to the SMPS have any continuity to each other.? And look very carefully as you mate the control/display boards with the main board, to see if any metallic surfaces come in contact with each other.?? And next time you turn it on to test with new parts, make sure and have a 200-250mA current limit set to avoid breaking something again if there is a fault still present.

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Stan KC7XE

I am also trying to diagnose a fried (?) board, Rev 4, ordered and built in the last couple weeks. wound for 9V.

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As posted elsewhere, i had an issue with a powerboard, replaced it, and it worked fine for about 12 hours.? made some FT8QSOs (two new DXCC for me even!), and some CW contacts and RBN showing well into europe and across US.

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But then it all just suddenly stopped.? All terminal diagnostics are pretty much exactly as shown in the operation manual, the only issue being no power out on pressing T.? What i did discover was continuity between the center pin (radiator) of the BNC connector and ground. I removed the BNC connector but the issue turns out to be with the main board. With the BNC connector removed, there is no continuity between the female center and the outer housing.

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Any suggestions on where to look first?

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Thanks in advance.?

JD

You will always see DC continuity from the BNC connector to ground through T507.

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For no power output, start with the BS170's. I just went through this, frying the BS170's, IC503 and Q507.

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73,

Steve, N2IC

Hi Steve.? Thanks.

So even the CENTER of the BNC?? the middle, radiating contact of the BNC is connected to ground via T507?? MyBS170s look fine.? Is there a way to test them?? It makes sense that they would pop first, but i've no idea how to test them.??

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Is there a single spot, somewhere on this forum, where it lists key measurements to check when troubleshooting?? I must have made Google a million dollars in eyeball hits in my searches over the last week.

?TNX again!

JD

On Wed, Apr 2, 2025 at 08:29 PM, VA2OJD wrote:

the middle, radiating contact of the BNC is connected to ground via T507??

Yes, of course.? In most radios with an internal SWR meter.? But this is DC ground.? RF flies through the transformer almost untouched.

On Wed, Apr 2, 2025 at 08:29 PM, VA2OJD wrote:

Is there a single spot, somewhere on this forum, where it lists key measurements to check when troubleshooting?

Have you looked at this forum's wiki?? There is a topic specifically for the BS170s.? It has some helpful diagnostic guidance.? And you can search messages in this forum for BS170 and find a good number of entries, with different people being guided in their diagnostic efforts.

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Since your QMX was working for a good amount of time, a good possibility is that you fried your BS170s - most likely just them and not the driver and Q507.? This can occur, especially if you are running FT8 at more than 12.0V (for a 12V build) or 9.0V (for a 9V build).? Or if your QMX puts out more than 5W on a band at the correct voltage.? And especially if you combine over-voltage with some higher SWR on your antenna (probably more than 2:1), which can happen intermittently on wire antennas if the wind is blowing.? Doing any of the above over an extended time has risk of gradually beating your BS170s into failure.? That's why Hans has in big red print in his manuals to a) not run above 12V and b) not run above 5W.? And this is especially true with constant carrier modes like FT8, which add heat in the finals to the mix.? CW is more forgiving, because it is lower duty cycle.? At 5W the BS170s are pretty much up to their rated max, so going above any of the parameters puts more stress on them.? [This is true of their use in (tr)uSDX and other QRP radios, too.]

Stan KC7XE

Stan - that was super helpful.? Thank you for taking the time to type that out for me/other hams that want to learn by doing (or by destroying).?

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After i read the earlier post about the pin to T507 i assembled enough non-panicky energy to actually go and look at the traces in the assembly manual.? Lo, what did I see?? Indeed a direct connection to T507.? I was operating at 7V for a 9V QMX, but indeed over extended period.

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The support of you, Steve, and others who have answered my questions over the years since my first QCX build gives us electronically-uneducated persons the readiness to venture forth and enjoy this world of home-finished qrp radios.

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It is all very appreciated. Thank you!

JD