Hello,
QMX? Ver. 2
12 volt build

After complete success with my low bands unit I built the hi band over Christmas.? Every connection was inspected with a loop during the build and everything was re-inspected before attempting to power up.? I used a current limiting supply set for 7volts and 250ma.? Upon pushing the left encoder I noticed immediate current limiting and the voltage pulled down to about 5.5 volts.? I let go of the encoder and current went back to zero.? There was no smoke or smell.

I've since been over the board several more times and can find no shorts I may have created.? The 3.3 volt rail on the main board now measures a direct short to ground.

Any pointers as to what I might check next would be greatly appreciated.

Thanks,
Ken, W4KAC

Hello Ken,

I suggest you separate all the boards and measure the resistance on the Vdd line of the main board.? If that is zero, you can do little other than check for shorts.? In my case, it was the MPU that shorted out.? The chip needs the QRP-Labs boot loader program to work, so replacing the chip is not yet an option, even if you can replace a large SMD device.

If the mainboard Vdd is not shorted, Check the other boards to see which one is the culprit.? I hope this is your case, as those are repairable.

73
Evan
AC9TU

Hello Evan, and thanks for the reply.? Yes, the test I mentioned above was on the main board alone.? There is a direct short from the vdd connection for the 3.3 volt switching supply board to ground.? Other than start pulling parts I don't see anything indicating a short so it may very well be the processor. I was afraid of that after reading some other threads here.

I had some hobby money in paypal so I've ordered another kit and will keep looking this one over.?

Thanks again and 72/73
Ken

开云体育

Seems like there are a lot of occasions where it turns out that the MPU is fried. I've encountered it what, two or three times?

The MPU had to be working to put in the boot loader; what could subsequently damage the MPU in a normal turn-on? Is it always over-voltage on VDD, or is it exposing some other connection to the MPU to over-voltage?

And I suppose importantly: are there design changes that would better protect the MPU? (Not critiquing Hans' design, he's done more than I would've!)

73, Paul -- AI7JR

Hello Ken,

I suggest you separate all the boards and measure the resistance on the Vdd line of the main board.? If that is zero, you can do little other than check for shorts.? In my case, it was the MPU that shorted out.? The chip needs the QRP-Labs boot loader program to work, so replacing the chip is not yet an option, even if you can replace a large SMD device.

If the mainboard Vdd is not shorted, Check the other boards to see which one is the culprit.? I hope this is your case, as those are repairable.

73
Evan
AC9TU

-- 
Paul -- AI7JR

Hello Paul,
I don’t know. ?ESD maybe? ? I work on a grounded mat but I don’t always wear the arm band.?

Thanks for the comment,
Ken

Could it be ESD perhaps?
I always use a static grounding wrist strap.

Sorry, I sent this before reading the earlier message saying much the same…

My experience with VDD shorted to GND on the main board is that it’s usually but not always in the MPU chip. ?You can determine that with a milliohm meter. ?Find where on the board the lowest resistance is from VDD to GND. ?Sometimes it’s a single pin on the MPU or sometimes it’s the supply lines to the MPU. ?The over voltage that causes this is often a mistake in building or a solder bridge or a probe slipping when measuring. ?If 5V or 12V gets put onto any of those MPU lines, it’s all finished. ?The MPU can be replaced. ?I’ve done it a couple of times. ?But other chips could be damaged as well. ?Also replacement MPUs are expensive and to date not available from Hans by themselves. ?I’ve always salvaged them from known working boards.

Jeff
W1NC

Thanks Jeff.? ? I'm still looking it over some each day trying to determine if I messed up somewhere, but so far, nothing.? The only voltage that has ever been applied was the first attempt to power it up.? ?I don't have a milliohm meter, so will keep looking for errors for now.? I may get it to you sometime if you're willing to take a look.

Thanks for the comments,
Ken

An alternative to a milliohm meter is to carefully unsolder and lift individual pins on ICs that connect to VDD. ?When the short goes away, you have your culprit. ?This is tedious and not easy. ?I use a drop of low temp solder on the pin then carefully lift with a tiny thin blade. ?Wear eye protection though!