开云体育

I recently finished my first build of the QMX and can't get it to power up properly. At first when I pressed the left encoder, the power consumption went up to 440 mA and then back down when the encoder was released. After some inspection I thought that I had blown Q101 because it was getting hot with the left encoder pressed. I replaced it and my consumption when down to 340 mA but it would still not power on.

?

Thinking it was a problem with the power board I ordered a new 3.3V power board and replaced it. I still get 340 mA when the left encoder is pressed. With a 7.0V supply I am getting the following voltages with the left encoder pressed.

?

+12V: 7.0V

LIN_REG_EN: 5.1V

VDD: 0.2V

IC101 VI: 5.0V

IC101 VO: 1.2V

IC101 GND: 0.5V

?

Would appreciate some thoughts if anyone has had a similar problem or might know what is going on

?

Thanks,

Jeremy Goates

KK7QVY

Jeremy,

?

I would test both power boards offline following QMX QMX_Plus Power supply test Process from the QRP Labs Wiki.

IC101 VI should be at least 5.3 V + 0.6 V (elevated GND of IC101). Your 5.0 V is not enough for 3.3 V output.

?

Without a running controller LIN_REG_EN is made by a voltage devider, 0.5 * "+12V".? Your 5.1 V is wrong. Please consult schematics page 1 to see where to check.

?

73 Ludwig

Sorry for my previous message (now deleted).?? Ludwig's value was correct, as always.

?

After removing the SMPS cards to test them, as Ludwig has recommended, also measure the resistance between Vcc and GND, and Vdd and GND on the main board, which should be a few k-ohms each.? And also measure Vcc to Vdd, and 12V to Vcc and Vdd - each of these should have a very high resistance, in the mega-ohm range.? You likely have a short or a failed component somewhere - and the first place to check is the voltage rails, because if that is where the short is, it can continue to cause additional failures.

?

For all further testing, until after a successful power-on is completed, please set a current? limit on your supply to about 250mA if possible.? Without a current limit, some types of issues can fuse your SMPS protection diodes or damage a main board component.? I hope your new 3.3V SMPS is still ok.? 440mA and 340mA are way too high, indicating something pulling down on the 3.3V power supply.

?

Stan KC7XE

?

?

?

On Thu, Mar 27, 2025 at 08:14 PM, Stan Dye wrote:

440mA and 340mA are way too high, indicating something pulling down on the 3.3V power supply.

Oops, I wrote too quickly again.? In your case it could also be something else.? A voltage very near your 7V input should be appearing at VI of IC101, rather than the 5V that you see.?? Check carefully the 12V input as it appears on the 3.3V SMPS.? Has it dropped to 5V at that point (being pulled down somewhere in the interconnect)?? Or is it dropping from 7V to 5V across Q102?? That measurement can give you a hint, if you don't find the error doing the independent SMPS tests.? I suspect (based on your measurement of LIN_REG_EN) that you will see it dropped to 5V at the input of Q102, which would mean that the 12V bus is not getting cleanly to your 3.3V SMPS via the connectors.

?

Good luck, Stan.