开云体育

Booted up the new QMX on my bench power supply, managed to flash firmware, hooked up a dummy load to test TX and forgot to increase current limit on the power supply, so it browned out on TX. Then it died, and I'm showing a short from 5V rail to ground that is not on the power supply daughter card, it's on the main PCB.... somewhere. Any tips for figuring out where it is (and why that would have happened) are appreciated.

I should add -

No current draw on initial powerup. Significant current is drawn while pressing left knob. Current goes back to zero after releasing left knob.

First rule out solder bridges and stray pieces of wire on your board. ?My experience in this kind of short on a QMX main board is that it’s from a failed chip. ?5V is supplied to the chips in the receive path (mostly), a few capacitors and the display. ?Remove the display first to rule that out. ?Then it’s a matter of elimination. ?Once you find the fault, then you have to ask why. Chips don’t fail without a reason.

I had the exact same initial boot sequence experience. After power cycling and pressing the left encoder button, the magic smoke was let out. I went through the first 2 steps of G5CTH Chris'? troubleshooting steps and determined that PS1 is damaged. I checked the main board interconnects from PS1 and sure enough I had a short to ground. Turns out I had a very fine [sic] solder bridge across C201 on the main board, it came off easily without the use of an iron. No more short to ground on pin 6 of JP101.?

It would appear that as a minimum Zener D106 on PS1 is damaged as a result of the short, maybe more. The QMX in its present state is able to respond to a power on key press and enter terminal applications mode.? So it is need of some repair on PS1 or replacement.?

Steve W6WU

The short is somewhere on the main board, verified by pulling out the PSUs and testing each board individually. I wish the PSU was the problem since it's easier to just order another power module than to troubleshoot a very dense main board with a bunch of very tedious labor put into it that I'd rather not repeat... As it is I'm going to borrow a thermal camera from a friend and see what heats up when I try to power up. I live pretty close to a digikey warehouse so I can get pretty quick shipping on replacements.

I'm willing to bet the failure went something like this -

* Bench supply goes into current limit, drops input voltage
* Duty cycle of buck regulators increase
* Bench supply comes out of current limit when TX ends, input voltage comes back up
* Known issue with CPU failing to respond to transients in input voltage fast enough to prevent rails from spiking
* Protection zeners fail open? I don't see any shorts in PSU boards
* Op amps are probably fine since they're all 30 volt parts, but the Tayloe detector, ADC, DAC, and LCD are all liable to have been killed by this.

I would propose the following modification to the power circuit to help prevent this issue:

https://photos.app.goo.gl/shkveUjM5gU6WNqP8

This would shut down the pwm in an overvoltage event much faster than the cpu can.

The ultimate solution would be to use an off the shelf buck converter IC with a frequency synchronization feature. An IC whose control loop runs as fast as its pwm will naturally have better impulse response than a CPU running a control loop at 1khz.

Update: Definitely a toasted zener on the 3.3V PSU board.

Hooked up 3.3 board to bench supply. High current draw, zener gets hot. Removed zener. Current draw now reasonable. Installed 3.3 board into motherboard. Connected JP101 pin 8 to to JP201 pin 8 to trick CPU into thinking 5v rail is healthy. Connected bench supply to JP101 pins 4 and 2. CPU seems happy, I see PWM on the PSU PWM pins. If anything powered by 3.3 failed it didn't fail short. Still waiting on my buddy with the thermal camera to see if I can ID what failed short on the 5V rail.