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I completed the 20-10 version a couple of days ago and on power-up the firmware loaded and all was working fine. It ran for over an hour while I went through menu settings, etc. I ran through all the diagnostics in terminal mode and all checked out, Tx test showed 2.5..4.? W on all bands.

Then I did a power-down, turned off supply (9V bench) and disconnected phones and keyer paddles to use the other radio for a sked.

When I tried to turn on the 20-10 QMX with an antenna to see if Rx working, it would not come on.

I have traced the pushbutton signal as working:
?- The power input to the 3.3V supply on the PCB2 pin 5 comes on while the button is pressed.
?- The power input also is detected at the junction of R206/R209 while the button is pressed.

The 3.3V supply does not receive an enable signal-it goes up from a few mV to .7V while the button is pressed. There is not 3.3V output.

There is not much more tracing I can think of. Has anyone else seen this sequence of completely working and then not?

Was the SWR protect circuit turned ON or OFF ? Might reload the firmware to factory default status and see if it comes ON.

73 Kees K5BCQ

The SWR protect was enabled, set to 3 during menu operation while it was working. The system does not even turn on to allow firmware loading when the forced mode is enabled with the jumper.

Troubleshooting update.
Observing that 3.3V linear supply was not enabled, and that the linear supply needs to come on before anything else happens, I tested the supply board alone by connecting the battery supply to the '+12' pin and 'GND' pins on the 2X4 socket on the board, leaving the enable open the 3.3V output appears on its pin. The enable signal taken to GND turns the 3.3V regulator OFF.
Next, I plugged the 3.3V board in via a 2X4 socket with long pins so that I could bend up the enable pin to leave it open, then pressing the control panel button turns on the front section of the other supply board to send the battery voltage to the 3.3V linear regulator. Still did not work. Then I noticed that the input current from the battery jumped to about 500+ mA as long as the button is pressed. Ohmmeter indicates approx 0.6 ohm between Vdd and GND. So it looks like the orignal failure was some damaged component on the Vdd bus caused by a turn-off transient.
I examined a number of components with a sensitive DVM directly across the pads of each component, hoping to find one that showed a larger voltage indicating that it was the location of the short. First tried across supply chokes L201 and L202 (no voltage rise with supply ON). Then several other of the many bypass caps, IC directly connected to Vdd. Not really successful. Nevertheless, I removed C501, then IC501, IC502 but Vdd still shorted.

Which power supply did you use during the last switch off from normal work?

On Sat, Feb 17, 2024 at 02:57 AM, @Ludwig_DH8WN wrote:

Which power supply did you use during the last switch off from normal work?

?When it was working I was using a linear-regulator, 3A, bench power supply at 9V, same as I use for my low-band QMX.

Did you switch off the QMX or the power supply?
Could you check the voltage from your power supply during a switch on and off using a oscilloscope and a resistor as a replacement of the QMX?
Switch in a first test the load and in a next test the supply.
In a next step if all was ok you could use your low band QMX as a load.
Did you read ?

On Sun, Feb 18, 2024 at 05:50 AM, @Ludwig_DH8WN wrote:

Did you switch off the QMX or the power supply?

I switched off the QMX first, then the power supply. I have (almost) always followed this order with the low-band QMX. The one time I did not do that with the low-band QMX, it showed no Ill effect.

Before I used this bench supply with the low-band QMX, I had been using a pair of 18650 Li-ion cells. So, when I got tired of recharging them every 3 or 4 days, I pressed my old bench supply into service and? before turning it on to the QMX for the first time, I had checked it for transients, noise, etc. I did not see any load-change transients for around 1A change from no-load to 1A. I do see a turn-off transient when it's unloaded, about +/- 1V oscillatory transient that lasts about 1 msec.

Could you check the voltage from your power supply during a switch on and off using a oscilloscope and a resistor as a replacement of the QMX?
Switch in a first test the load and in a next test the supply.
In a next step if all was ok you could use your low band QMX as a load.
Did you read ?

I checked the supply again, and there is no load-change transient turning the low-band QMX on or off. If there isn't one with 1A load shift, I did not expect to see one with a 200 mA transient from the QMX.

I had not read the post you refer to, but I don't think my situation is comparable. In any case, D108 is not shorted.