Arduino UNO with CH340G not recognized

I bought two Arduino UNO boards with the CH340G chip. I installed the drivers on Windows 10. I turned on the first board, Windows recognized a COM3 port connected and I could load one program into the board with the Arduino IDE. The program ran correctly.
When I turned on the second board the ON LED lights and the L LED blink. May be this indicate the board is working fine. But the COM3 port did not appear in Windows.
I have nothing connected to the boards.
Could this be a defect in the CH340G chip or in the crystal oscillators? How can I test those?

But the COM3 port did not appear in Windows.

Did any port appear? The port number won't always be the same (though it usually will be if you only connect one board at a time).

Make sure the USB cable is completely plugged into both the Arduino board and the PC.

You may have a damaged/defective or charge-only USB cable. Try a different cable.

Either of the above can result in the power connections being made (and thus LED lights on the board), but no data connections (and thus no port for the board).

Usually boards come with the 'blink' sketch preloaded.

So the blinking L led is to be expected; your other board probably also did that but you might not have noticed.

Further see @pert's reply.