Hi Amogh,
Thanks for your advice. I deleted the profiles ( two identical found !) , also deleted all entries of all WiFi routers in the Win 7 Registry. This step did not resolve the issue.
However I found the reason for the connection trouble and the solution is as follows:
In the Wireless Router being connected to (TP-LINK) , the Wireless Settings --> Channel was set to "AUTO" (which is automatically selecting 20MHz or 40MHz) .
In the Dell WLAN 1703 Device driver (Properties --> Advanced --> 802.11 n Bandwidth , the value was set to (ie the Atheros default is !!) "20MHz".
Once I set the value to option "20MHz / 40MHz Auto" , the connection sprang to life.
What a relief !!!
The matter is now completely resolved. Thank you for all of you who contributed.
Best regards Dzsino.