I've been working on almost exactly the same thing (an Avios IP Power 9258)

PS should be:

socket = new TCPSocket(false);
socket.onConnect = function(){
socket.write("GET /Set.cmd?CMD=SetPower+p61=1t HTTP/1.0 \r\n\r\n");
}

socket.connect('192.168.1.3', 80, 3000);

Just check that the port number is the same (80 in the above case) / if you have to specify it.

Also, should there definitely be a t after 1 in 'SetPower +p61=1t' ? (not needed on my device otherwise the syntax is identical.)

Also in my case I have to use the authentication of login and password - if you do too you'll need to add....:

Authorization: Basic YWRtaW46MTIzNDU2Nzg=

....before the HTTP/1.0 business... Above authorisation is converted to Base64 and is for default login/password.

If you have changed your login/password you will need to add your own base64 conversion of your login/password - google a site that does it for you.

Cheers

Zed