I have problems with the focus on my Westinghouse H-251 TV. Can not get the picture to focus properly, focus is best when the focus control is at maximum setting. Also the sound is weak and oscillates sometimes depending on the volume setting.
All paper capacitors and electrolytics are replaced and some bad tubes have been replaced also.
What can the problem be?

I would start by measuring the B+ to see if it is too low. It could be a weak LV rectifier. (5U4 or some such) I think this set is too old to use seleniums.

The B+ is OK. There are two 5U4G and they are tested and have very good emission.

You could have a bad resistor somewhere in the focus circuit.

It uses magnetic focus so as long as the coil is good it would probably have to be a voltage problem.

I would start at the focus control and check all the resistors in the circuit, chances are some of them have drifted up in value.

Do you have a schematic for it?

I have checked the resistors there and they are OK. I have a schematic for it, but it is in poor quality and therefore difficult to read.

Fredrik, I have a spare copy of what Sams published for this set. (they didn't offer a full packet, just schematics) I could mail it to you if you want. Send me a PM with your address if interested. Might take awhile to get to where you are, but I guess it took awhile for the Westinghouse to make it to!