I have a Sony Trinitron Color TV - Model no. KV-8AD11
The picture tube kept getting smaller and smaller until now it is just a single line. Occasionally (or if I tap the top robustly) it will expand back to normal. Any ideas?

Dave

Sounds like something is overheating.

Don't know the product, but when I used to repair TV 90% of the time it was replacing a valve (tube). Occasionally a resistor would change value and replacing solved the problem.

For starters you would need a schematic, be able to identify the circuit and trace it from there.

How old is the set? How much did it cost?

To be honest electronics is so cheap, if this isn't something easy and quickly fixed. Bin it and buy new (Unless your local TV repair person is cheap)

This will most likely be dry (cracked solder joints) on the frame output IC (if horizontal line) and line output stage if vertical line) Should be cheap and easy to repair. If attempting to repair yourself ensure the set is disconnected from the power as these are high voltage circuits.

Regards

Totally agree with kdb. Sony doesn't use tubes in any of their products and never have. TV has vertical problem. By model number you gave this is an 8 inch TV that is very easy to work on, but unless you have electronic knowledge you probably won't be able to identify vertical circuit of TV and find loose connection. This model uses a IC vertical output.
LA------ or something like that. About the size of a quarter.
Good luck