In article , M.Joshi.1krwy2
@news.diybanter.com says...
When using the built-in antenna, there are diagonal lines
across the screen for each channel. On certain channels it is not
as bad.

The strange thing is that, when an external antenna (Outdoors) is
connected, these diagonal lines appear to go away?

I assumed that this may be a problem with either the tuning or I.F
circuitry?

Not familiar with your particular model, but in general, TV's
have wide range AGC in the IF amplifier stage(s). With weak
signal from built-in antenna, the IF is probably operating at
or near full gain, and therefore most susceptible to picking
up noise or whatever. With stronger signal from external
antenna, IF gain is much reduced and therefore less prone
to interference. And in this day of cable almost everywhere,
many tuners and IF's are marginal performers to start with.
Make sure all grounds are well bonded and all shields in place,
and IF alignment may help or not.