Very Strange Antenna Problem?
 

Hello,

I am trying to diagnose a problem with a 14" Grundig CUC7303 television. 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?

Hi!

I didn't see two postings...

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

It is possible that your TV set generates enough interference internally to
cause it to show up on the screen as you watch it. The interference could be
generated by something else in the same room or on the same electrical
circuit as the TV. Have you tried taking the set to another room (even
outdoors would be OK for a test) or a different electrical circuit?

The interference going away when an external antenna is hooked up suggests
that something nearby the TV is actually the culprit.

William

"William R. Walsh" wrote in
message news:rd9Sd.33058$tl3.23595@attbi_s02...
Hi!



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

With respect; why don't you leave the external antenna connected?
Sounds like a signal strength issue to me.

Pete

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.

"M.Joshi" bravely wrote to "All" (20 Feb 05 18:08:50)
--- on the heady topic of "Very Strange Antenna Problem?"

M. From: M.Joshi
M. Xref: aeinews sci.electronics.repair:11029

M. I am trying to diagnose a problem with a 14" Grundig CUC7303
M. television. When using the built-in antenna, there are diagonal lines
M. across the screen for each channel. On certain channels it is not as
M. bad.

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

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


M.Joshi,

It is possible the TV's built-in antenna is picking up stray RF from
the TV's own microcontroller. I have a Sansui TV which was bad for
this. I had to isolate one particularly noisy output line and add
little series resistance to limit the risetime. It was a little tricky
because it was the line to turn on/off the psu. Too much resistance
and it wouldn't turn off and too little and the rfi increased a lot.

However, many compact fluorescent lamps generate a non-negligable
amount of RFI. If you have one of these near the TV try replacing it
with an ordinary filament type.

Good luck,

A*s*i*m*o*v

.... Digital circuits are made from analog parts.

Thank you to everyone who has replied so far with possible suggestions.

Rubbishrat - I am repairing this T.V for someone else so they would be using it with the built-in antenna.

I don't if this is any help but, the original problem with the television was power related (Blown fuse, 1N4007 bridge diode S/C and MJF18001 als S/C). I purchased a reliability kit for this common power supply problem for the chassis (CUC7303) which includes replacement parts for the diode bridge, a BUT11A instead of the MJF84001, some zener diodes, electrolytic capacitors and resistors.

Could one of the replacement parts indirectly be related to the diagonal lines problem?

Thanks.

In article , M.Joshi.1ktrm3
@news.diybanter.com says...
Could one of the replacement parts indirectly be related to the
diagonal lines problem?

A long shot, but did any little ferrite beads come for the
transistor legs? Sometimes switchers can have little
parasitic oscillation chirps that radiate.

I think there may be a ferrite bead near the LOPT but that was not changed. I will check though.

Thanks.