Strange problem with a colour tv I have here. A very loud 50Hz-ish
sounding buzzing noise has been causing severe interference to low
freqs, LW MW radio and conventional telephone which
I traced its source back to the TV. Noise only appears when set is
connected to aerial/scart which acts as a radiating aerial. TV works
perfectly, but with a 'scope looking at both the inner and outer of
the coax socket, there appears a significant waveform. It doesn't
alter with picture changes so I presume it's not line timebase. Any
ideas? It's a Grundig T70-1020.

Richard
UK

"radio10" wrote in message
oups.com...
Strange problem with a colour tv I have here. A very loud 50Hz-ish
sounding buzzing noise has been causing severe interference to low
freqs, LW MW radio and conventional telephone which
I traced its source back to the TV. Noise only appears when set is
connected to aerial/scart which acts as a radiating aerial. TV works
perfectly, but with a 'scope looking at both the inner and outer of
the coax socket, there appears a significant waveform. It doesn't
alter with picture changes so I presume it's not line timebase. Any
ideas? It's a Grundig T70-1020.

Richard
UK


Usually, anything nasty being radiated from a TV set these days, is
switching noise from the PSU. You may have a cap starting to go bad on one
of the rails. It is very common for electrolytics to go poor ESR in
switching power supplies, and the effect of this is not always obvious in
the performance of the equipment, until they fail completely. Is there
definitely not even the slightest herringbone interference on the picture ?
Sometimes, this can be the first sign of a failing cap, and is most
obviously visible on large single colour areas of the pic. Another clue can
be a slight reluctance of the set to come out of standby, or to come up into
standby from a cold start.

Be careful 'scoping around looking for a waveform responsible for the
trouble. A grounded 'scope plus a switcher connected directly to the mains,
can equal a big bang ... The set should be on an isolation tranny when you
conduct these tests, and bear in mind that the set's general ground (
metalwork, tuner can etc ) is NOT psu primary ground, which is the " - " of
the bridge or its smoothing cap.

Arfa

Is there
definitely not even the slightest herringbone interference on the picture ?

...yes, there's a narrow horizontal noise bar slowly wandering up and
down the picture.


Sometimes, this can be the first sign of a failing cap, and is most
obviously visible on large single colour areas of the pic. Another clue can
be a slight reluctance of the set to come out of standby, or to come up into
standby from a cold start.


No probs there, as yet.


Be careful 'scoping around looking for a waveform responsible for the
trouble. A grounded 'scope plus a switcher connected directly to the mains,
can equal a big bang ... The set should be on an isolation tranny when you
conduct these tests, and bear in mind that the set's general ground (
metalwork, tuner can etc ) is NOT psu primary ground, which is the " - " of
the bridge or its smoothing cap.

Yes, thanks for the safety reminder. My scope is not earthed.

Thanks, Richard

On 10 Jun 2006 05:01:39 -0700, "radio10"
put finger to keyboard and composed:

Is there
definitely not even the slightest herringbone interference on the picture ?

..yes, there's a narrow horizontal noise bar slowly wandering up and
down the picture.

Looks like mains ripple. I'd check the main filter capacitor on the
primary side of the switching transformer. It would have to be pretty
bad, though, for ripple to make it across to the secondary side.

- Franc Zabkar
--
Please remove one 'i' from my address when replying by email.

Franc Zabkar wrote:
On 10 Jun 2006 05:01:39 -0700, "radio10"
put finger to keyboard and composed:

Is there
definitely not even the slightest herringbone interference on the picture ?

..yes, there's a narrow horizontal noise bar slowly wandering up and
down the picture.

Looks like mains ripple. I'd check the main filter capacitor on the
primary side of the switching transformer. It would have to be pretty
bad, though, for ripple to make it across to the secondary side.

- Franc Zabkar
--
Please remove one 'i' from my address when replying by email.



Fault now found. As you guys suggested, it was in the SMPSU. I
discovered a 0.1 400v decoupling C sitting at about 30 degrees to
horizontal. You've guessed it, one pin was disconnected. Very strange
as the solder pad it was supposed to have been fixed to looked good.
One quick resolder and, presto, end of problem. Thanks guys.