Strange Noise from Grundig TV

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

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

No probs there, as yet.

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

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

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

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.