Tektronix TDS540 doesn't start up anymore

Those are the symptoms for bad electrolytics on virtually any reasonably modern piece of electronics which employ a switch mode power supply - as I think this item does. It's extremely common for switch mode power supplies to refuse to restart after they've been left 'off-power' for a couple of weeks whilst an owner is away on holiday, for instance. The only realistic way to check electrolytics for this kind of failure, is to run over them all with an ESR meter. This will quickly reveal any that are out of spec, without having to remove them. Suspect any that are located close to components which run hot, or show any signs of their tops bulging or splitting. Changed one just today in my dear old mum's VCR. That too was refusing to start up for several minutes from a full power off, and then when it finally did, the display was locked and corrupt, due to the noise on the rail from the failed filter cap.

Arfa

If that has the fluorescent type display, you may want to check the inverter for the hight voltage to the display.. if it's a LCD., it should have lamps and that also has an inverter that can cause the same problem..

But I would look first at the inverter circuit. Aging crimps inside of Electrolytic caps give for poor connection and generate ESR. Switching supplies have issues with this because you'll have in effect a soft response on the cap and it may not start or not generate the desired effect in over all performance of the circuit depending of where it's located.

It's not common to have ESR meters lying around, even though the actual construction of one is very simple and low part count.

An alternative would be if you have a square wave generator and a second scope lying around. Place a low value R from the output of the generator that fits with in the specs of the unit, like a 50 ohm for example. What you want to do is make a L network, R coming from the generator in series and the DUT (device under test) in this case a suspected Cap, to the other end of the R then to common. Attach a Scope across the Cap and monitor the wave being generated. What you're looking for is the base line of the square wave. Adjust the output frequency of the generator so that you can see a full charge on the cap before it reaches the right side of the ON duty cycle of the square wave.

When the duty cycle reverts to off, you should see a near perfect 0.0 volts. This is because the response time of the square wave is fast enough to cause the cap to create near perfect 0 ohm condition how ever, if there is internal issues with the cap, you'll see a non base line effect or even noise generated at the initial skew point of the wave..

years ago, I made up a test jig box that has a square wave generator in it and employs an analog meter along with a scope signal so that I could debug either way.. I used a bridge to detect the differential offset via 2 small caps to take a reading only at the falling edge of the square wave. This results would go into a sample and hold amp.

Have fun..

"

Thanks Arfa and Jamie. I'm going to get hands on an ESR meter or build one. I'll try your procedure on one of those caps once I'll get my oscilloscope back to life. I already feel better with the idea that fixing it might not be that difficult after all. And I'll probably change all those small electrolytic SMD caps as advised as I don't want to have to deal with leaking caps in the future, it seems quite a job to clean.

Thanks again,

David.

The new version of Bob Parker's ESR meter, called the "Blue" is now available from Anatek Corp if you're in the US. This is an updated version of the original, and retains all the same features. It is fully automatic, and a real value for money performer, especially if you take it in kit form. I recently did a build and review article on it for a mag here in the UK, and I can absolutely recommend it as an easy build, and excellent performer. You'll wonder what you ever did without one.

Arfa

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There's a tekscopes group on Y-a-h-o-o- that's very helpful