Leaky Electolytics in Philips 'scope

I should have said the caps I refer to are the small, low voltage ones on the signal boards, NOT the large ones in the PSU section.

Unlikely caps problem. Hard (ie black stained)quickly blown fuses or slowish soft blown? How do the fuses fare if you try running off a variac at say 60 percent mains?

T-rated fuses. In fact I was running off a Variac when the replacement fuse blew at only about 80V! I've got one of the PSU covers off now and have identified a suspect component. It's some sort of capacitor with the following markings:

and it has a fine web of cracks over its encapsulation. I had one of these go short-circuit once before in a microscope power supply and that was the cause of it blowing fuses too. I don't understand the markings on the component, though. IIRC, "fo" refers to the self-resonant frequency and I don't see what purpose there would be in that at 50hz mains where it's located in that part of the device pre-PSU!

I suspect you are correct. This is a brand new meter I'm entirely unfamiliar with, first time I've used it, and the readout messages aren't terribly grammatical. But I've moved on now and narrowed the fault down to the pre-PSU area. There's one of those aluminium cased mains filters next inline down from the 2A fuse compartment and it's measuring only 1ohm across it! However, measuring the resistance looking into the scope circuitry after disconnecting this results in an open circuit reading, so replacing the filter alone will not fix the problem. Probably the filter just blew and took out something further down the line. More investigation required....

Cursitor Doom schrieb:

Search for "X capacitor", e. g.

here:

HTH

Reinhard

Simple experiment.

Depending on the circuit, remove it and see if the fuse still blows.

When fuses are blowing, that is not a good idea. You often melt the transformer or something else in adition to what blows.

Fuses are often rated at somewhat above the normal current draw. With a very bad overload , they will blow quickly enough, but at just a slight overload, they will not blow and often the transformer or other components will also overheat and go bad.

For simple circuits like lighting circuits or where there is lots of big heavy duty components, I have used a light bulb in place of a fuse. Just sort of use a bulb that has a low wattage compaired to the normal current draw.

Thanks for that. Interesting, but it still doesn't answer what the relevance of the "fo=3Mhz" parameter is. Neither does the Wiki page on the subject!

Well, if that makes you nervous, run it at much lower current. Only takes a couple of watts to make something warm enough to detect by touch.

Had a laptop I couldn't fix because the power supply wouldn't stay up. had to put a DC supply downstream of the inductors and stuffed an amp into the board. Took a millivoltmeter to trace the short to a SMT cap under a heat sink. Was really shorted, didn't get warm at all.

Interesting point there I'd not considered before: if something's

Yep, in this case, I had to move the injection point twice because the isolation inductors in the path were getting too hot for comfort.

That's one of the problems fixing the old Tektronix scopes. Those tantalum caps are REALLY shorted. Of course, the one that fails most often requires removing the backplane just to get at something to test.

Such caps across the AC supply act as an EMI filter so their high frequency characteristics are important.

Above 3MHz, the impedance of that cap will begin rising and its effectiveness lost. A lower value cap will have a higher resonant frequency.

Allow about 15nH inductance and you will easily see how he game works.

... Phil

Taking a resistance measurement into a circuit can lead to abnormally low o r high resistance readings depending on the dmm used, and sometimes you can recheck them and get totally different readings. I would try feeding fuse d AC into it past the disconnected filter and see what happens.

Do'h! You're right of course; a massive oversight on my part. I don't have any schematics as yet; I've tried searching and the only hit I got was some French forum where you have to join to get anything. I'm considering it...

Excellent suggestion - but with a fuse in line too, naturally. I'll give it a go and report back later.

Thanks for that. I'll try simulating it later. And this is for what purpose exactly? To prevent noise on the mains from other appliances getting into the scope, or to prevent noise from the scope getting into the mains?

Turns out to be 1M ohm or thereabouts now I've remeasured it with a decent DVM.

Have now done this and it isn't blowing fuses any more. However, I'm up to 100V on the variac and no sign of life from the scope yet (230V main here). Unfortunately the PSU section is sandwiched in the middle of other boards, so probing for voltages within it is not possible without removal (which looks a right PITA). I'll have to have a good think about how to proceed further.

It's a class X mains filter capacitor. The appearance you describe (mesh of cracks) is common when they fail. Usually they go with a bang and a lot of smoke.

It will be ok to test the oscilloscope with out it in circuit to see if it is the cause of the blowing fuses, but you should replace it eventually. Note again this is class X, and should only be replaced with another class X.

The power board is very easy to remove, a couple of screws and plugs and it pulls out the rear of the scope. I suggest you get the service manual. Philips power supplies all have dried out axial caps and won't start up, you wont get anywhere without replacing them. That's usually all you need to do, they are well built apart from that.

How is 115/230 volts mains selected? Is it in the line filter/fuse/IEC connector housing? Or...?

Of course when bypassing the line filter you're inserting your mains voltage to the proper transformer taps...?