Scope keeps blowing fuses

Did the caps fail Open or Short?

Peter Wieck Melrose Park, PA

Did you do any resistance checks like for a shorted rectifier(s) or power transistor like Q1050? FWIW, installing a new fuse and waiting for smoke is a terrible idea.

I can't see enough detail in that diagram to be sure of what every component is, but as a general principle my first suspects would be any suppression capacitors across the incoming mains; they always seem to be the first to go and they invariably fail short-ciircuit.

If the fuse is glass bodied with a simple wire element, the state of it after it has blown will give you some idea of the type of fault. A dead short will blacken the inside of the glass (and may break it), a substantial overload will blow the wire to bits and leave blobs of metal scattered around inside the glass, a prolonged gentle overload will leave most of the wire intact but sagged and broken in the middle. This may give you some idea of the sort of fault you are looking for and the most likely place to find it.

These are X2 caps and supposed to fail short. However, for whatever reason, they both failed open.

Yes, well, my options are limited due the to the *dumb* design implementation here. Plus I can't probe the test points or look for voltage drops when the power supply is constantly interrupted within miliseconds of switch-on. Looks like I'll have to break out my dim bulb tester (if I can find it).

I am going to vote for C1001 & C1002. Either fails short and that fuse will blow. Both fail short and that blow will be instant. That or a bad primary winding - which could have been eaten by repeated refusing without proper diagnosis.

A dim-bulb tester is a reasonable tool for this purpose as a rubber spoon makes a reasonable carving knife. Just not fine enough for any kind of accurate diagnosis.

Peter Wieck Melrose Park, PA

Thia might help:

I don't see that as being an issue, though (the re-fusing bit) since whatever has gone low-resistance is clearly drawing current away from the other healthy components, no? And if said component has gone low-res (as we assume it must) then cooking it a bit more doesn't matter in the least since it's going to have to be replaced anyway. The important thing is not to do it to such an extent that the PCB becomes heat-damaged - in my submission anyway.

I'll check out the caps you suggested, however. Sounds like a good call. Thanks,

CD.

I did actually download this yesterday and that's where the snippet of the wiring diagram I posted comes from. I find, however, that service manuals in this form are so hard to navigate. I'd much rather have the physical hard copy; so much more user-friendly.

Well, as they say, the only good short is a dead short. I'd start by checking all the caps and semiconductors on the line side instead of continually smoke testing it. I wouldn't rule out the transformer either. Shouldn't be too hard to find. I'd also disconnect all the loads. Perhaps the folks on the yahoo tek scopes group might have more targeted information - some sharp ppl there and all the do is scopes.... Good luck

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Those Rifa caps are bound to self destruct at some age, but two at once is suspicious.

.... Phil

Two capacitors in parallel can cause a problem when they both fail. I had a computer power supply that had four smoothing capacitors in parallel. It worked with just one so all four had to fail before the fault was detected.

Seems a tad overly pessimistic. If I powered it up via a DBT, I'd be able to unplug all the PSU outputs one by one until the bulb went out (or went dim). That's on the alternative possibility that the fault doesn't lie within the PSU board itself of course.

It is already almost a certainty that the issue is in the power-supply. All that rubber spoon will tell you is that the issue is in the power-supply. At which point, you will need to troubleshoot at the component level.

Each time you apply power to what amounts to a dead-short, you may be damaging other components downline. When the second fuse blows, *STOP THERE*. Or the first, if there is any sort of associated symptoms such as heat, smoke, noise or a vaporized fuse.

Peter WIeck Melrose Park, PA

Well, I'm inclined to agree it's most likely in the PSU itself. However, if it comes down to troubleshooting at component level then I'm stuffed - certainly if I have to do it powered-up, because access is very poor indeed on this model. On previous Tek scopes I've worked on, it's been possible to pull the board out and work on it live without much difficulty, but the 'plumbing-in' as it were of the wiring on this one without some dedicated extender board is going to be very challenging indeed.

OK, I have! When a new fuse blew after I'd replaced the 2 RIFAs, I posted here and haven't attempted another power-up since. Apart from the RIFAs, there's no sign of physical damage at all to the other components. :-/

a) Do you have a VOM? And, if so, how good is it? b) Not that testing capacitors in-situ generally accurate, but you can usually tell the difference between an open cap and a dead-shorted cap using the Ohms scale. c) Similarly, transistors. d) And it is possible that the RIFAs were symptoms, not the cause.

Which would be a start.

Peter Wieck Melrose Park, PA

Several! Some professional ones too.

It would be very useful to know, but I cannot tell when they blew and if they blew together or seperately. I can say is that they both went catastrophically and blasted crap all over the surrounding components. But we all know these caps 'let go' after a certain time in service anyway. You can pretty much guarantee it.

Yes. On the face of it, this should be simple to find and fix, but the layout of the PSU on this model is just awful. Just one example, they didn't leave enough room for the board! it's tight interference fit with the casing and can only be withdrawn and inserted with considerable force.

If the alternative is landfill, then heroic measures are justified.

If the alternative is 'sending it out', unless you wish to did deeper - and all indications are that you do not - then go ahead and send it out.

There is no magic bullet.

Peter Wieck Melrose Park, PA

I totally get where you're coming from here.

I'm not sure it's worth the expense, because there's another fault with this scope that will most likely still be present even if the present one is fixed. In fact, this 'other fault' may well have caused the current one. This 'other fault' involved the displayed traces suddenly turning extremely bright, accompanied by lines of dots. It materialised only rarely, but I was out of the room when the scope went *phut* so have no idea if there's any link to the current fault or not. I'm beginning to suspect those caps may have failed at some time in the past, because for them to go *bang* with that amount of crap sprayed everywhere must surely have resulted in a great deal of acrid smoke being released - yet there was none discernable. And if those caps fail open circuit (which they both did despite being X2 safety ones which are supposed to fail short) then the scope can contintue to function pretty well as normal so long as the mains supply is reasonably clean.

How true!