***BANG!***

The capacitors on the AC line input crack and adsorb moisture, Then the exp lode, along with the smell of burning plastic. They are a common failure in older HP test equipment. Once the short clears, the equipment will operate , but not in a safe manor. Most people replace the entire filter, rather th an the capacitors. The ones that fail are mostly the same brand. There are a lot of comments about these on

I used to repair test equipment at Hughes Aircraft. I was surprised to find that HP used Sprague Black Beauty capacitors in their older equipment. Regardless of the reason it was on the bench, standard procedure was to replace all them.

Thanks for that, Michael. I think I've found the cap you refer to, straddled across the mains incoming supply, looking rather jaded, bulgy and cracked. I'd have thought they'd have used potted, integrated filters like the Tek scopes of the same era did. I think IIRC I've had issues with these caps before on other equipment. It's buried somewhat in the guts of the device immediately behind the mains socket but I can just about see it. 250V it says on it. Not much headroom with our 240V mains here! How about I just snip it out and not bother replacing it? It's a PITA to get at.

Or you could just throw it in the trash. If it's not worth fixing, it's not worth keeping.

They are X or Y rated safety capacitors. I would not omit them. The 250 volt marking is AC volts, not DC. The HP manual should have the specs for the bad caps.

Just think of all the Ebay sales you could have made to deluded musicians, with those used BB caps! come to think of it, I had a bag of NOS BBs that came from an old shop that went out of business. Maybe I'll dig them out and list them. :)

Have you seen the 'Nano VNA' handheld units? Under $50 at many places.

I have a box full of Wima X and Y caps here (which are supposed to be superior) so sourcing a replacement for this failed Rifa is not an issue. It's getting access that's the thing.

What's life, without a challenge? :)

Elsewhere down thread you mention "RIFA" capacitors, sounds like the gold coloured 0.47uF/250V ones that seem to disgrace various 1980's ASTEC branded power supplies (BBC Computer, Apple II computer, ICL One Per Desk and others) all of which work fine for a while, and then crackle-phut-burnt toffee flavoured smoke, but keep on working.

I guess I got lucky, they didn't fail shorted, but they definitely cracked and bulged.

They're "safety capacitors" in respect of the fact that they can be connected across a (reasonably) high energy source; that's all. If they are omitted it's NOT a safety issue. Obviously *if* one is going to connect a capacitor across the mains supply it needs to be X rated, but

I bought one a while back and paid $ 57 for it. They have came down.

While not lab quality, they do work well enough for an expirmenter on a low budget.

Around 20 years or less ago , they would almost have been lab quality. All they would need is a high stable and accurate frequency .

There is lots of stuff comming out of China that works well enough for people like me that don't have a lot of money to buy things with but still like to expirment and learn.

Those little component testers for around $ 20 really get to me. They seem to work really well in most cases as to identifying components and their values.

A challenge has to be, by definition, ultimately achievable. Removing this cap is *not* achievable. I would have to destroy the entire VNA to get it out. What a rotten design.

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I bought a Nano VNA for $42.87 with some Ebay bucks that were about to expi re, but I haven't tried it yet. I am currently trying to get another door a nd a handicap ramp installed at my home. The current exits all have too man y steps to go up and down. I haven't been in my shop building for several y ears, because it is too much pain to get out to it, and stand at the workbe nch. Once the modifications are done, I will have room for a small bench in that room.

I am retired, so I no longer have access to the multiple workbenches of tes t equipment that I used on my last job.

I agree about those component testers being useful. I have a couple. Anothe r one that is handy is a $10 VGA monitor test generator.

X capacitors are to filter RF from the power lines. Y capacitors are for safety.

Those plastic capacitors were supposed to be high rel, but they didn't expect the cases to craze, and fail. What model is your VNA?

It's the 8754A. It tops out at 1.3Ghz which was, a long time ago when I purchased it, was a respectably high frequency! In fact it's still the highest practicable frequency band allocated for radio hams, which was another good reason for me to choose it.

I suspect you're going to have a few contradictory views about that sweeping statement from some of our not-so-polite regulars here! ;-)

I can *just* access enough to snip the blown cap out and discard it. I think I'll fit a new one to the end of the power lead instead; it's my only option realistically.

But did it cause a bang loud enough to be heard in a shower?

I had a TV do that back in the 80s, and what it did was launch an electroly tic clean off the board. I found some paper packing and eventually the cap itself. The TV still worked and the only evidence left on the board was tw o very clean leads where the cap used to sit.

Even though you found a pretty good suspect, I'd look the rest of it over v ery carefully for a couple of leads and no part between them.