Re: explosion mystery on my 7104


Brian
 

I agree with that , the line filter in my 7104 went . Fortunately the earth leakage breaker went before any explosion in that instance but other instruments using that same age and brand of line filter have all needed replacement as a result of explosions of one sort or another - sometimes mild others not .
My 2465A needed the RIFA's replaced , the scope told me so with a loud hissing noise and a cloud of grey smoke blown out the fan exhaust about 30minutes after I unpacked it several years ago , the smell lasted for about a week .

On Saturday, 19 June 2021, 13:37:50 BST, Carsten Bormann <cabocabo@gmail.com> wrote:

On 2021-06-19, at 01:19, Sparky99 <jnolan@iprova.com> wrote:

brown tar like goo
That is most likely the result of an exploded Schaffner line filter — the ones built in the 1980s used to have internal RIFA (or RIFA-like?) caps across line and neutral, and these RIFA caps do explode, the occasional deniers here notwithstanding.

If you want to see how this looks like for non-enclosed RIFA caps, see e.g.:

https://youtu.be/c2dgI7duA7w?t=840
https://youtu.be/XAbrU17hLTM?t=160

(Humidity and 230 V are contributing, but not necessary, factors; don’t be fooled.  References to the 250 V working voltage are irrelevant, as the caps were *designed* to operate with that; it is the crazing and the moisture ingress that wasn’t part of the design, and that is pretty much independent of the voltage.
Of course, the short can be two to four times more violent at 230 V...)

When the internal cap shorts and explodes inside the Schaffner filter, there is no way for the heat and the plasm/gases to get out, so the tar-like potting of the Schaffner filter (which turns out *not* to be blocking the humidity) melts/sublimates and is spewed out with the RIFA gases.  The resulting jet of molten tar and stinky RIFA gases can go anywhere, often outside the unit (the line filter being installed in the back panel); mine went about a foot in a straight line before it hit furniture.

PUBLIC SERVICE ANNOUNCEMENT:
Replace on sight: the 1980s RIFA caps in transparent epoxy blobs (replace even if you can’t see the cracks yet — they are sometimes on one side only and hard to see), and Schaffner inlet filters from ~ the 1980s (newer Schaffners are fine, as are other brands).

Grüße, Carsten

Join TekScopes@groups.io to automatically receive all group messages.