Re: 2245A mains filtering

Chuck Harris
 

Although the allowable range of capacitors for a line
filter are quite large, not all X and Y class filter caps
are used in the line filters. They are quite often used
as snubber capacitors in switching circuitry... where their
value is somewhat critical to efficient operation.

There should be absolutely no problem getting exact matches
for the X and Y capacitors in a Tek scope. Go to DigiKey,
or Mouser. Use film type rather than paper type. Your biggest
issue will be size. You may have to bend the leads on the
newer smaller capacitors to fit the holes in the circuit board...
Or, alternatively, you can specify higher voltage rating
capacitors, as they will be more likely to be the exact physical
dimensions.

Why are you singling out the X and Y caps? In the spectrum of
part failures, they are more reliable, and less important to the
function of the scope, than any electrolytic capacitor. I have
never had one that I *HAD* to replace because it failed... I have
had many that I replaced because of cracked epoxy coatings, and
because customers insisted I do so...

Corcom is a very good filter company. I have never had
one of their products fail electrically. I have seen a few
that were ripped out of the panel, but I don't think that was
a fault of manufacture... it looked deliberate.

Schafner, on the other hand....

It is also rare for any power line filter module to fail if it
is run from US 120V power... But quite common if it is run from
EU 240V power, especially if the utility runs to the high side
of tolerance, like the UK.

-Chuck Harris

Brian Cockburn wrote:

Hi,

I'm looking at replacing the three Y class and two X class capacitors in the PSU of my (new to me) 2245A as a very first step to keeping it in tip-top condition. How critical are the values of these capacitors? The Y class are 1.5nF (1500pF) and the X class are 150nF (0.15uF). I'm guessing that '22', '33', and '47' would be OK replacements for '15' if these are what's available at a reasonable price and (low enough) quantity.

Secondly, the mains inlet filter. I've read that there can be problems with these things. My unit has a Corcom filter (250 VAC, 3A, 50-60Hz). It has 8734 on it which seems likely for the date of manufacture given the filter is original to the 'scope. Do I need to worry about the age of the filter and fit a new one or are these units built to last?

Thanks, Brian.



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