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2245A mains filtering

Brian Cockburn
 

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.

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.



Brian Cockburn
 

Chuck,

The caps in my 2245A's PSU that I'm considering changing are the filtering after the Corcom and before the bridge rectifier. The three Y class look a bit RIFA, although without taking them out I can't be sure. The two X class are yellow Philips units. And as I said this is a first step. PSU electrolytics are next if/when there are DC level or ripple problems.

I have had an X class fail - actually went bang in my hands. Although that was on a router (power tool, not network infrastructure).

Ah, Schafner, is that the only brand to worry about?

Here, tonight, it's 240.2 VAC, so yeah I live in a 'high voltage' area. And my 'scope served its time in the UK as far as I'm aware (ex Uk military).

Thanks, Brian.

Chuck Harris
 

Hi Brian,

As far as I know, Schafner is a bad brand during the
80's, through early 2000's. They work ok in the USA,
but in the 240V world, they like to let their smoke out.

The RIFA sort that are a problem have a yellowish clear
molded epoxy outer coating, over a paper dielectric
capacitor. The epoxy shrinks, cracks, and lets moisture
into the paper capacitor, where it can become explosively
leaky.

Generally, tektronix scopes are made of good stuff. You
can leave most of it alone, until it fails, without setting
yourself up for great collateral damage.

The exceptions are SMD electrolytic capacitors in some units,
and Schafner filters when used in 240V land..

-Chuck Harris

Brian Cockburn wrote:

Chuck,

The caps in my 2245A's PSU that I'm considering changing are the filtering after the Corcom and before the bridge rectifier. The three Y class look a bit RIFA, although without taking them out I can't be sure. The two X class are yellow Philips units. And as I said this is a first step. PSU electrolytics are next if/when there are DC level or ripple problems.

I have had an X class fail - actually went bang in my hands. Although that was on a router (power tool, not network infrastructure).

Ah, Schafner, is that the only brand to worry about?

Here, tonight, it's 240.2 VAC, so yeah I live in a 'high voltage' area. And my 'scope served its time in the UK as far as I'm aware (ex Uk military).

Thanks, Brian.



Brian Cockburn
 

Chuck,

Maybe I'll just stop worrying.

Thanks, Brian.