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Electronics => Repair => Topic started by: TimInCanada on January 23, 2017, 02:32:46 am

Title: Test Eqpt Restoration: Adding caps & MOVs to AC Line Inputs?
Post by: TimInCanada on January 23, 2017, 02:32:46 am
Hello everyone,

I've collected up some test gear, averaging about 40+ years old, and am starting to tear down pieces, cleaing them up, replacing electrolytics and doing obviously needed repairs.  The collected restoration links in this forum have been a great help.  While I'm in there (the most dangerous restoration words... ) I was wondering about updating the AC line input with X & Y safety capacitors and MOVs?  A couple dollars in parts per instrument should do it.  I'm not an EE, so your feedback and advice would be appreciated.

Let me know if the following seems reasonable.

Put a MOV between line and earth ground and another between neutral and ground.  The one connected to the line would be after the unit's power switch and the fuse.  What specs on the MOVs would be appropriate for 120VAC residential home supply?

Put a Y2 rated safety capacitor between line and ground and another between neutral and ground.  EMI line filters seem to use ceramic caps with values between 2200 - 4700 pF, so that would be my guess for appropriate size.

Put a X2 capacitor between line and neutral, also after the power switch.  EMI line filters seem to use values of 0.1 uF, so use this size.

Thanks much,


Title: Re: Test Eqpt Restoration: Adding caps & MOVs to AC Line Inputs?
Post by: blueskull on January 23, 2017, 02:44:28 am
For 120V AC, you should expect up to 132V if assuming 10% voltage error (US standard is 5%, but who knows, just in case), then multiply by 1.414 to get peak value, you are expecting 187V.
I would say putting a 200V working voltage MOV should work.
For X caps and Y caps, if your gears are powered by switching mode power supply, then replace with exactly the same value. If they are mains transformer powered, then no X/Y caps are needed.
X/Y caps are used to shunt switching noise from power supply. For a linear power supply, you don't need them.
X/Y caps may also be good at filtering external noise from going into the device, but how well it works, I don't know.
One thing to keep in mind is Y caps can cause leakage current from mains to earth, so if your gear is non-grounded, you may shock yourself. Even if you grounded it properly, they may trigger GFCI.
Title: Re: Test Eqpt Restoration: Adding caps & MOVs to AC Line Inputs?
Post by: james_s on January 23, 2017, 04:40:02 am
I don't like MOVs personally. They tend to fail silently, or they can fail in a partially conductive state that causes them to heat up. I've seen more than one cheap surge protected power strip melt down, one at work several years ago burned a big hole in the carpet after a loose cable in one of the panels caused one phase to rise up high enough for the MOV to conduct. I'm skeptical of their value in protecting equipment too, unless you live somewhere with a lot of nearby lightning strikes or something.

I wouldn't add capacitors across the line either unless it was originally designed with them. Suppression capacitors are not a new technology, my 1941 Philco radio has one. Generally I assume that the engineers who designed a piece of equipment knew what they were doing and this is especially true of test equipment. My advice is unless you know there is a design fault that can benefit from correction and you know exactly what you're doing, don't modify the design. Just replace any defective components and call it good.
Title: Re: Test Eqpt Restoration: Adding caps & MOVs to AC Line Inputs?
Post by: TimInCanada on January 24, 2017, 01:56:23 am
Thanks for those replies.  After some more reading I think James' advice is right.  This old test gear lasted decades in industrial environments that were bound to have more powerline noise and transients than a house will have. 

This page ( is the only thing I found that mentioned measuring transients in a house.  When induction motors shut down they found a maximum 2V peak. 

As blueskull pointed out, Y capacitors (line to ground) introduce a leakage current to ground.  I want to put my little workbench on a GFI circuit and building such leakage into equipment would likely prevent that.