Capacitor outer foil test box....

[jaysg]

So I built one yesterday and it doesn't work for me. I ran it into my 18Watt, which is in perfect running order, and I couldn't tell the difference with several caps -- P715, M150, and MPP.

I have construction questions: The Boobird drawing showed a guitar cable attached to the On/Off switch. I used an isolated jack on an aluminum Bud box. The thing hums like a you-know-what when I touch the DPDT which has a metal handle, or the box. Any suggestions?

[Phil_S]

Yeah, I built one that doesn't work either. I haven't been able to get back to figure out what's wrong. I built mine in a plastic switch box -- the kind used in home construction. I figured no metal was a good idea on this thing, but who really knows?

[M Fowler]

Mine works just as instructed hold onto the cap with one hand and it will create a noiser response in one direction. The switch is mounted side ways so that you follow the direct the switch bat is in to know the out of foil side. Perhaps your testing poly caps that do not have out of foil.

[Richie]

http://www.youtube.com/user/SoundGarage ... P6ZIHXq6Yg

[paulster]

I've never tried the amp method as I always use a scope but the difference is night any day on the scope, so it should be equally audible on an amp too.

[Sonny ReVerb]

I found this on another forum:

Easy test:
If there is doubt about which cap lead is outer foil it is easy to find out with an oscilloscope set on high gain. Connect the cap across the input terminals of the scope vertical axis. When the noise grows and 60 Hz hum appears when the fingers are touched to the outside of the cap it is clear that the outer foil connection is hooked to the hot lead of the scope input. If the outer foil is on the ground connection the noise on the screen will not increase. If you do not have an o-scope you could use the mag phono input of your stereo system preamp and listen for hum. Keep that volume control down!

Is that the method you use?

Here's the original website about the tester.

[jaysg]

I replaced the isolated jack with a normal one. I have a scope to compare to...I can only get an audible difference if I put it across the amp's AC cord. Touching it makes it harder to tell...strange.

[paulster]

I grip the cap between thumb and forefinger and connect my scope probe and ground flying lead to the cap. Then I disconnect them and connect them round the other way whilst still holding the cap in the same position and the difference between the noise level on the display makes it really obvious which way round is which.

[brewdude]

I built one and it works great.

[Raybob]

...Here's the original website about the tester.

According to that site, "For capacitors that are used in tone stacks, connect the outer foil to the ground." I believe that's backwards. The outer foil is supposed to go to the low impedance side. In the case of tone stack caps, 100K from plate resistor is much less than 1M from the volume pot. The outer foil should be on the B+ side.

Raybob

[jaysg]

I built one and it works great.

What amp do you use for this, and does it have to be turned way up? I have to crank it to 10 and I think Gabi does in his video.

[paulster]

I believe that's backwards. The outer foil is supposed to go to the low impedance side. In the case of tone stack caps, 100K from plate resistor is much less than 1M from the volume pot. The outer foil should be on the B+ side.

Exactly. There are spots where it's really obvious which way round they should go and others where you need to make a judgement on what the likely source impedance will be in order to make an informed decision.

Pots are one of the common areas where, with a 1M pot, you could have a source impedance of zero ohms at one end and anything over 500K in the electrical middle of the travel.

[M Fowler]

jaysg,

For mine I just have a very small SS combo amp that I plug into for this test. Or use a scope.

[brewdude]

I built one and it works great.

What amp do you use for this, and does it have to be turned way up? I have to crank it to 10 and I think Gabi does in his video.

I used a small SS practice amp. I don't remember how loud I had it. Some caps showed a much bigger difference than other "identical" caps. I tested and marked every cap I had and have not used again since.

[Structo]

I remember watching that video of Gabbi doing that.

I never did build the switch thing though.

It didn't really appear that his amp was very loud when he did it.