Need an easy test for outer foil connection. Tried connecting cap to scope input to see hum when holding cap in fingers and got no difference when either end was connected to ground. I'm recapping an old healthkit RF sig. gen. and it may not be that critical, but I thought that I would give it a go.
I was able to identify the outer foil using an audio signal generator. Thanks for the tip. The modern caps are physically so much smaller than the vintage caps. I think the lack of surface area makes the measurement more difficult. As I stated, I don't know how important the issue is: but Heathkit did indicate in the assembly manual the direction and I'm not an RF guy.
They may have indicated the direction because it matters, or because they wanted to reduce support calls asking which way to wire the cap. It could be that the orientation does not matter in an absolute sense, but affects dial calibration by changing stray capacitance - if so a modern replacement may create problems too. Only change faulty caps would be my advice.
To my knowledge, the best DIY outer foil tester project may currently be the one published on August 2018 by the guy that runs the youtube channel "Mr Carlson's Lab". It is available only to the subscribers of the associated Patreon account. I haven't built one myself but the documentation is convincing and the schematic looks good.
I use a simpler method. I connect the foil capacitor under test to the input of a high impedence headphone amplifier (100k input impedence) with crocodyle clips, then I grab the capacitor body with two fingers without touching the leads. I take notice of the hum intensity, then I reverse connections and repeat the test - it is important to keep the capacitor body orientation the same on both tests. On some brand/types the difference is usually quite evident, far more so than by looking at a screen. On other capacitor brand/types I hear no difference, I believe that they use a different internal construction to avoid the outer foil shortcoming when one leg of the capacitor is at very high impedence.
I use a simpler method. I connect the foil capacitor under test to the input of a high impedence headphone amplifier (100k input impedence) with crocodyle clips, then I grab the capacitor body with two fingers without touching the leads. I take notice of the hum intensity, then I reverse connections and repeat the test - it is important to keep the capacitor body orientation the same on both tests. On some brand/types the difference is usually quite evident, far more so than by looking at a screen. On other capacitor brand/types I hear no difference, I believe that they use a different internal construction to avoid the outer foil shortcoming when one leg of the capacitor is at very high impedence.
- Status
- This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.