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furly 17th January 2006 12:11 AM

Help with diaphragm coating
Hey guys,

Just tried testing the resistance of a small piece of Mylar coated with some powdered graphite, but i am not getting any readings on either of my meters. Its like it is not conducting.

Any thoughts on the matter would be much appreciated.



SY 17th January 2006 12:28 AM

What kind of meter and probe are you using?

And do the films look gray?

I_Forgot 17th January 2006 04:12 AM

Put the film, graphite side up, on a hard surface. Drop two coins onto the film. Place your meter's probes on the coins. Check the resistance.

If you have a real cheapo analog meter it may not read high resistances well.

Are you sure you used graphite?


furly 17th January 2006 04:29 AM

Ok, I have it conducting now.

Wow do you ever have to rub it in hard and I am finding it difficult to get it evenly so that it conducts in the whole area. I also ripped it. This may be difficult.

What resistance should I be aiming for? Around 20K maybe? And how important is uniformity? If i have resistance values that range from 10K to 50K for example what effect will it have?

In Sander's book he says that he can do a 6 square feet of diaphragm in 1 minute with an amount of graphite that would only fill half a pea.
This seems odd to me, but i guess i just need some more practice. :xeye:


I_Forgot 17th January 2006 12:17 PM

How are you rubbing it in? I use cotton balls- sprinkle a little graphite over the surface (instead of dropping a lump in one spot) and use the cotton to spread it everywhere.

Resistance that is measured in tens of megaohms is best, but that is almost impossible with graphite. Just get it as high as you can. It will work.

Have you considered Licron? It is easier to apply and gives very high resistance.

The coating does not need to be uniform. The charge applied to the diaphragm will spread itself out over the surface as long as the surface is even a little bit conductive.


Bazukaz 17th January 2006 01:43 PM

I have had one strange problem with graphite.I used a graphite rod from the art supply shop.
I couldn't get it conductive ! speaker didn't work at all , and neon lamp on EHT supply didn't flicker.After applying some soap to the membrane, it started operating !
The same with graphite grease ... I Don't understand this.

I also tested shoe polish.It seems to have a high resistance , but , except my EHT supply , i have no tools to measure conductivity.

I_Forgot 17th January 2006 08:23 PM

A graphite rod from the art store is probably mostly clay, as is pencil lead. Also, even if it were graphite, you would have a hard time grinding it fine enough to use for coating the speaker diaphragm.

Shoe polish will work, as will dish detergent, as will stove black, but not for very long. I tried stove-black about a year ago and the stuff worked for about 6 weeks, then quit.

Use lock lubricant graphite, or better yet, get some Licron.


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