[R-390] Mech filter REPAIR

[Tim Shoppa]

ToddRoberts2001 at aol.com wrote:
> I think the next stage of R-390A filter repair would be to have the ability 
> to rewind the input or output transducer coils if needed. I am sure there are a 
> number of the coils with an open winding somewhere in the middle, not just at 
> the ends. 

Exactly. Don't forget not just opens, but several of my "bad" filters
have either shorts to the can or a strongly-temperature-dependent
leakage.

> be freed up by a soak in paint remover or some kind of solvent. It may ruin the 
> wire insulation but at least you could still count the number of turns in the 
> coil. If it is impossible to count the turns then knowing the inductance of a 
> good coil can lead to a target figure by trial and error of number of turns 
> of a rewind.  One of the old MoReCo Morris Hand Coil Winders would be nice to 
> have for rewinding the small bobbins.

Even those would be too big. I think we're into watchmaking territory.

>       I believe the disc resonators should probably last forever unless the 
> filter was dropped hard or physically damaged. There is really nothing there 
> that can wear out except the wire coil or connections inside the filter and of  
> course the original foam insulation that eventually turns into a sticky 
> corrosive mess.

That, indeed, is the hope.

Tim.