[volt-nuts] Wanted wirewound precision resistors and ESI DB877

[Frank Stellmach]

> It seem so because the direct resistance measurement seem ok,
> and the current once connected to the fluke 343 source
> returns perfect value, but now I fear to repeat the process.
> ...
> Can you advice a tecnique to check the other resistors?

I think, you should better measure the resistance by a high resolution 
DMM, on its normal OHM range.
Thats much more stable and less power is dissipated. A comparison 
between two of them is also much easier.

>  I opened one broken resistor ( they are open, with only a tape
>  on the outside) and the wire is oily, there was also somethin like
>  wax in some points on the outside. I think you are right that the
>  wire insulation is probably compromised.

I have seen wax inside very old ww resistors from COHU and KINTEL, on 
the soldering junction between resistance wire and leads.
Obviously to protect the solder from corrosion.
Some drops of oil may be used to increase high volt isolation as it 
ingresses between the windings. I assume, your resistors are tubular 
ones, with plastiv caps around, and they are not hermetically tight, 
therefore they cant be filled completely for corrosion protection and 
thermal  conductivity, like in the Vishay metal foil VHP type.

Besides electrical measurement (on OHM range!), I 'd recommen optical 
inspection of the case, and if its possible, of the windings for 
burnt/overheated areas. Corrosion may be easily identified, also by 
slightly stained parts of the wire.

 From your description, the resistors seem to be very old, from the 
70ties perhaps, therefore the solder junctions may also be defect, if 
you find an "open" resistor. Careful resoldering might help.

Frank