"Jeff Liebermann" wrote in message
...
On Fri, 23 Jan 2009 00:41:46 -0500, "Barry L. Ornitz"

wrote:
"Unless the resistors are protected by a coating that the fluid cannot
permeate, such as an epoxy, these fluids cause the resistance of the Type
AS
resistors to increase. With some silicones, this increase is as little
as
10%; with some mineral oils this can be as much as 100%. Generally, the
resistance will rise as the fluid permeates the resistor body, and it
will
finally stabilize..."

For a Carborundum resistor, if the mineral oil causes the resistance
to increase, I would guess(tm) that it would corrode or otherwise
remove material from the resistive element. That might [make] it a one
way
trip, where subsequent solvent cleaning, heating, and drying would
probably not return the resistor to its original value. Doubling the
value suggests that a considerable amount of material was corroded
away.

If the resistor material were [like] carbon composition [resistors], the
mineral
oil would act as an insulator between carbon grains, reducing the number
of points of contact, and also increasing the resistance. However,
this should be recoverable by cleaning and baking.

Are either of these mechanisms probable?

From what I know about the original Acheson Process and the later Lely and
Modified Lely Processes for making silicon carbide, and about subsequent
sintering processes, I suspect the true mechanism is somewhere in between
the two extremes that Jeff suggests, but the second mechanism is more
likely. Silicon carbide and graphite are quite inert to organic solvents.
It would even take something like hydrofluoric acid to attack the material
at such low temperatures. Knowing exactly how the material is sintered
would help with the explanation. Three main methods of sintering silicon
carbide are used. One uses glass frit or metal as a bonding agent. The
second uses graphite and silicon metal which is reaction bonded to the
silicon carbide grains. Finally boron carbide is used as a sintering aid
for very high temperature applications. Since sintering only bonds a
composite material in discrete points (as opposed to fully melting the
material), an insulating liquid could diffuse between grains and increase
the bulk resistance.

Have you tried to "repair" the load resistor?

No. Once I discovered that the transformer oil I had been given was
contaminated with PCB's, I disposed of the Cantenna. The PCB containing
oil was burned in an EPA-rated incinerator, and the Carborundum resistor
was sent to a hazardous waste landfill. I had the original HN-31, so I
rinsed the metal parts with solvent that was burned with the oil. I never
tried washing the resistor, and I was afraid to vaporize the trapped
mineral oil because of the PCB contamination.

By this time, I had purchased a high power Bird termination load at a
hamfest for $5. It had originally been used as a termination for a TACAN
system and was marked accordingly with a metal plate stating that it was
rated for a kilowatt in the frequency range of 900 to 1250 MHz. The seller
had dropped the price in increments all day and yet hams were not
interested thinking it would only work in that frequency range. I realized
that this was a TEM and not a filled waveguide load and I got an
exceptional deal. The seller had not bothered to read the resistance with
an ohmmeter! Not wanting to pay Bird's high prices for a Type-N adapter, I
made one myself.

73, Dr. Barry L. Ornitz WA4VZQ
{transpose the digits to reply}