[Amps] ML!-2500 Filter Cap Replacement

[Tom W8JI]

>
> I've never found one that aged down in value, always the 
> ones I've checked have gone up in resistance. Even when 
> they've never been installed, and that includes brand new 
> military ones in NATO sealed packages.
> It's quite amazing how well some of the old gear still 
> works with resistors that were 20% tolerance to start with 
> and in many cases, are now 50% tolerance.
> But I agree whole heartedly about not using them for 
> voltage equalising or dropping.

Aging down is actually a very common so I'm quite surprised 
you never saw it! It is even in the resistance cycle curves 
supplied by some manufacturers.

The normal cycle with a carbon comp is to decrease 
resistance with prolonged exposure to heat until the carbon 
gets so hot it cracks, at which point resistance increases 
rapidly. This is because the carbon changes inside from heat 
and makes a better "connection" along the length. It's very 
common to find "shorted" resistors, and it was the cause of 
all the Sylvania TV set fires in the 70's. Sylvania stupidly 
used a few 2 watt carbons to drop 350V B+ down to collector 
voltage for some RGB driver transistors, and over time the 
resistors aged down in value. This increased current, and 
the resistors ran hotter. Eventually they would get so hot 
the phenolic circuit board would ignite.

We also used to find certain Bendix aviation radios commonly 
with "zero ohm" carbons, caused by the very same effect, and 
I've seen hundreds of Dentron and Amp Supply amps with the 
same issue. The original AL80 design had the very same 
problem. Of course that design was scrapped with only a 
hundred sold, and people were given a free new amp by Prime 
Instruments when Prime took the Ameritron mess from Amp 
Supply over during a "debt adjustment".

There is a second problem. That's where the leads contact 
the carbon. Moisture will cause a bad connection and 
increase the resistance. That's common in carbons that have 
sat a long time cold and especially damp....but the aging 
down in resistance is very common.

I suppose the perspective varies if someone mainly looks at 
unused stored parts or parts from service in the field.

73 Tom