From: mmm@cup.portal.com (Mark Robert Thorson)
Subject: Re: Making up odd resistor values required by filters
Organization: The Portal System (TM)
Lines: 3

One way to make cheap trimmable resistors is to use a high-wattage carbon
composition resistor, and saw a notch in it with a triangular file.
Of course, you can only increase the resistance, not decrease it.