CGS Steiner Parker problem: no control over cutoff

codenamerofl · Joined: May 26, 2013 Posts: 9 Location: south carolina

hi all,

built a CGS ver 1.3 steiner parker sythacon filter on protoboard. no shorts, no smoke, it's filtering, has good output, resonance changes gain and is working from what i can tell, and the trimmer adjusts the lowest cutoff point.

however, the cutoff pot will not change the cutoff frequency. i am stumped as to why. i have quadruple checked the transistors and they are wired up right. i have tried different pots, and still nothing. the 1uF cap at the junction is correctly orientated.

anyone have a guess as to why the frequency will not change? my brain is fried. been staring at it for 6 hours for two days now.

thanks for any help

prgdeltablues · Posted: Fri Jul 19, 2013 4:07 am Post subject:

Have you tried disconnecting the cut-off freq pot and the CV mod pot, and apply a few volts directly to the end of the 220k resistor (where the wiper from the pot would have connected)? And check that you have resistance across the span trim pot - if that is shorted, then obviously the base of the first npn would be being held at ground. Always hard to tell without calculating in a complete circuit, but you should have a bit under 1k across that trimmer.

Peter