Century 22 Audio Filter Mod

From the Ten-Tec Reflector March 4, 1999

Recently I acquired a nice Century 22. This rig uses an audio filter between the detector and audio amplifier for all of it's selectivity. When I advanced the "filter" control for maximum selectivity, I noticed that the peaked or resonant frequency of the filter was very low, around 350hz. This did not correspond to the manual's specification of 750hz and made for uncomfortable listening.

Check the schematic of the RCVR/CONTROL ASSEMBLY and observe the 4 pole audio filter across the top. Mine shows an LM3900 with 8 capacitors, all being .0033uf. This seemed strange since the Delta 580 and Omni D use the same component values yet the capacitors are all .0022uf. The ARRL Handbook also indicates the caps should all be .0022uf. Now to see what was really inside the rig. Sure enought, .0033uf!

I removed all 8 capacitors and replaced them with .0022uf. Now the filter peaks beautifully at 750hz. Turning the filter to max results in a clean 750hz tone on cw notes.

I would appreciate some feedback on this. What does your manual show and what is actually in your rig? Do other C22's have the correct value?

73 Steve Ellington N4LQ

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