Century 22 Model 579 Audio Tweeks

From the Ten-Tec Yahoo! Group - April 6, 2008

I love my recently acquired C 22 specially the beautiful QSK but the pleasure was marred a bit by poor audio which increases fatigue in my experience. I confess to being a bit of an audiophile and my other hobby is tube audio amplifiers and other esoterica so I really do notice poor audio. Plenty of rice burner radios have poor audio too but we don't worry about them here do we? My other radio is a Corsair 2 and its darned good!

My C22 is an early one so I had to re-centre the narrow filter audio pass band to about 750Hz by fitting 0.0022 uf caps in place of the existing 0.0033's fitted by the factory but I think this mod is well known about nowadays

First audio problem is that the wide band filter tends to dominate until the filter pot is nearly fully CCW. The result is a "wall of sound" that's hard on the ears when you want to listen out for CQ's etc This is easy to improve, simply add a resister say 10k in series with the lead to the centre pin of the filter pot. This makes the narrow filter dominate and the wide band noise etc is reduced to a much more comfortable level even when the control is at its widest position.

Next concern was noticeable distortion and hash, audio IMD etc so I looked at the audio circuitry and my attention was drawn to the low pass filter that cleans up the product detector output. Lo, ten tec had fitted plain old electrolytic caps and unbiased to boot resulting in high ESR and lots of cross over distortion. I replaced them with miniature axial 0.47 MFD film caps that I had lurking in my components collection. Result is an wonderfully clear sound with all that nasty high grass and hash gone for good. The sound is now very musical as a good DC RX should be. I tend to use good neodymium magnet DJ type headphones and I can really hear the difference now. Good quality headphones are always best for CW operating, forget about so called communications 'phones.

Result is a much nicer radio to listen to while winkling out the DX on 40M at night.

Hope this helps. I attach a photo with the offending filter caps circled.

73 Alan G0KMC

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