Was wondering if anyone in the group has ever heard of or tried the mod
described below and if so could you share your comments about wheter it was
worthwhile doing.
Regards,
George
Subject: [omni_V_562] product detector improvement
The CA3053 product detector (U3 on the IF/AF board) has been the subject of
several mods intending to improve intermodulation distortion in the audio
passband. W3TS has written about this at length.
The CA3028 is the same part as the CA3053, so I did some research and found a
Harris Application Note AN5337 discussing the CA3028 in RF applications. In
studying this info I noticed that Harris always biases pin 5 at 2/3 of Vcc, as
opposed to 1/2 Vcc in the O-V. The datasheets also show this if you look close
enough.
R14 and R15 form a voltage divider to provide this bias, so I put a 10K
resistor in parallel with R14 to get the bias level to 2/3 of Vcc. The voltage
at pin 5 in my radio is now about 5.6v, up from about 3.6v before the change.
The result is that the distortion goes away. Pile-ups now sound so clean and
clear that it's hard to believe one little resistor can make such a difference.
Try it! You'll like it!
_______________________________________________
TenTec mailing list
TenTec@contesting.com
http://lists.contesting.com/mailman/listinfo/tentec
|