Because the TA-51 exciter uses a microphone preamp, the audio can get distorted and noisy from the gain stage that precedes the limiter. This gain is not necessary when driven with high audio levels like what's available from a repeater controller. Also, the limiter may not be necessary is some cases and the associated circuitry and filtering can be eliminated or bypassed. This modification will also alleviate some, if not all, of the inherent "tinny" sound this transmitter has from the factory.
Beware: The peak clipper (that doesn't really work) is eliminated, so don't use this modification on a 15 KC split 2 meter machine without substituting a better peak clipper. Since the Low Pass Filter is not eliminated, it is not necessary to add another.
This page deals with a modification to ham radio equipment, and is not FCC type accepted.
The circuit shown below is a modification to the original Mike Preamp and some of the Low Pass Filter to allow higher levels of audio to be applied to the TA-51-144 or TA-51-220 exciter. This modification will result in better transmitted audio, however be aware that no peak limiting will occur.
Remove or cut out CR-2 in the limiter section. The audio will now be applied at the junction of R11 and R12.
R11 is changed in value to 4.7 K.
The addition of a 10 K pot and 0.3 uF capacitor as shown in the schematic completes the modification.
I use three 0.1 uF capacitors in parallel to make a 0.3 uF.
The added 10 K pot will be used as a "Gain" control to set-up correct audio to the "Deviation Control"
This circuit modification was designed by Kevin Custer W3KKC in June 1994.
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