override post excerpt length with substr

add_filter( 'jnews_module_excerpt', 'override_jnews_module_excerpt', 99, 4 );

function override_jnews_module_excerpt( $excerpt, $post_id, $excerpt_length, $excerpt_more ) {

    $excerpt  = mb_substr( $excerpt, 0, 100, 'UTF-8' ); /* You can replace the value 100 with the number of characters you want to display in the post excerpt. */
    $excerpt .= ' ' . $excerpt_more;

    return $excerpt;
}