Faulty Validation Logic in Java Base Conversion

Question

I have been doing research on this for a while, but I'm either not understand what I'm reading, or I am searching for the wrong thing. The program I am writing is to convert a number from base to base (2-36) without using a regex method, and validating that the value is valid for the original base before converting it to the new base. My program runs; however, the validation logic is wrong and says every that every valid is incorrect.

Here is my validation logic

// Contract: returns true if iValue is valid expression in base
public static boolean isValidInteger(String iValue, int iBase){
    iValue.toUpperCase();
    while(iBase >= 2 && iBase <= 36){
        for(char character : VALID){
            for(int index = 0; index < iValue.length(); index++){
                if(iValue.charAt(index) != character){
                    System.out.println("Input value is invalid for chosen base!");
                    System.out.print("Please re-enter the value to be converted: ");
                    Scanner sc = new Scanner(System.in);
                    iValue = sc.nextLine();
                }
                else
                    return true;
            }
        }
    }   // End of outer if statement
    return true;
}       // End validation method

I'm honestly at a loss as to where my logic went south. I would greatly appreciate any guidance or feedback on this problem.

Here is the complete code as I have it so far.

import java.math.BigInteger;
import java.util.Scanner;

public class BaseConversionCalculator {
    public static char[] VALID =
            {'0', '1', '2', '3', '4', '5', '6', '7', '8', '9', 'a', 'b', 'c', 'd',
                    'e', 'f', 'g', 'h', 'i', 'j', 'k', 'l', 'm', 'n', 'o', 'p', 'q', 'r',
                    's', 't', 'u', 'v', 'w', 'x', 'y', 'z'};
    private static String iValue = "";
    private static int iBase = 0;
    private static String fValue = "";
    private static int fBase = 0;

    public static boolean isValidInteger(String iValue, int iBase) {
        iValue.toUpperCase();
        while (iBase >= 2 && iBase <= 36) {
            for (char character : VALID) {
                for (int index = 0; index < iValue.length(); index++) {
                    if (iValue.charAt(index) != character) {
                        System.out.println("Input value is invalid for chosen base!");
                        System.out.print("Please re-enter the value to be converted: ");
                        Scanner sc = new Scanner(System.in);
                        iValue = sc.nextLine();
                    } else
                        return true;
                }
            }
        }    // End of outer if statement
        return true;
    }        // End validation method

    /**
     * Main method
     */
    public static void main(String[] args) {
        // Create Scanner object
        Scanner input = new Scanner(System.in);
        //  Greeting
        System.out.print("Welcome to the Base Conversion Calculator!\n");
        System.out.print("This program will ask you for a value to be converted" +
                "and the base (2-36) that the value is in," + "\nthen it will ask for" +
                "the desired base (2-36), do the conversion and display the new value.\n");
        /*
         * Input Section
         */
        // User Input
        System.out.println("Please enter the value to be converted: ");
        String iValue = input.nextLine();
        System.out.println("Next, please enter the original base (2-36): ");
        int iBase = input.nextInt();
        System.out.println("Finally, please enter the desired base (2-36):");
        int fBase = input.nextInt();
        /*
         * Conversion Section
         */
        // Validate the user input
        isValidInteger(iValue, iBase);

        // Convert initial input to BigInteger
        BigInteger bValue = new BigInteger(iValue, iBase);
        System.out.println(iValue + " is " + bValue + " in decimal.");
    }
}