It is good practice to always specify the VARCHAR length. Try this:
SELECT '2000038028654' + '0000' + CMTrxNum +
CASE WHEN TRXAMNT < 0 THEN LTRIM(STR(-1 * TRXAMNT, 20, 2))
ELSE LTRIM(STR(TRXAMNT, 20, 2))
END + CONVERT(VARCHAR(

, TRXDATE, 112) +
CASE WHEN VOIDED = '1' THEN 'V' ELSE '' END
FROM CM20200
WHERE CHEKBKID = 'UPTOWN TRUST'
AND SOURCDOC IN ('PMPAY', 'PMCHK');
--
Plamen Ratchev
http://www.SQLStudio.com