I have read Williams repair manual. some suggestions to try . 1 make sure sound board properly earthed. 2 could try changing sb resistor R18 from 22o ohm to 100 ohm . it says this resistor controls neg feedback to the TDA2002 but also it will reduce overall volume level. but the above only applies to sys 3/4 type 1 sound boards. On type 2 Sb check the TDA2002 for cracked solder joints . These are just some things that you could check depending on the type of SB but worth a try
