Magnavox T991-01 hiss on sound.(IC Type No)

[BRC2000]

Hi All I have a Magnavox T991-01 on the bench which is probably the only one in the UK. It was made in Greeneville Tennessee in May 1977. It looks unused and after repairing the vert plug in board it is working and producing a very good NTSC colour picture.
I now have the last problem hiss on the sound even when set to zero volume.
I do not have a diagram for the sound board type 210915-3 it only has one IC on it which has the numbers 76-Z date code 7716 on it. I can probably repair it if I had the ICs Manufacturers type number.
The other ICs and transistors in this TV seem have only Magnavox part numbers.
Thank You
Keith