I have a mark 4A. if I remember correctly mine done the same thing. it was one of the chips, maybe ic802. I know one of them is a chip that was programmed by browning when they were made, ( maybe the pll). this ic is very very hard to find. the other one that mine needed was a binary multiplier. I bought a few extras, but I cant remember the part number. if you want to look on it and post the part number, I will look I may have a spare for ya. the one my rig used was not a special ic, it was plug and play. please forgive me if I am off a little, im trying to go by memory. I rebuilt mine about 4-5 years ago. it is still going strong. it had sat in a garage for 25-30 years, in the boxes. I bought it not working and done a total recap, and a few other things. it looks like new it is a beauty, and I love talking on it.
good luck