I don't know how similar/different the -50 sound board is compared to the -51 but this site helped me to track down which chip went bad in my HG. http://www.pinball4you.ch/okaegi/rep_soundold.html#programmable1
If that doesn't help and no one assists, barakandl may be able to repair it or he has a new repro https://pinside.com/pinball/market/classifieds/ad/46360
Eric