I recently acquired a late 70s Ford 3600 diesel and have been having some issues with the battery staying charged. I purchased a new battery a couple months ago and it started without any problems for a few times. When it wouldn’t start I checked the voltage on the battery and it was reading around 8V. I charged the battery and the same thing happened again. I had the alternator tested at a repair shop and it checked out ok. I then replaced the voltage regulator. Forgot to mention that I checked voltage before starting and after the tractor is running and also with increasing the rpms and the voltage stays the same around 12.4 (doesn’t appear to be getting charge from the alternator...but it tested good according to shop). What should I check next? Any help is appreciated!