It is not at all uncommon for these 70s receivers to have all the bulbs burned out. I have seen this many times. You may want to pop the cover and see what bulbs they are and look for replacements. If you have a meter you can check for voltage at the socket or even check continuity through the bulb (i.e. just measure the resistance). If you get no resistance reading the bulb is burned out.
I did check the voltage getting to the bulbs and they all have about 6.8 volts so I order some LED's to replace them. I expect them early next week and am excited to see what it looks like with everything properly illuminated.