Hi, thank you for contacting JustAnswer.com. My name is Russell. I will do my best to provide the right answer to your question.
The power supply cannot be found period - that modem is far too old.
Look on the manufacturer's label - if it is not too obscured.
Can you find it? can you read, for instance, the model # for the modem off of it?
Then it may indicate the AC V or the DV C or the V AC or the V DC on it... and perhaps mention W or Watts also. Let me know what you find out... that info will help!
9 V AC is a likely figure - I encountered it also when I did the research. But be it noted: another modem might have another voltage. There's nothing like having the specs right off the modem's manufacturer's label, which without the actual adapter, is as good an authority as you could find.