Technics EAH-420 Headphones

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I bought a Set of Technics Headphones off of EBay 3 week’s ago they look in mint condition but don’t work at all. They are Technics Model EAH-420 made in 1975. Where can I find the Service Manual for them? I can’t find it anywhere. These were there Top of Line Model. They are Hudge & Heavy & made good but someone dropped them I can hear like a Screw loose inside. I don’t want to touch them till I know how to open them up because there is so much crome on them & don’t want to make a mistake.
Any ideas where a Service Manual can be found?

 

[sarasota2k4]

Not sure if it is the same model, but this video on YouTube looks like the only access may be through removing the cushions first.