MEGURO MN-446A Noise Meter repairs

Hi,
Just wondering if someone might happen to have a Service Manual or schematics for the MEGURO MN-446A Noise Meter please?
I have one of these units and its developed an internal noise fault that is leading to erratic readings.

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The new owners of MEGURO DENKI, Japan, no longer carry any information for historic MEGRUO products.
Thanks in advance.
A.

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