MEGURO MN-446A Noise Meter - Service Manual / Repair help?

[Kings.Warren]

Hi all,

I have a MEGURO MN-446A Noise Meter that's suffering from an internal oscillating noise problem - the issue manifeasts itself with an offset reading appearing when using the ranges 100mV and below.
On the 10uV range the meter needle swings and hovers around the midaway position of the meter. In all cases a short circuit is applied to the input.
The weird part is that the meter needle slowly oscillates back and forth (approx 0.5 Hz) - the needle is not steady reading.

(The instrument hasn't been used for 5 years and was working to spec before then.)

I did reachout to Meguro Electronics Corp's new owners in Japan, KEISOKU GIKEN Co., Ltd. They gave me the runraround, at first it was a yes, we will email the service manual  for repairs - they sent the user manual with the schematics removed from the backpages.  I offered to pay their costs - No: we've never given out Meguro Service Manuals or schematics was the reply - strange I have two other Meguro User/Service Manuals each with schematics in the back sections and servicing tips.

A Service Manual with schematic diagrams would be very much appreciated.

Thanks!



 

[Kings.Warren]

...MEGURO MN-446A Noise Meter

Photos show the front external.

With noise shield (Al.) attached

Control Board with battery backed settings register - not IEEE-488/HP-IB - just binary bit settings to control the range, weighting filters and the meter rectifier/detection circuits.

Last photo shows thw PSU board in the forground, with the weighting filter board behind.

If you ever wondered who ever used the old RCA/Harris/Intersil - CA3100E - these guys loved them!