Lexicon / Murata filters "De-potted" Detailed picture included.

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jwhmca

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Hi Guys,

I'm rebuilding a Lexicon 480L. One of the Murata LPF filters is bad. I was able to "de-pot" it. Neat little guy...

Does the substrate have anything to do with the circuit? It looks like there is  little "pads" of carbon (or something) instead of SMD resistors?
 

jwhmca

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Here is a close up of the filter.

Are those little "laser trimmed " resistors?
 

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gyraf

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Yup, looks like laser-trimmed thickfilm resistors.

Browns are probably ceramic capacitors

Now is a great time to reverse-engineer.  Would be valuable information for anyone with a dead lex filter.

There's a very good chance one of the 4560 opamps is the damaged part. Remove them.

- With 4560's out of the way, draw schematic.

- Referring to schematic, measure capacitors and resistors: As the active stuff is out of the way, many points will be "hanging loose" and many component values can be read in-circuit.

- Refer to previously-drawn schematic to find the places where parallel loading disturbs component value read-out.

- As a last resort, desolder capacitor to read value.

- Use analogue filter logic to figure out/estimate the remaining parts.

- Solder back in three new 4560's and test for functionality.

...and oh, don't forget to document the whole process here !!!  ;D

Jakob E.
 

mrzoo

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Apr 12, 2006
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Hi jwhmca,

I got a 480L with multipy filter that are leaking, but the are all working at the moment.
So the need to be cleaned, a tedious work I think.
Did you use acetone to de-potted the Murata filter?
Or is that bad for it as it could damage it?

/Peter
 
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