Electronics > Metrology

Beautiful precision from the golden age - Fluke 720 oil can teardown

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martinr33:
Thanks for doing this. I expected wafer resistors, but we have naked bobbins.

Using four resistors in each location allows Fluke to correct second and third order tempo effects. These resistors have to hold steady to 8 digits to deliver  the 0.1 ppm resolution.

The two resistors at the top are part of the bridge used to balance the resistor values.

This thread does give me an idea. The 0 - 0.1 setting has no adjustment pot, and is the reference for the whole instrument. If that string is too far off, it forces many modifications to the other strings. Therefore, we could simply switch it with another string, and make balancing easier.


RoadDog:

--- Quote from: martinr33 on May 29, 2022, 10:46:50 am ---This thread does give me an idea. The 0 - 0.1 setting has no adjustment pot, and is the reference for the whole instrument. If that string is too far off, it forces many modifications to the other strings. Therefore, we could simply switch it with another string, and make balancing easier.

--- End quote ---

A0 is adjusted by the bridge balance so I’d think as long as you can balance the bridge it should be fine. I don’t know if I’d want to start messing with the harness. You could also adjust the trim resistor inside with another in parallel or series I’d think similar to what you and Dr Frank have done.

martinr33:
Adding the extra parts will be messier than changing the leads. Of course, lead lengths may also be a factor.

Adjusting the bridge balance works - but it can also shift the set points for the other resistors out of range.

At the end of the day, there are many ways to recenter these devices when they run out of room. However, you do need a plan of attack based on drift across all ranges.

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