[Test-Equipment] Bridge Parts

[Robert Rolf]

By choosing a new %1 R that is just below the value he needs, he can add 
a small value trimpot in series to dial in the EXACT value he needs.

Or he can 'make' almost any 1% value by paralleling resistors.

1/54580-1/54900=1/9.36 meg in parallel.
Use a slightly bigger initial R (56k) and then the second resistor 
doesn't have to be quite so large a value.

If you have a good assortment of precision resistors, you can use 3 or 
more if you don't want a trimpot.

R

k2cby wrote:

> Three suggestions:
> 
> First:     Don’t change anything. I looked at the schematic and it appears
> you can use an external 
>             generator as the signal source. Use one set to 50Hz and you’re
> in business.
> 
> Second:  As the old saying goes, you’ve got the keys to the jail in your
> pocket. This is a capacitance bridge, 
>               so just get a bunch of high quality mylar capacitors close to
> .02 µF and .04 µF and measure them 
>               on the bridge at 50 Hz (or measure at 60 Hz without using the
> auto-null feature) until you find one within 1%.
> 
> Third:    The frequency-critical circuit in the detector is a bridged T
> designed so that C411 and C412 each have a 
>             reactance at the operating frequency (131k) twice the resistance
> R422 and R423; and C413 has a 
>             reactance (67k) of twice the resistance of R424. 
> 
>             You can get the same result for 60 Hz by leaving the 50 Hz
> capacitance values in place and 
>             changing R422 and R423 to 54,580 (closest 1% value is 54900) and
> changing R424 to 27,922 
>             (1% value = 28000).
>             
> 
>