Sergei Bezrukov writes:
Some multimeters use 9V block batteries which do not last long and are relatively expensive compared with other alkaline battery types. If the multimeter is used extensively one need to replace batteries pretty often. A more practical solution would be to power the multimeter from AA or AAA batteries and use a DC-DC converter to obtain 9V from 3V.
The converter is based on the power supply controller TL499A manufactured by Texas Instruments and the schematics follows the standard one from the data-sheet. The only difference is attaching a LF filter to pin 3 consisting from a 10 Ohm resistor and a 1μF capacitor. This idea is taken from a similar project published in June 2007 issue of Everyday Practical Electronics and it significantly reduces the voltage peaks at the output. The inductor is Murata 18R473C.
9V DC-DC converter for a multimeter – [Link]
3-to-9V booster – [Link]