Battery alternative for HP 403B wideband vintage AC voltmeter

[trobbins]

The HP 403B vintage transistor AC voltmeter may not be alone in using internal NiCad battery modules which need replacement over the decades.  I just restored a 403B by replacing the 4x 6.5V 225mAh NiCad modules with four 6.0V active TL431 shunt regulators in series.  An external battery can be connected if needed, such as 2x 12V 4Ah monoblocs.  Maintaining internal batteries is always a pain, especially if the meter is only used intermittently.  This meter only uses <10mA rail feeds, so the TL431 provides a nice quiet low impedance voltage reference to emulate a battery module.

The 403B may be vintage and only average responding, but it does have a 1-2MHz bandwidth so can be convenient to keep an eye on performance when other DMM's would not show a reading.

It does seem to have a display face that cracks and peels and can restrict needle movement, and the e-caps need replacing, but otherwise is pretty simple to work on.  My meter movement has points of stiction, which seems to result from bearing slop (which I can't see how to repair), so that may be a show-stopper for some.

[geggi1]

How about using 3 9V battery cells.
You will get 27V

[bdunham7]

Very good idea.  One of the issues with this unit is that it uses the battery pack as a voltage divider to provide multiple supply voltages, so the batteries need to be installed, working and at least nominally charged for the meter to function properly at all.  They're very handy AC voltmeters, especially for those signals that are just a bit too small or fast for even a top-notch DMM, like 100uV @ 1MHz. They can be found on eBay for very little money--but the battery packs would cost 2-3X what the meter costs! 

b/t/w, I might be able to scrounge up a spare meter assembly that I could send you.

[trobbins]

Yes internal batteries can be a maintenance nightmare, and the batteries used in the 403B are not common now, so it is quite a cost to outlay, and even then the meter may not end up in cal or have other issues like the cracking/curling display.

Thanks for the meter option but my meter is mainly a restoration journey and I may not use it much anyway so will just keep it for the off chance that I need to keep an eye on a HF voltage and can't use my HP3400 or a scope.  The meter generally calibrates ok but I haven't put it on external batteries yet to confirm the 1mV FS range cal, or set up a 300V source (I need to scrounge around in my valve amp output transformer collection, including PP OPTs with an open primary half winding).

[trobbins]

It was fairly easy to add a resistor divider across the DCV generated for the meter, so that an external battery powered DMM can be connected to give a digital readout of meter voltage, for both the 0-3 and 0-1 range dials.  I use two Aneng AN8009 for general benchwork and connecting one of them to either a 0-3 or 0-1 range output makes it easy to use due to 10Meg input resistance and sufficient accuracy and resolution for reading display.  Some restoration notes are in link: https://www.dalmura.com.au/static/HP403B%20AC%20meter.pdf

Battery operation certainly suppresses mains voltage pickup in the first amp section of the circuitry, as there is quite a significant meter signal generated for a shorted input when using mains power.  Battery powering provides residual noise to be under 1% for shorted input.

[nssouza]

Excellent suggestion! the 4 TL431 are programmed for 6.25V and installed directly in place of old batteries?

[jdragoset]

I used 4 Ea 2 AAA cell and 4Ea. 3 AAA cell holders with 20 Ea. Ni-MH cells.
Four groups of five cells all in series.