HP 403B Volt / dB meter problem - diyAudio
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Old 5th May 2012, 05:38 PM   #1
sherpa is offline sherpa  United States
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Default HP 403B Volt / dB meter problem

So i picked up this 403B meter and the Battery in it is shot, Instead of replacing it would a string of 6.5v Zener diodes work as a bypass? i have no need of operating it on batteries and no urge to spend $60 on batteries. Is there another cheap way of bypassing the batteries?
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Old 5th May 2012, 05:58 PM   #2
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The battery is a series string of four 6.5 volt nicads, centertap ground with +13v, -13V, and -6.5V taps. Suggestions since the meter will not seem to work unless the batteries have enough charge.
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Old 5th May 2012, 06:04 PM   #3
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Couldn't you build the required supplies using LM317/LM337's .
You need three supplies + / - 13.5 V and -6.5 V . It appears to be a simple straight forward solution . Am I missing something ?
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Old 5th May 2012, 06:10 PM   #4
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Prolly not but i'm still trying to teach myself most of this stuff and i haven't dealt with the voltage regulators yet.
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Old 5th May 2012, 06:27 PM   #5
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OK, sorry about that. The circuit would be a straight forward application of whats recommended in the application manual. I can't put that up now. Maybe tomorrow morning as it's time for me to go to bed now ! Meantime someone would have had time to put one up for you.
I checked your meter casing. It might take a board on the top ( inside the case) or you can attach the assembly to the rear of the meter itself on the outside ! You can mull over this till I see what others have suggested tomorrow !
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Old 5th May 2012, 06:32 PM   #6
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Awesome! sounds good, thank you. i may be able to fit a small board inside the case where the batteries are now.
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Old 5th May 2012, 06:35 PM   #7
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Velleman 1A 1.5V-35V Power Supply Kit 320-216
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Old 7th December 2013, 04:33 PM   #8
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The 403B Transformer isn't center tapped, and the HP403B power supply actually produces 3 voltages, +/-13V and -6.5V, and a ground reference you'll need, a schematic is shown below.

Click the image to open in full size.

You can use a BUF634 to derive an artificial ground, and the 3 voltages a la:

Click the image to open in full size.

The resistor values for the LMXXXL regulators are comparatively high -- this helps the bypass capacitor knock down noise.

I came upon this as I am replacing the diode rectifier circuit of my 403B with a true rms detector.

Last edited by jackinnj; 7th December 2013 at 04:42 PM.
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Old 7th December 2013, 05:17 PM   #9
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I've solved this problem in similar test equipment by just replacing the batteries with a big electrolytic, bypassed with a suitably rated zener diode to match the original battery voltage. Just measure to be sure you know what the charging and operating currents are.
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Old 9th December 2013, 04:11 PM   #10
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I got these from a battery supplier who had 23 in stock, and they were discontinuing the line. No solder tabs.

The energy density of a Lithium Ion battery pack would be much higher -- or you could just use "AA"s and 2x the 8-AA holder from Radio Shack.

Click the image to open in full size.
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