Here's another piece of my collection of antique measurment equipment:
The HP 400F AC Millivoltmeter.
![](https://www.eevblog.com/forum/index.php?action=dlattach;topic=5016.0;attach=15551;image)
I had to open it to clean the contacts on the range switch. As you can see on the picture below
they had some severe oxidaton problems, it had gotten worse in the last few months so I decided
it was time to have a look inside the thing.
![](https://www.eevblog.com/forum/index.php?action=dlattach;topic=5016.0;attach=15553;image)
I can now date my unit to about 1973/74 from date codes on the Sprague electrolytics inside.
According to information found on the www it was in production from 1966 to about 1985 in
the US, England and Germany. Mine's got the "Made in W. Germany" label on the back.
![](https://www.eevblog.com/forum/index.php?action=dlattach;topic=5016.0;attach=15555;image)
The double sided PCB is of highest quality as you would expect from an instrument such as this.
Circuitry is completely discrete, not a single IC in sight.
![](https://www.eevblog.com/forum/index.php?action=dlattach;topic=5016.0;attach=15557;image)
![](https://www.eevblog.com/forum/index.php?action=dlattach;topic=5016.0;attach=15559;image)
The scale is calibrated to use dBm (1mW @ 600R) and as such is ideally suited for audio work.
Specified frequency range reaches up to 4MHz, fullscale error depends on frequency and selected
range, it varies between 0,5% and 2%.
The meter can be powered from 115/230V AC or from batteries.
My unit was last calibrated in 1988, at least that's what the sticker on it says. It still works fine,
even though I don't bother to have my meters calibrated. As long as they all more or less
agree with each other it's OK with me
![Cool 8)](https://www.eevblog.com/forum/Smileys/default/cool.gif)
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