HP-400D voltmeter

Check all the capacitors for their ESR specs, and for their values to start with. In these old units, it is very normally to have many bad caps. Some of the resistors can also be off value from age.

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Greetings,

Jerry Greenberg GLG Technologies GLG ========================================= WebPage

I picked up an old HP-400D voltmeter on a garage sale. Having cleaned most of the grime away from the equipment (it must have set in a basement for years) I plugged it in. The tubes all light up and the power supply voltage is around spec, but the meter moves always past its maximum in every range. The second amplifier chain, after the attenuator, is oscillating at about 1MHz, that is what the meter is showing. What could be the cause of the oscillation? Bad tubes? I am also thinking about bad ground connections, as if (I have to check this) grounds are connected to solder lugs that are riveted to the frame. There could be some corrosion at those rivets.

Thanks, Peter

-- Peter E. Orban National Research Council of Canada e-mail: snipped-for-privacy@nrc.ca

Peter,

I'd start by cleaning all of the tube pins, as they are likely to have corrosion on them from sitting all these years. Clean the range selector switch too. I suggest leaving the tube pins dry (no contact treatment) after cleaning. Sometimes 400 grit (more or less) sandpaper is the best way to clean tube pins.

Regards, Tim Schwartz Bristol Electronics

"Peter E. Orban" wrote:

The most common problem with these is the coupling caps between stages. Each stage uses a molded case paper capacitor to couple the signal from the plate of the previous stage to the grid of the next. These caps are almost all leaky by now and this throws the grid bias way off.

In the 400D you will find 4 of those caps. One of them is in the power supply and the other three are for interstage coupling. The power supply one isn't a problem, but the other 3 should just be replaced. IIRC, I could hook a DC uAmmeter between the grid end of each cap and chassis and measure the leakage of each one. The resistors in the grid circuit are high enough that it doesn't take much leakage to make for several volts of grid bias change.

Getting rid of the leakage will pull down the grid voltages and reduce the plate currents, taking a big load off the tubes and making everything in there run quite a bit cooler, too.

I replaced mine (all 4) with nice axial lead mylar caps maybe 15 years ago. It made a world of difference. I found a couple of other problems, too, but I had to get it close before I could even see those problems.

----------------------------------------------- Jim Adney snipped-for-privacy@vwtype3.org Madison, WI 53711 USA

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