On Mon, 10 May 2004 14:52:34 -0400 "Peter E. Orban"
wrote:

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.

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.

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Jim Adney
Madison, WI 53711 USA
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