HP-400D voltmeter

P

Peter E. Orban

Guest
Hi Everyone,

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: peter.orban@nrc.ca
 
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.

--

Greetings,

Jerry Greenberg GLG Technologies GLG
=========================================
WebPage http://www.zoom-one.com
Electronics http://www.zoom-one.com/electron.htm
=========================================


"Peter E. Orban" <peter.orban@nrc.ca> wrote in message
news:409FCF72.7E76F184@nrc.ca...
Hi Everyone,

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: peter.orban@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:
Hi Everyone,

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: peter.orban@nrc.ca
Click to expand...
 
On Mon, 10 May 2004 14:52:34 -0400 "Peter E. Orban"
<peter.orban@nrc.ca> 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.
Click to expand...
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 jadney@vwtype3.org
Madison, WI 53711 USA
-----------------------------------------------
 
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