HP 400 VTVM question

[R. D. Davis]

Should the meter in an HP 400 VTVM deflect to the far right, pegging,
then, go all the way to the far left, as far is it can physically go,
and then return to zero, when switching from one scale to another?

The meter is giving off a very faint burning smell, like the smell of
heating elements in a toaster or electric heater, but looking inside,
I don't see any smoke or anything that looks charred. Didn't pinpoint
the source of this yet.

--
Copyright (C) 2003 R. D. Davis The difference between humans & other animals:
All Rights Reserved an unnatural belief that we're above Nature &
rdd@rddavis.org 410-744-4900 her other creatures, using dogma to justify such
http://www.rddavis.org beliefs and to justify much human cruelty.

 

[Dbowey]

rdd asked:

Should the meter in an HP 400 VTVM deflect to the far right, pegging,
then, go all the way to the far left, as far is it can physically go,
and then return to zero, when switching from one scale to another?

Every one I ever used did that. Irritating, but it didn't result in harm to
any of them.

Don

 

[Jim Adney]

On Fri, 21 Nov 2003 03:57:34 -0000 rdd@rhiannon.rddavis.org (R. D.
Davis) wrote:

Should the meter in an HP 400 VTVM deflect to the far right, pegging,
then, go all the way to the far left, as far is it can physically go,
and then return to zero, when switching from one scale to another?

Hmmm, I don't recall the exact cause of that, but I think it may be a
rather standard symptom wiht the HP 400. It's in the manual. What's
the serial number on yours?

The meter is giving off a very faint burning smell, like the smell of
heating elements in a toaster or electric heater, but looking inside,
I don't see any smoke or anything that looks charred. Didn't pinpoint
the source of this yet.

Is it possible that you're just not used to tube operated gear? These
can put out a bit of a smell from the hot tubes, especially if they
have not been fired up for a long time.

The most common problem on these old units is that there are 4 large
axial lead molded paper capacitors in there and they are probably all
leaky. 3 of these are AC coupling capacitors in the meter amplifier,
while the 4th is in the back, part of the power supply.

The amp ones are paired with some resistors and mica caps. If these
are leaky they will bias the control grids of the next amp stage high
and cause poor gain as well as tubes that run extra hot, as well as
shortening their lifetimes.

Let me know your serial number and I may be able to send you a
schematic with voltage values to check. I generally find that LOTs of
things start to work well on these once I get the leaky caps replaced.

There's a very nice old HP 400H sitting right behind me. ;-)

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Jim Adney jadney@vwtype3.org
Madison, WI 53711 USA
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