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419A Chopper Replacement

 

Good morning,

I have a 419A Nulling Voltmeter that I just finished replacing all the
electrolytic capacitors in. This cleared up my 'after warming up no
operation' problem. However, I also had to really have my way with
both the chopper freq adj and neon current adj pots to get them to
fire _something_ like reasonably. I can't get the frequency to be
correct (too slow by close to half), and the waveform is pretty
distorted over most of the range, and just _somewhat_ distorted over
the rest. This makes me think I probably need new choppers. I assume
these are unobtainum.

1. is my logic sound, or is there more likely a problem with the
driver instead?
2. what will a somewhat slow, distorted chopper waveform do to the
correct operation of this otherwise-excellent meter?

Thanks very much!
J Forster
 

Many Ne bulbs have minute amounts of radioactive material in them to
stabilize the firing point. As the radioactive emission rate decays, the
bulbs become erratic. Put in new bulbs, if you can.
Best,
-John



Christian A Weagle wrote:

Good morning,

I have a 419A Nulling Voltmeter that I just finished replacing all the
electrolytic capacitors in. This cleared up my 'after warming up no
operation' problem. However, I also had to really have my way with
both the chopper freq adj and neon current adj pots to get them to
fire _something_ like reasonably. I can't get the frequency to be
correct (too slow by close to half), and the waveform is pretty
distorted over most of the range, and just _somewhat_ distorted over
the rest. This makes me think I probably need new choppers. I assume
these are unobtainum.

1. is my logic sound, or is there more likely a problem with the
driver instead?
2. what will a somewhat slow, distorted chopper waveform do to the
correct operation of this otherwise-excellent meter?

Thanks very much!
 

--- In hp_agilent_equipment@..., J Forster <jfor@...> wrote:

Many Ne bulbs have minute amounts of radioactive material in them to
stabilize the firing point. As the radioactive emission rate decays, the
bulbs become erratic. Put in new bulbs, if you can.
That was my first impulse; however, the manual (paragraph 5-27) states
that "physical alignment and selection of the neon bulbs are critical"
and that replacement of the entire assy is the only thing to do.

Can you speak to this at all? If I _do_ replace the neons, are
standard NE-2 bulbs the right thing to use?

Thanks for your help!
J Forster
 

I don't know about the alignment. My experience was with the opto-chopper in
the 410C. Mine was replaced as a unit by the company cal lab.

There are only a very few wire pigtail Ne's made. I think there was a means of
ID'ing which is in there by looking very carefully at the glass pinch area.
There is something stamped into the glass as I remember. You might ask someone
like BulbDirect that specializes in lamps.

Best,
-John



Christian A Weagle wrote:

--- In hp_agilent_equipment@..., J Forster <jfor@...> wrote:

Many Ne bulbs have minute amounts of radioactive material in them to
stabilize the firing point. As the radioactive emission rate decays, the
bulbs become erratic. Put in new bulbs, if you can.
That was my first impulse; however, the manual (paragraph 5-27) states
that "physical alignment and selection of the neon bulbs are critical"
and that replacement of the entire assy is the only thing to do.

Can you speak to this at all? If I _do_ replace the neons, are
standard NE-2 bulbs the right thing to use?

Thanks for your help!
John Day
 

At 04:22 PM 11/3/2007, you wrote:

I don't know about the alignment. My experience was with the opto-chopper in
the 410C. Mine was replaced as a unit by the company cal lab.

There are only a very few wire pigtail Ne's made. I think there was a means of
ID'ing which is in there by looking very carefully at the glass pinch area.
There is something stamped into the glass as I remember. You might ask someone
like BulbDirect that specializes in lamps.
Or ask on the NEON-NIXIE Yahoo group, a lot of expertise and
experience there too.

John



Best,
-John

Christian A Weagle wrote:

--- In
<mailto:hp_agilent_equipment%40yahoogroups.com>hp_agilent_equipment@...,
J Forster <jfor@...> wrote:

Many Ne bulbs have minute amounts of radioactive material in them to
stabilize the firing point. As the radioactive emission rate decays, the
bulbs become erratic. Put in new bulbs, if you can.
That was my first impulse; however, the manual (paragraph 5-27) states
that "physical alignment and selection of the neon bulbs are critical"
and that replacement of the entire assy is the only thing to do.

Can you speak to this at all? If I _do_ replace the neons, are
standard NE-2 bulbs the right thing to use?

Thanks for your help!
Harvey White
 

On Sat, 03 Nov 2007 15:22:26 -0500, you wrote:

I don't know about the alignment. My experience was with the opto-chopper in
the 410C. Mine was replaced as a unit by the company cal lab.

There are only a very few wire pigtail Ne's made. I think there was a means of
ID'ing which is in there by looking very carefully at the glass pinch area.
There is something stamped into the glass as I remember. You might ask someone
like BulbDirect that specializes in lamps.
I remember NE2-D and NE2-J as well as the NE2; different wattages,
IIRC.

Harvey



Best,
-John



Christian A Weagle wrote:

--- In hp_agilent_equipment@..., J Forster <jfor@...> wrote:

Many Ne bulbs have minute amounts of radioactive material in them to
stabilize the firing point. As the radioactive emission rate decays, the
bulbs become erratic. Put in new bulbs, if you can.
That was my first impulse; however, the manual (paragraph 5-27) states
that "physical alignment and selection of the neon bulbs are critical"
and that replacement of the entire assy is the only thing to do.

Can you speak to this at all? If I _do_ replace the neons, are
standard NE-2 bulbs the right thing to use?

Thanks for your help!
 

deane kidd who has tektronix parts might have the neon bulbs you need

----- Original Message -----
From: Harvey White
To: hp_agilent_equipment@...
Sent: Saturday, November 03, 2007 9:07 PM
Subject: Re: [hp_agilent_equipment] Re: 419A Chopper Replacement


On Sat, 03 Nov 2007 15:22:26 -0500, you wrote:

>I don't know about the alignment. My experience was with the opto-chopper in
>the 410C. Mine was replaced as a unit by the company cal lab.
>
>There are only a very few wire pigtail Ne's made. I think there was a means of
>ID'ing which is in there by looking very carefully at the glass pinch area.
>There is something stamped into the glass as I remember. You might ask someone
>like BulbDirect that specializes in lamps.

I remember NE2-D and NE2-J as well as the NE2; different wattages,
IIRC.

Harvey

>
>Best,
>-John
>
>
>
>Christian A Weagle wrote:
>
>> --- In hp_agilent_equipment@..., J Forster <jfor@...> wrote:
>> >
>> > Many Ne bulbs have minute amounts of radioactive material in them to
>> > stabilize the firing point. As the radioactive emission rate decays, the
>> > bulbs become erratic. Put in new bulbs, if you can.
>>
>> That was my first impulse; however, the manual (paragraph 5-27) states
>> that "physical alignment and selection of the neon bulbs are critical"
>> and that replacement of the entire assy is the only thing to do.
>>
>> Can you speak to this at all? If I _do_ replace the neons, are
>> standard NE-2 bulbs the right thing to use?
>>
>> Thanks for your help!
 

-----Original Message-----
From: hp_agilent_equipment@...
[mailto:hp_agilent_equipment@...]On Behalf Of Christian A
Weagle
To: hp_agilent_equipment@...

I have a 419A Nulling Voltmeter that I just finished replacing all the
electrolytic capacitors in. This cleared up my 'after warming up no
operation' problem. However, I also had to really have my way with
both the chopper freq adj and neon current adj pots to get them to
fire _something_ like reasonably. I can't get the frequency to be
correct (too slow by close to half), and the waveform is pretty
I don't know the 419A. For what it's worth, my Fluke 895A differential
voltmeter had a distorted chopper waveform that was about half normal
frequency. Turned out to be a short on one of the power supplies that
the chop oscillator drives in addition to the choppers.

Dave Wise
 

Well, here I am answering my own question(s). This one was sort of my
fault. To wit:

When I got this unit, I tried to turn it on, no luck. Seeing the
battery charge pushbuttons, I feared the worst (leaky cells corroding
the PCBs), and opened it up. Found some wacky NiCds (basically, four
'cells', each the size of sub-Cs, each labelled as 6V 225mAh (so
probably 5 big fat coil cells in fancy wrapping), chained up to
provide +/- 12V, and dated in the early 80's) that were truly dead -
no leaks though. I removed them as some stickers on the back of the
unit implied that they were trickle charged whenever it was plugged
in, and they probably wouldn't like that in their current state!

Changed a bunch of the electrolytic caps too. I then proceeded to
operate the unit, sort of. It was all drifty, lots of random brief
full-scale deflections, couldn't cal the photochopper, etc... Changed
the rest of the caps. No help.

After a close perusal of the power supply page of the schematic,
though, I saw what was wrong. The only place that this meter gets
its' internal ground is from the center of that battery stack! It
basically runs from, and continuously charges them from, the top and
bottom of a full-bridge rectifier. Ground is sort of virtual - if
there are no batteries, there is no fixed ground. Hacking in a pair
of 12-cell NiMH packs makes everything better - no drifts or abrupt
swings, and I could cal the photochopper.

The packs are somewhat higher voltage, but I think they should
regulate down to the correct 12V after a while of running off them to
discharge them, before plugging the unit in.

What an unusual way of doing business!
 

they can be replaced with zener diodes and bypass caps

----- Original Message -----
From: Christian A Weagle
To: hp_agilent_equipment@...
Sent: Thursday, November 08, 2007 9:20 PM
Subject: [hp_agilent_equipment] Re: 419A Chopper Replacement


Well, here I am answering my own question(s). This one was sort of my
fault. To wit:

When I got this unit, I tried to turn it on, no luck. Seeing the
battery charge pushbuttons, I feared the worst (leaky cells corroding
the PCBs), and opened it up. Found some wacky NiCds (basically, four
'cells', each the size of sub-Cs, each labelled as 6V 225mAh (so
probably 5 big fat coil cells in fancy wrapping), chained up to
provide +/- 12V, and dated in the early 80's) that were truly dead -
no leaks though. I removed them as some stickers on the back of the
unit implied that they were trickle charged whenever it was plugged
in, and they probably wouldn't like that in their current state!

Changed a bunch of the electrolytic caps too. I then proceeded to
operate the unit, sort of. It was all drifty, lots of random brief
full-scale deflections, couldn't cal the photochopper, etc... Changed
the rest of the caps. No help.

After a close perusal of the power supply page of the schematic,
though, I saw what was wrong. The only place that this meter gets
its' internal ground is from the center of that battery stack! It
basically runs from, and continuously charges them from, the top and
bottom of a full-bridge rectifier. Ground is sort of virtual - if
there are no batteries, there is no fixed ground. Hacking in a pair
of 12-cell NiMH packs makes everything better - no drifts or abrupt
swings, and I could cal the photochopper.

The packs are somewhat higher voltage, but I think they should
regulate down to the correct 12V after a while of running off them to
discharge them, before plugging the unit in.

What an unusual way of doing business!

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