Electronics Repair (sci.electronics.repair) Discussion of repairing electronic equipment. Topics include requests for assistance, where to obtain servicing information and parts, techniques for diagnosis and repair, and annecdotes about success, failures and problems.

Reply
 
LinkBack Thread Tools Search this Thread Display Modes
  #1   Report Post  
Damian Menscher
 
Posts: n/a
Default HV Probe construction advice

A pet project for the past several years is repairing an ancient
Tektronix 674 oscilloscope. (The high voltage circuit was going into
overload protection, I think as a result of leakage from the diodes
(5647 tubes). Replacing them with solid-state diodes seems to have
fixed that issue (I wish I could keep it in its original form, but I
simply gave up after being unsuccessful for a few years.).

Anyway, with the reduced load for lighting the tubes, I think this
needs to be recalibrated (intensity suggests it's got too much HV
now). The service manual indicates a HV test point, at which I
should read 2.2kV. Problem is, I'm just a hobbyist with no HV probe.

After reading specs on a 6kV Fluke HV probe, I realize they're really
just voltage dividers (the Fluke uses a 75MOhm resistor and a 75kOhm
resistor). So I figure I could save myself $75 by building my own.
Off I go to radioshack.com, where I find 5-packs of 1MOhm, 1/2W
resistors for $1. Recalling high school physics, I work out that one
of these should be able to drop 707V at .707mA. So if I put 10 in
series, I should be able to measure up to 7kV, and simply measure off
the last one and multiply the voltmeter reading by 10.

One problem: Rat Shack says these have a "max working voltage" of
350V (see http://www.radioshack.com/search.asp?find=271-1134). Are
they just arbitrarily throwing a fudge factor of 2 away, or are they
mislabeling 1/4W resistors as 1/2W resistors, or what? (They list
10Ohm 1/2W resistors as 350V max also, so I'm guessing this is safe
to ignore.)

Any other warnings I should heed before doing this? I realize I'll
lose some accuracy (they're only accurate to 5%). I guess I also
have to worry about affecting the circuit when I draw .7mA out of it?
I think my measurement error should be small, since my multimeter has
a 10MOhm imput impedence. Is that good enough, or should I use a
smaller final resistor in the voltage divider to reduce measurement
error?

Finally, my crazy idea of protecting myself from electrocution is to
encase this thing in the plastic barrel of a Bic pen. They seem
about the right size, plastic doesn't conduct, and as long as I hold
it at the low-voltage end, it seems reasonably safe. Am I asking
for trouble here, or does that seem reasonable for measuring only up
to 7kV? I suppose I could build a safer handle onto it if needed.

I'd appreciate any advice that could improve my life expectancy.

Damian Menscher
--
-=#| Physics Grad Student & SysAdmin @ U Illinois Urbana-Champaign |#=-
-=#| 488 LLP, 1110 W. Green St, Urbana, IL 61801 Ofc217)333-0038 |#=-
-=#| 4602 Beckman, VMIL/MS, Imaging Technology Group217)244-3074 |#=-
-=#| www.uiuc.edu/~menscher/ Fax217)333-9819 |#=-
-=#| The above opinions are not necessarily those of my employers. |#=-
  #2   Report Post  
NSM
 
Posts: n/a
Default


"Damian Menscher" wrote in message
...
| A pet project for the past several years is repairing an ancient
| Tektronix 674 oscilloscope. (The high voltage circuit was going into
....
| One problem: Rat Shack says these have a "max working voltage" of
| 350V (see http://www.radioshack.com/search.asp?find=271-1134). Are
....
| Finally, my crazy idea of protecting myself from electrocution is to
| encase this thing in the plastic barrel of a Bic pen. They seem
| about the right size, plastic doesn't conduct, and as long as I hold
| it at the low-voltage end, it seems reasonably safe. Am I asking
| for trouble here, or does that seem reasonable for measuring only up
| to 7kV? I suppose I could build a safer handle onto it if needed.

1. I'd be reluctant to push the voltage too far. If one flashes over they
all will. You're probably OK but be cautious.

2. Don't even THINK of using a hand held probe. Hook the divider up to the
circuit, turn the scope on and check the voltage. Don't touch anything
except the scope on/off switch or however you control this setup.

3. 5% is near enough.

4. If you want to own a probe, buy the real thing on eBay. Remember: "One
flash and you're ash"!

N




  #3   Report Post  
James Sweet
 
Posts: n/a
Default


"Damian Menscher" wrote in message
...
A pet project for the past several years is repairing an ancient
Tektronix 674 oscilloscope. (The high voltage circuit was going into
overload protection, I think as a result of leakage from the diodes
(5647 tubes). Replacing them with solid-state diodes seems to have
fixed that issue (I wish I could keep it in its original form, but I
simply gave up after being unsuccessful for a few years.).

Anyway, with the reduced load for lighting the tubes, I think this
needs to be recalibrated (intensity suggests it's got too much HV
now). The service manual indicates a HV test point, at which I
should read 2.2kV. Problem is, I'm just a hobbyist with no HV probe.

After reading specs on a 6kV Fluke HV probe, I realize they're really
just voltage dividers (the Fluke uses a 75MOhm resistor and a 75kOhm
resistor). So I figure I could save myself $75 by building my own.
Off I go to radioshack.com, where I find 5-packs of 1MOhm, 1/2W
resistors for $1. Recalling high school physics, I work out that one
of these should be able to drop 707V at .707mA. So if I put 10 in
series, I should be able to measure up to 7kV, and simply measure off
the last one and multiply the voltmeter reading by 10.

One problem: Rat Shack says these have a "max working voltage" of
350V (see http://www.radioshack.com/search.asp?find=271-1134). Are
they just arbitrarily throwing a fudge factor of 2 away, or are they
mislabeling 1/4W resistors as 1/2W resistors, or what? (They list
10Ohm 1/2W resistors as 350V max also, so I'm guessing this is safe
to ignore.)



They can flash over, that's your main worry. I've seen people make HV probes
with simple resistors though, the best way seems to be to chain them
together and then place them in a piece of plastic conduit then fill that
with mineral oil. You can make a reasonably professional HV probe that way,
and you're only measuring a few Kv so you should be fine. That said, I got a
real Fluke 40 Kv HV probe on ebay for under $30 a couple years ago, I've
seen the 6 Kv ones go for under $20 but with a 40 Kv you can measure CRT
anodes.


  #4   Report Post  
aurgathor
 
Posts: n/a
Default


"Damian Menscher" wrote in message
...

After reading specs on a 6kV Fluke HV probe, I realize they're really
just voltage dividers (the Fluke uses a 75MOhm resistor and a 75kOhm
resistor). So I figure I could save myself $75 by building my own.
Off I go to radioshack.com, where I find 5-packs of 1MOhm, 1/2W
resistors for $1. Recalling high school physics, I work out that one
of these should be able to drop 707V at .707mA. So if I put 10 in
series, I should be able to measure up to 7kV, and simply measure off
the last one and multiply the voltmeter reading by 10.


In high voltage circuits like that there's not a whole lot
current, and 10 Meg could be a significant load on it;
however, that's probably no big deal.

One problem: Rat Shack says these have a "max working voltage" of
350V (see http://www.radioshack.com/search.asp?find=271-1134). Are
they just arbitrarily throwing a fudge factor of 2 away, or are they
mislabeling 1/4W resistors as 1/2W resistors, or what? (They list
10Ohm 1/2W resistors as 350V max also, so I'm guessing this is safe
to ignore.)


Not exactly. With voltages in that range theres's a possibility
of a flash-over. You definitely want to make the divider so
the resistors are phisically arranged in a series like:
--####---####---####---
and don't use your HV probe when the humidity is too high.

Also, I think those high value resistors are composite, so put
one or two metal films in there -- if something bad happens,
the metal film will evaporate, so in essence, it will act as a
fuse.

If you ever get hit by a CRT, 2.2kV isn't that high of a voltage,
but it can be very inconvenient, or deadly if the PS can supply
enough juice.


  #5   Report Post  
mike
 
Posts: n/a
Default

Damian Menscher wrote:
A pet project for the past several years is repairing an ancient
Tektronix 674 oscilloscope. (The high voltage circuit was going into
overload protection, I think as a result of leakage from the diodes
(5647 tubes). Replacing them with solid-state diodes seems to have
fixed that issue (I wish I could keep it in its original form, but I
simply gave up after being unsuccessful for a few years.).

Anyway, with the reduced load for lighting the tubes, I think this
needs to be recalibrated (intensity suggests it's got too much HV
now). The service manual indicates a HV test point, at which I
should read 2.2kV. Problem is, I'm just a hobbyist with no HV probe.

After reading specs on a 6kV Fluke HV probe, I realize they're really
just voltage dividers (the Fluke uses a 75MOhm resistor and a 75kOhm
resistor). So I figure I could save myself $75 by building my own.
Off I go to radioshack.com, where I find 5-packs of 1MOhm, 1/2W
resistors for $1. Recalling high school physics, I work out that one
of these should be able to drop 707V at .707mA. So if I put 10 in
series, I should be able to measure up to 7kV, and simply measure off
the last one and multiply the voltmeter reading by 10.

One problem: Rat Shack says these have a "max working voltage" of
350V (see http://www.radioshack.com/search.asp?find=271-1134). Are
they just arbitrarily throwing a fudge factor of 2 away, or are they
mislabeling 1/4W resistors as 1/2W resistors, or what? (They list
10Ohm 1/2W resistors as 350V max also, so I'm guessing this is safe
to ignore.)

Any other warnings I should heed before doing this? I realize I'll
lose some accuracy (they're only accurate to 5%). I guess I also
have to worry about affecting the circuit when I draw .7mA out of it?
I think my measurement error should be small, since my multimeter has
a 10MOhm imput impedence. Is that good enough, or should I use a
smaller final resistor in the voltage divider to reduce measurement
error?

Finally, my crazy idea of protecting myself from electrocution is to
encase this thing in the plastic barrel of a Bic pen. They seem
about the right size, plastic doesn't conduct, and as long as I hold
it at the low-voltage end, it seems reasonably safe. Am I asking
for trouble here, or does that seem reasonable for measuring only up
to 7kV? I suppose I could build a safer handle onto it if needed.

I'd appreciate any advice that could improve my life expectancy.

Damian Menscher


Your resistors are too small. Too much load on a typical high voltage
circuit. You're putting too much voltage on each resistor. When you
buy resistors from RS, you have no idea what you're getting.
When you use a pen as a housing, you have no idea what impurities are
in there and whether it will arc over or through.

If you have regular need for a HV probe, put a watch on EBAY for one
and snag one that goes at a low price. Whole bunch of 'em have sold
near the $10 mark recently.
IF you only need this one measurment, borrow a probe and be safe.

I wouldn't risk my life on a probe that cost me $8 to build when I could
have bought a safe one at a swapmeet for $10.
mike

--
Return address is VALID.
Wanted, Slot 1 Motherboard
500MHz Tek DSOscilloscope TDS540 Make Offer
http://nm7u.tripod.com/homepage/te.html
Wanted, 12.1" LCD for Gateway Solo 5300. Samsung LT121SU-121
Bunch of stuff For Sale and Wanted at the link below.
http://www.geocities.com/SiliconValley/Monitor/4710/



  #6   Report Post  
Sam Goldwasser
 
Posts: n/a
Default

mike writes:

Damian Menscher wrote:
A pet project for the past several years is repairing an ancient
Tektronix 674 oscilloscope. (The high voltage circuit was going into
overload protection, I think as a result of leakage from the diodes
(5647 tubes). Replacing them with solid-state diodes seems to have
fixed that issue (I wish I could keep it in its original form, but I
simply gave up after being unsuccessful for a few years.).
Anyway, with the reduced load for lighting the tubes, I think this
needs to be recalibrated (intensity suggests it's got too much HV
now). The service manual indicates a HV test point, at which I
should read 2.2kV. Problem is, I'm just a hobbyist with no HV probe.
After reading specs on a 6kV Fluke HV probe, I realize they're really
just voltage dividers (the Fluke uses a 75MOhm resistor and a 75kOhm
resistor). So I figure I could save myself $75 by building my own.
Off I go to radioshack.com, where I find 5-packs of 1MOhm, 1/2W
resistors for $1. Recalling high school physics, I work out that one
of these should be able to drop 707V at .707mA. So if I put 10 in
series, I should be able to measure up to 7kV, and simply measure off
the last one and multiply the voltmeter reading by 10.
One problem: Rat Shack says these have a "max working voltage" of
350V (see http://www.radioshack.com/search.asp?find=271-1134). Are
they just arbitrarily throwing a fudge factor of 2 away, or are they
mislabeling 1/4W resistors as 1/2W resistors, or what? (They list
10Ohm 1/2W resistors as 350V max also, so I'm guessing this is safe
to ignore.)
Any other warnings I should heed before doing this? I realize I'll
lose some accuracy (they're only accurate to 5%). I guess I also
have to worry about affecting the circuit when I draw .7mA out of it?
I think my measurement error should be small, since my multimeter has
a 10MOhm imput impedence. Is that good enough, or should I use a
smaller final resistor in the voltage divider to reduce measurement
error?
Finally, my crazy idea of protecting myself from electrocution is to
encase this thing in the plastic barrel of a Bic pen. They seem
about the right size, plastic doesn't conduct, and as long as I hold
it at the low-voltage end, it seems reasonably safe. Am I asking
for trouble here, or does that seem reasonable for measuring only up
to 7kV? I suppose I could build a safer handle onto it if needed.
I'd appreciate any advice that could improve my life expectancy.
Damian Menscher


Your resistors are too small. Too much load on a typical high voltage
circuit. You're putting too much voltage on each resistor. When you
buy resistors from RS, you have no idea what you're getting.
When you use a pen as a housing, you have no idea what impurities are
in there and whether it will arc over or through.

If you have regular need for a HV probe, put a watch on EBAY for one
and snag one that goes at a low price. Whole bunch of 'em have sold
near the $10 mark recently.
IF you only need this one measurment, borrow a probe and be safe.

I wouldn't risk my life on a probe that cost me $8 to build when I could
have bought a safe one at a swapmeet for $10.
mike


He's talking about the miserable almost no current output of a scope
high voltage multiplier! Everyone is going safety crazy. I'm all
for safety - there are enough references to it in the FAQs but for
this, a simple voltage divider really is sufficient and safe. At
worst, it will fry his meter if something bad happens.

This is NOT a microwave oven.

This is NOT projection TV.

It's only 2 or 3 kV at a few hundred microamps max.

2 or 3 kV isn't going to flash over 6 inches!

See: http://repairfaq.ece.drexel.edu/sam/hvprobe.htm

However, I do agree that if you plan to do more HV stuff, be on the lookout
for a proper HV probe.

--- sam | Sci.Electronics.Repair FAQ Mirror: http://repairfaq.ece.drexel.edu/
Repair | Main Table of Contents: http://repairfaq.ece.drexel.edu/REPAIR/
+Lasers | Sam's Laser FAQ: http://repairfaq.ece.drexel.edu/sam/lasersam.htm
| Mirror Sites: http://repairfaq.ece.drexel.edu/REPAIR/F_mirror.html

Note: These links are hopefully temporary until we can sort out the excessive
traffic on Repairfaq.org.

Important: Anything sent to the email address in the message header above is
ignored unless my full name is included in the subject line. Or, you can
contact me via the Feedback Form in the FAQs.
  #7   Report Post  
Sam Goldwasser
 
Posts: n/a
Default

See: http://repairfaq.ece.drexel.edu/sam/hvprobe.htm

Building a voltage divider out of 10, 10M ohm resistors (and suitable
math!) is sufficient and safe.

However, I do agree with the other posts that if you plan to do more HV
stuff, be on the lookout for a proper HV probe.

--- sam | Sci.Electronics.Repair FAQ Mirror: http://repairfaq.ece.drexel.edu/
Repair | Main Table of Contents: http://repairfaq.ece.drexel.edu/REPAIR/
+Lasers | Sam's Laser FAQ: http://repairfaq.ece.drexel.edu/sam/lasersam.htm
| Mirror Sites: http://repairfaq.ece.drexel.edu/REPAIR/F_mirror.html

Note: These links are hopefully temporary until we can sort out the excessive
traffic on Repairfaq.org.

Important: Anything sent to the email address in the message header above is
ignored unless my full name is included in the subject line. Or, you can
contact me via the Feedback Form in the FAQs.
  #8   Report Post  
mike
 
Posts: n/a
Default

Sam Goldwasser wrote:
See: http://repairfaq.ece.drexel.edu/sam/hvprobe.htm

Building a voltage divider out of 10, 10M ohm resistors (and suitable
math!) is sufficient and safe.


And if that's what he'd proposed, I probably wouldn't have chimed in at
all. Reread the original post. He said "10 'one meg' resistors in series".
I maintain that the value is too low.

I err on the side of safety simply because there are so many people here
recommending generally unsafe behavior without sufficient explanation of
why it might be OK in THIS case. Note that people (even famous repair
gurus) often fail to read
what's written, as suggested by your own comment above.

Remember that these posts hang around in the archive for a LONG time.
It's easy to imagine some kid reading a random post out of context,
generalizing the snippet of information and killing himself off.
Anybody who has to ask how to build a voltage divider shouldn't be using
their first attempt on a 2KV measurement inside an instrument that has
other sources of lethal voltages...especially when there are a bunch of
commercial HV probes on ebay for cheap. Save your death wish for
something that can save you some real money.

While I'm on a rant, I'll suggest a general process to be used when
posting advice to the web. You're familiar with the game "telephone".
Tell someone what you're about to post. Tell them to pass it on.
Come back a day later, pick someone 3 layers down the chain and see
if what they heard bears any resemblance to what you said. If it's
still safe, post it to the web. Soon you'll get to where you can do
this in your head quickly by reviewing your proposed post with a blank
mind. (I pride myself on my blank mind) The reader has no idea what
you meant.
All they know is what you wrote...then interpreted in their own unique way.

Just cause I got away with it for 30 years,
doesn't mean It's safe to recommend it to a newbie.

mike


However, I do agree with the other posts that if you plan to do more HV
stuff, be on the lookout for a proper HV probe.

--- sam | Sci.Electronics.Repair FAQ Mirror: http://repairfaq.ece.drexel.edu/
Repair | Main Table of Contents: http://repairfaq.ece.drexel.edu/REPAIR/
+Lasers | Sam's Laser FAQ: http://repairfaq.ece.drexel.edu/sam/lasersam.htm
| Mirror Sites: http://repairfaq.ece.drexel.edu/REPAIR/F_mirror.html

Note: These links are hopefully temporary until we can sort out the excessive
traffic on Repairfaq.org.

Important: Anything sent to the email address in the message header above is
ignored unless my full name is included in the subject line. Or, you can
contact me via the Feedback Form in the FAQs.




--
Return address is VALID.
Wanted, Slot 1 Motherboard
500MHz Tek DSOscilloscope TDS540 Make Offer
http://nm7u.tripod.com/homepage/te.html
Wanted, 12.1" LCD for Gateway Solo 5300. Samsung LT121SU-121
Bunch of stuff For Sale and Wanted at the link below.
http://www.geocities.com/SiliconValley/Monitor/4710/

  #9   Report Post  
Jim Yanik
 
Posts: n/a
Default

mike wrote in :

Sam Goldwasser wrote:
See: http://repairfaq.ece.drexel.edu/sam/hvprobe.htm

Building a voltage divider out of 10, 10M ohm resistors (and suitable
math!) is sufficient and safe.


And if that's what he'd proposed, I probably wouldn't have chimed in
at all. Reread the original post. He said "10 'one meg' resistors in
series". I maintain that the value is too low.


I agree,and it WILL load the HV circuit.Even a 100 Mohm probe does.
I used 4 22.5Mohm or 9 10Mohm 1/2w precision metal film resistors in series
for a simple probe for a 10 Mohm input Z DMM. It works great.


HV ------[90meg]------[DMM 10meg]-----/gnd

If you do the true divider way,you have to compensate for the DMM input
Z,if you want the probe to be reasonably accurate.

--
Jim Yanik
jyanik-at-kua.net
  #10   Report Post  
Asimov
 
Posts: n/a
Default

"Sam Goldwasser" bravely wrote to "All" (16 Dec 04 08:57:46)
--- on the heady topic of " HV Probe construction advice"

SG From: Sam Goldwasser

SG See: http://repairfaq.ece.drexel.edu/sam/hvprobe.htm

SG Building a voltage divider out of 10, 10M ohm resistors (and suitable
SG math!) is sufficient and safe.

SG However, I do agree with the other posts that if you plan to do more
SG HV stuff, be on the lookout for a proper HV probe.

The high voltage divider in a broken flyback can be used in a pinch.
There are high value resistors in it along with high value adjustable
pots for the screen and focus. It may be hard to get the potted stuff
out though...

A*s*i*m*o*v

.... You mean 15" sparks are SUPPOSED to come out of this thing?!?



  #11   Report Post  
Sam Goldwasser
 
Posts: n/a
Default

"Asimov" writes:

"Sam Goldwasser" bravely wrote to "All" (16 Dec 04 08:57:46)
--- on the heady topic of " HV Probe construction advice"

SG From: Sam Goldwasser

SG See: http://repairfaq.ece.drexel.edu/sam/hvprobe.htm

SG Building a voltage divider out of 10, 10M ohm resistors (and suitable
SG math!) is sufficient and safe.

SG However, I do agree with the other posts that if you plan to do more
SG HV stuff, be on the lookout for a proper HV probe.

The high voltage divider in a broken flyback can be used in a pinch.
There are high value resistors in it along with high value adjustable
pots for the screen and focus. It may be hard to get the potted stuff
out though...


If it's a one time measurement, no real need to disassemble it.

--- sam | Sci.Electronics.Repair FAQ Mirror: http://repairfaq.ece.drexel.edu/
Repair | Main Table of Contents: http://repairfaq.ece.drexel.edu/REPAIR/
+Lasers | Sam's Laser FAQ: http://repairfaq.ece.drexel.edu/sam/lasersam.htm
| Mirror Sites: http://repairfaq.ece.drexel.edu/REPAIR/F_mirror.html

Note: These links are hopefully temporary until we can sort out the excessive
traffic on Repairfaq.org.

Important: Anything sent to the email address in the message header above is
ignored unless my full name is included in the subject line. Or, you can
contact me via the Feedback Form in the FAQs.
  #12   Report Post  
Jim Adney
 
Posts: n/a
Default

On Thu, 16 Dec 2004 03:24:44 +0000 (UTC) Damian Menscher
wrote:

A pet project for the past several years is repairing an ancient
Tektronix 674 oscilloscope.


674? What's that? Is there a typo there?

Anyway, with the reduced load for lighting the tubes, I think this
needs to be recalibrated (intensity suggests it's got too much HV
now). The service manual indicates a HV test point, at which I
should read 2.2kV. Problem is, I'm just a hobbyist with no HV probe.


That's a pretty low voltage for a Tek scope, so my guess is that this
has already been divided down. This makes it more important that you
make a pretty high impedance divider.

Off I go to radioshack.com, where I find 5-packs of 1MOhm, 1/2W
resistors for $1. Recalling high school physics, I work out that one
of these should be able to drop 707V at .707mA. So if I put 10 in
series, I should be able to measure up to 7kV, and simply measure off
the last one and multiply the voltmeter reading by 10.


Could you buy 10 MOhm resistors, and then just use your 10 MOhm meter
as the last resistor in the chain? That would reduce the load on your
test point by a factor of 10, which would be a good thing.

One problem: Rat Shack says these have a "max working voltage" of
350V (see http://www.radioshack.com/search.asp?find=271-1134). Are
they just arbitrarily throwing a fudge factor of 2 away, or are they
mislabeling 1/4W resistors as 1/2W resistors, or what? (They list
10Ohm 1/2W resistors as 350V max also, so I'm guessing this is safe
to ignore.)


That's just the voltage where they are likely to flash over. Since
you're proposing to work with just 200 V on each one you should be
fine. It's probably still a good idea to solder these together first
and then wipe off the resistor bodys with alcohol to remove finger
oils which can make surface leakage or flashover problems.

Any other warnings I should heed before doing this? I realize I'll
lose some accuracy (they're only accurate to 5%). I guess I also
have to worry about affecting the circuit when I draw .7mA out of it?
I think my measurement error should be small, since my multimeter has
a 10MOhm imput impedence. Is that good enough, or should I use a
smaller final resistor in the voltage divider to reduce measurement
error?


Don't touch any of this while you're doing the measurement. .7mA is
probably too much for this circuit; .07mA is likely to be fine.

Finally, my crazy idea of protecting myself from electrocution is to
encase this thing in the plastic barrel of a Bic pen. They seem
about the right size, plastic doesn't conduct, and as long as I hold
it at the low-voltage end, it seems reasonably safe.


It's safe only until they flash over. Then it will destroy your meter
and would probably not kill you but really make you wish you'd used a
real HV probe.

Damian Menscher
--
-=#| Physics Grad Student & SysAdmin @ U Illinois Urbana-Champaign

There are probably 50 HV probes within a quarter mile of where you
work. Ask around.

-
-----------------------------------------------
Jim Adney
Madison, WI 53711 USA
-----------------------------------------------
  #13   Report Post  
Damian Menscher
 
Posts: n/a
Default

Jim Adney wrote:
On Thu, 16 Dec 2004 03:24:44 +0000 (UTC) Damian Menscher
wrote:


A pet project for the past several years is repairing an ancient
Tektronix 674 oscilloscope.


674? What's that? Is there a typo there?


Sorry, it's the 647. And the tubes are 5642s, not 5647s. Apparently
it doesn't work to remember numbers after it's been 2-3 years.

Anyway, with the reduced load for lighting the tubes, I think this
needs to be recalibrated (intensity suggests it's got too much HV
now). The service manual indicates a HV test point, at which I
should read 2.2kV. Problem is, I'm just a hobbyist with no HV probe.


That's a pretty low voltage for a Tek scope, so my guess is that this
has already been divided down. This makes it more important that you
make a pretty high impedance divider.


This is the "HV test point" (-2.2kV). The anode of the CRT gets the
higher voltage you're expecting: +11.8kV.

Off I go to radioshack.com, where I find 5-packs of 1MOhm, 1/2W
resistors for $1. Recalling high school physics, I work out that one
of these should be able to drop 707V at .707mA. So if I put 10 in
series, I should be able to measure up to 7kV, and simply measure off
the last one and multiply the voltmeter reading by 10.


Could you buy 10 MOhm resistors, and then just use your 10 MOhm meter
as the last resistor in the chain? That would reduce the load on your
test point by a factor of 10, which would be a good thing.


Radio Shack doesn't have anything larger than 1 MOhm. Hence my idea
to use so many of them. I suppose I could add more to the chain,
though. Or I could just get a real HV probe....

Finally, my crazy idea of protecting myself from electrocution is to
encase this thing in the plastic barrel of a Bic pen. They seem
about the right size, plastic doesn't conduct, and as long as I hold
it at the low-voltage end, it seems reasonably safe.


It's safe only until they flash over. Then it will destroy your meter
and would probably not kill you but really make you wish you'd used a
real HV probe.


Thanks to you and everyone else for the advice. I'll certainly think
more carefully about this (and probably just use a real HV probe).

Damian Menscher
--
-=#| Physics Grad Student & SysAdmin @ U Illinois Urbana-Champaign |#=-
-=#| 488 LLP, 1110 W. Green St, Urbana, IL 61801 Ofc217)333-0038 |#=-
-=#| 4602 Beckman, VMIL/MS, Imaging Technology Group217)244-3074 |#=-
-=#| www.uiuc.edu/~menscher/ Fax217)333-9819 |#=-
-=#| The above opinions are not necessarily those of my employers. |#=-
  #14   Report Post  
NSM
 
Posts: n/a
Default


"James Sweet" wrote in message
news:Ll7xd.258$h.247@trnddc04...
|
| "NSM" wrote in message
news:F37xd.5807$KO5.1825@clgrps13...

| If you ever have any doubts about the power of electricity, watch the
| episode of "The Beverly Hillbillies" where the bear eats granny's love
| letter and Jethro comes up with a plan to X-ray the bear.

| I would hardly think Beverly Hillbillies to be a good source of
information
| on electricity safety, anything in a TV show like that is pure
fabrication,
| it may coincide with real life but no more likely than what any random
| person would assume to be true.

After you see Jethro stick his tongue in the electric lamp socket (to show
what the "dumb old bear" will do) you'll understand why the prospect of the
electric chair fills him with dread.

N


  #15   Report Post  
Tom MacIntyre
 
Posts: n/a
Default

On Sun, 19 Dec 2004 05:06:25 GMT, "NSM" wrote:


"James Sweet" wrote in message
news:Ll7xd.258$h.247@trnddc04...
|
| "NSM" wrote in message
news:F37xd.5807$KO5.1825@clgrps13...

| If you ever have any doubts about the power of electricity, watch the
| episode of "The Beverly Hillbillies" where the bear eats granny's love
| letter and Jethro comes up with a plan to X-ray the bear.

| I would hardly think Beverly Hillbillies to be a good source of
information
| on electricity safety, anything in a TV show like that is pure
fabrication,
| it may coincide with real life but no more likely than what any random
| person would assume to be true.

After you see Jethro stick his tongue in the electric lamp socket (to show
what the "dumb old bear" will do) you'll understand why the prospect of the
electric chair fills him with dread.

N


Sheesh...it was bad enough when I was stupid and drunk enough to
tongue-test a 10.6 VDC, 850 mA cellphone adapter...

Tom


  #16   Report Post  
Tom MacIntyre
 
Posts: n/a
Default

On 16 Dec 2004 08:57:46 -0500, Sam Goldwasser
wrote:

See: http://repairfaq.ece.drexel.edu/sam/hvprobe.htm

Building a voltage divider out of 10, 10M ohm resistors (and suitable
math!) is sufficient and safe.


I've heard, though, that it isn't a good idea to handle resistors that
high with bare hands, due to the possibility of hand oils conducting
current across the resistors.

Tom


However, I do agree with the other posts that if you plan to do more HV
stuff, be on the lookout for a proper HV probe.

--- sam | Sci.Electronics.Repair FAQ Mirror: http://repairfaq.ece.drexel.edu/
Repair | Main Table of Contents: http://repairfaq.ece.drexel.edu/REPAIR/
+Lasers | Sam's Laser FAQ: http://repairfaq.ece.drexel.edu/sam/lasersam.htm
| Mirror Sites: http://repairfaq.ece.drexel.edu/REPAIR/F_mirror.html

Note: These links are hopefully temporary until we can sort out the excessive
traffic on Repairfaq.org.

Important: Anything sent to the email address in the message header above is
ignored unless my full name is included in the subject line. Or, you can
contact me via the Feedback Form in the FAQs.


  #17   Report Post  
Sam Goldwasser
 
Posts: n/a
Default

Tom MacIntyre writes:

On 16 Dec 2004 08:57:46 -0500, Sam Goldwasser
wrote:

See: http://repairfaq.ece.drexel.edu/sam/hvprobe.htm

Building a voltage divider out of 10, 10M ohm resistors (and suitable
math!) is sufficient and safe.


I've heard, though, that it isn't a good idea to handle resistors that
high with bare hands, due to the possibility of hand oils conducting
current across the resistors.


Yes, once everything is soldered, they should be cleaned. Although, at
100M total, it probably isn't that significant.

--- sam | Sci.Electronics.Repair FAQ Mirror: http://repairfaq.ece.drexel.edu/
Repair | Main Table of Contents: http://repairfaq.ece.drexel.edu/REPAIR/
+Lasers | Sam's Laser FAQ: http://repairfaq.ece.drexel.edu/sam/lasersam.htm
| Mirror Sites: http://repairfaq.ece.drexel.edu/REPAIR/F_mirror.html

Note: These links are hopefully temporary until we can sort out the excessive
traffic on Repairfaq.org.

Important: Anything sent to the email address in the message header above is
ignored unless my full name is included in the subject line. Or, you can
contact me via the Feedback Form in the FAQs.
  #18   Report Post  
Jim Adney
 
Posts: n/a
Default

On Sun, 19 Dec 2004 03:38:11 +0000 (UTC) Damian Menscher
wrote:

Jim Adney wrote:
On Thu, 16 Dec 2004 03:24:44 +0000 (UTC) Damian Menscher
wrote:


A pet project for the past several years is repairing an ancient
Tektronix 674 oscilloscope.


674? What's that? Is there a typo there?


Sorry, it's the 647. And the tubes are 5642s, not 5647s. Apparently
it doesn't work to remember numbers after it's been 2-3 years.


How about 547? Does that sound any more familiar? ;-)

Could you buy 10 MOhm resistors, and then just use your 10 MOhm meter
as the last resistor in the chain? That would reduce the load on your
test point by a factor of 10, which would be a good thing.


Radio Shack doesn't have anything larger than 1 MOhm. Hence my idea
to use so many of them. I suppose I could add more to the chain,
though. Or I could just get a real HV probe....


If you really want to do this, let me know and I'll see if I can come
up with 90 MOhms of resistors to send you. (In the suggestion above, I
meant to suggest that you use nine rather than ten 10 MOhm resistors
in series with your 10 MOhm voltmeter.

Does your voltmeter have a 10 MOhm input impedance? What model is it?

-
-----------------------------------------------
Jim Adney
Madison, WI 53711 USA
-----------------------------------------------
  #19   Report Post  
Damian Menscher
 
Posts: n/a
Default

Jim Adney wrote:
On Sun, 19 Dec 2004 Damian Menscher wrote:
Jim Adney wrote:
On Thu, 16 Dec 2004 Damian Menscher wrote:


A pet project for the past several years is repairing an ancient
Tektronix 674 oscilloscope.


674? What's that? Is there a typo there?


Sorry, it's the 647. And the tubes are 5642s, not 5647s. Apparently
it doesn't work to remember numbers after it's been 2-3 years.


How about 547? Does that sound any more familiar? ;-)


No, it's definitely a 647. Got it right here in front of me. Looks
to be one of the earlier ones, judging by the serial number.

It has a strange malfunction which I've been trying (unsuccessfully)
to correct for the past 10 years or so. I'll start a new thread about
that, though, since it has little to do with this thread.

Could you buy 10 MOhm resistors, and then just use your 10 MOhm meter
as the last resistor in the chain? That would reduce the load on your
test point by a factor of 10, which would be a good thing.


Radio Shack doesn't have anything larger than 1 MOhm. Hence my idea
to use so many of them. I suppose I could add more to the chain,
though. Or I could just get a real HV probe....


If you really want to do this, let me know and I'll see if I can come
up with 90 MOhms of resistors to send you. (In the suggestion above, I
meant to suggest that you use nine rather than ten 10 MOhm resistors
in series with your 10 MOhm voltmeter.


You'll have to explain to me how to use the meter as part of the
chain... that doesn't really make sense to me. Also, doesn't that
impact the safety of the measurement?

Does your voltmeter have a 10 MOhm input impedance? What model is it?


Yes, it does. It's a BK Precision 2703B. Spec sheet is available
online at:
http://www.bkprecision.com/www/np_pdf.asp?m=2703B
and shows a 10 MOhm resistance for DC measurements (which is what I'm
planning to do with it.

Damian Menscher
--
-=#| Physics Grad Student & SysAdmin @ U Illinois Urbana-Champaign |#=-
-=#| 488 LLP, 1110 W. Green St, Urbana, IL 61801 Ofc217)333-0038 |#=-
-=#| 4602 Beckman, VMIL/MS, Imaging Technology Group217)244-3074 |#=-
-=#| www.uiuc.edu/~menscher/ Fax217)333-9819 |#=-
-=#| The above opinions are not necessarily those of my employers. |#=-
  #20   Report Post  
Jim Adney
 
Posts: n/a
Default

On Wed, 22 Dec 2004 04:47:04 +0000 (UTC) Damian Menscher
wrote:

Jim Adney wrote:


How about 547? Does that sound any more familiar? ;-)


No, it's definitely a 647. Got it right here in front of me. Looks
to be one of the earlier ones, judging by the serial number.

It has a strange malfunction which I've been trying (unsuccessfully)
to correct for the past 10 years or so. I'll start a new thread about
that, though, since it has little to do with this thread.


Is this a solid state scope?

You'll have to explain to me how to use the meter as part of the
chain... that doesn't really make sense to me. Also, doesn't that
impact the safety of the measurement?


Think of ten 10 MOhm resistors in series. Each one sees 1/10th of the
total voltage. Now replace the bottom resistor with your meter, which
just looks like a 10 MOhm resistor when you put it in a circuit.

Now your meter reads out the voltage it sees across itself, which is
1/10th of the total. So the total voltage is 10 times the meter
reading.

As long as you keep your fingers away from the upper resistors, I
don't think you're in any real danger at the voltage levels you're
talking about here. Of course you need to make sure that the low side
test lead of your meter is securely grounded before you start this.
Otherwise, the meter itself will float up to the full voltage.

A better probe for high voltages would actually have a 990 MOhm
resistor. Then you get to multiply by 100, and there is even less
loading of the test point. But resistors for this will be much harder
to find. A probe made to work to 40 kV would be made like this.

-
-----------------------------------------------
Jim Adney
Madison, WI 53711 USA
-----------------------------------------------


  #21   Report Post  
Damian Menscher
 
Posts: n/a
Default

Jim Adney wrote:
On Wed, 22 Dec 2004 04:47:04 +0000 (UTC) Damian Menscher
wrote:
Jim Adney wrote:


How about 547? Does that sound any more familiar? ;-)


No, it's definitely a 647. Got it right here in front of me. Looks
to be one of the earlier ones, judging by the serial number.

It has a strange malfunction which I've been trying (unsuccessfully)
to correct for the past 10 years or so. I'll start a new thread about
that, though, since it has little to do with this thread.


Is this a solid state scope?


Yup, but it still uses tubes as rectifiers in the CRT supply. It's
kind of a weird hybrid, since everything else is solid state. I'm
guessing they either didn't have solid state diodes that could handle
the HV, or they thought the tubes would give a smoother output.

I should take some pictures of it... I get the feeling this was a rare
one. Even Google the Omniscient doesn't have pics of it.

Damian Menscher
--
-=#| Physics Grad Student & SysAdmin @ U Illinois Urbana-Champaign |#=-
-=#| 488 LLP, 1110 W. Green St, Urbana, IL 61801 Ofc217)333-0038 |#=-
-=#| 4602 Beckman, VMIL/MS, Imaging Technology Group217)244-3074 |#=-
-=#| www.uiuc.edu/~menscher/ Fax217)333-9819 |#=-
-=#| The above opinions are not necessarily those of my employers. |#=-
  #22   Report Post  
Damian Menscher
 
Posts: n/a
Default

Damian Menscher wrote:
Jim Adney wrote:
On Wed, 22 Dec 2004 04:47:04 +0000 (UTC) Damian Menscher
wrote:
Jim Adney wrote:


How about 547? Does that sound any more familiar? ;-)

No, it's definitely a 647. Got it right here in front of me. Looks
to be one of the earlier ones, judging by the serial number.


Is this a solid state scope?

Yup, but it still uses tubes as rectifiers in the CRT supply. It's
kind of a weird hybrid, since everything else is solid state. I'm
guessing they either didn't have solid state diodes that could handle
the HV, or they thought the tubes would give a smoother output.


I should take some pictures of it... I get the feeling this was a rare
one. Even Google the Omniscient doesn't have pics of it.


Here are pictures, for anyone interested:
http://www.uiuc.edu/~menscher/Tek647/

Now that it's working, I'll have to think about what to do with it!

Damian Menscher
--
-=#| Physics Grad Student & SysAdmin @ U Illinois Urbana-Champaign |#=-
-=#| 488 LLP, 1110 W. Green St, Urbana, IL 61801 Ofc217)333-0038 |#=-
-=#| 4602 Beckman, VMIL/MS, Imaging Technology Group217)244-3074 |#=-
-=#| www.uiuc.edu/~menscher/ Fax217)333-9819 |#=-
-=#| The above opinions are not necessarily those of my employers. |#=-
Reply
Thread Tools Search this Thread
Search this Thread:

Advanced Search
Display Modes

Posting Rules

Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are On
Pingbacks are On
Refbacks are On


Similar Threads
Thread Thread Starter Forum Replies Last Post
Risk Management/Shop Safety and Advice (long) charlie b Woodworking 8 June 9th 04 10:51 PM
Newbie to Veneering Needs Advice Jack Schumacher Woodworking 6 June 9th 04 08:39 PM
Appraisal, refinance, and "under construction" question DanceRat Home Ownership 2 April 19th 04 10:01 PM
Single Wall Construction Extension - Problems?? SuzySue UK diy 2 January 8th 04 01:16 PM


All times are GMT +1. The time now is 07:06 PM.

Powered by vBulletin® Copyright ©2000 - 2024, Jelsoft Enterprises Ltd.
Copyright ©2004-2024 DIYbanter.
The comments are property of their posters.
 

About Us

"It's about DIY & home improvement"