CRT anode discharge


Dave Peterson
 

Thank you for saving my life!

Or at least a painful reminder.

Just experienced my first healthy CRT anode discharge arc. Thanks to advice I've read here I: grounded the chassis, kept one hand behind my back, used a pair of insulated pliers to gently twist and pull the anode connection out, and before I could even consciously try, ZAP! To ground. Big 'ol blue nasty looking arc. But not even a mark on the chassis.

I'll have to see if my Fitbit registers a sudden spike in heart rate! :P

Been getting "comfortable" working around the HV stuff, but this is a good reminder to keep my guard up. That was a nasty looking arc. Spark seems too mild a word. But I'm fine. Scope appears fine.

Credit to those here who've espoused save HV practices.
Dave


Tom Lee
 

Being the discharge path for a scope CRT is certainly unpleasant, but not particularly dangerous if the unit isn't powered up (and if you're in reasonably good health). Sure wakes you up, though, doesn't it?

If you do discharge the HV with a shorting tool, be aware that you'll need to keep the shorting tool in place a good deal longer than you'd think. If you remove the tool soon after hearing or seeing the discharge, you'll be unpleasantly surprised to find that the voltage soon climbs back up to some non-negligible percentage of the original value. A "discharged" HV cap (including ones that may be integral with the CRT) can still bite you.

Tom

--
Prof. Thomas H. Lee
Allen Ctr., Rm. 205
350 Jane Stanford Way
Stanford University
Stanford, CA 94305-4070
http://www-smirc.stanford.edu

On 3/14/2021 13:40, Dave Peterson via groups.io wrote:
Thank you for saving my life!

Or at least a painful reminder.

Just experienced my first healthy CRT anode discharge arc. Thanks to advice I've read here I: grounded the chassis, kept one hand behind my back, used a pair of insulated pliers to gently twist and pull the anode connection out, and before I could even consciously try, ZAP! To ground. Big 'ol blue nasty looking arc. But not even a mark on the chassis.

I'll have to see if my Fitbit registers a sudden spike in heart rate! :P

Been getting "comfortable" working around the HV stuff, but this is a good reminder to keep my guard up. That was a nasty looking arc. Spark seems too mild a word. But I'm fine. Scope appears fine.

Credit to those here who've espoused save HV practices.
Dave




Dave Peterson
 

Being a former technician getting bit is something I've experienced, but never anything more than line voltage. Military 400Hz power has a definitely different flavor than civilian 60Hz. I don't like being bit at all. So far no scars.

I suspect it just as likely that being bit by 40KV (or whatever it decays to in 5 minutes) can cause cardiac arrest out of shear fright. This is not the first CRT I've pulled, but the first time I've ever experienced such an arc. Still feel some lingering adrenaline metabolizing. Who needs a fourth cup of coffee on a Sunday?

Yeah, I put it back in the sheath while my shakes settled down. Found it still snapped when removed again. Left it wedged to the chassis for several minutes.

This is a pre SN250K 465. Could that have anything to do with it retaining charge more than the other post SN250K 465s I've pulled?

I went ahead and posted this as a cautionary tale for any new arrivals. Too bad we can't have sticky posts at the top of the list. HV safety is always a good thing. Complacency is sometimes corrected - harshly.

Stay safe folks!
Dave

On Sunday, March 14, 2021, 01:55:06 PM PDT, Tom Lee <tomlee@ee.stanford.edu> wrote:

Being the discharge path for a scope CRT is certainly unpleasant, but
not particularly dangerous if the unit isn't powered up (and if you're
in reasonably good health). Sure wakes you up, though, doesn't it?

If you do discharge the HV with a shorting tool, be aware that you'll
need to keep the shorting tool in place a good deal longer than you'd
think. If you remove the tool soon after hearing or seeing the
discharge, you'll be unpleasantly surprised to find that the voltage
soon climbs back up to some non-negligible percentage of the original
value. A "discharged" HV cap (including ones that may be integral with
the CRT) can still bite you.

Tom

--
Prof. Thomas H. Lee
Allen Ctr., Rm. 205
350 Jane Stanford Way
Stanford University
Stanford, CA 94305-4070
http://www-smirc.stanford.edu

On 3/14/2021 13:40, Dave Peterson via groups.io wrote:
Thank you for saving my life!

Or at least a painful reminder.

Just experienced my first healthy CRT anode discharge arc. Thanks to advice I've read here I: grounded the chassis, kept one hand behind my back, used a pair of  insulated pliers to gently twist and pull the anode connection out, and before I could even consciously try, ZAP! To ground. Big 'ol blue nasty looking arc. But not even a mark on the chassis.

I'll have to see if my Fitbit registers a sudden spike in heart rate! :P

Been getting "comfortable" working around the HV stuff, but this is a good reminder to keep my guard up. That was a nasty looking arc. Spark seems too mild a word. But I'm fine. Scope appears fine.

Credit to those here who've espoused save HV practices.
Dave





Sean Turner
 

I got zapped once by the residual charge in a 7904 crt. I would characterize it as no worse than a very strong static shock of the type that are common on a windy days in the American southwest (I've had some painful ones). However, this jumped a *much* larger gap!! Fortunately, the current is very low. If you're following standard good practices for working around this stuff like the OP was, very unlikely to kill you unless you have certain health problems already.

Now, on the other hand, the B+ supplies in 500 series scopes we all love are low impedance and can supply significant current. Those should be treated with the utmost respect, as they will most certainly not suffer you to make the same mistake twice.

Sean

On Sun, Mar 14, 2021 at 01:54 PM, Tom Lee wrote:


Being the discharge path for a scope CRT is certainly unpleasant, but not
particularly dangerous if the unit isn't powered up (and if you're in
reasonably good health). Sure wakes you up, though, doesn't it?

If you do discharge the HV with a shorting tool, be aware that you'll need to
keep the shorting tool in place a good deal longer than you'd think. If you
remove the tool soon after hearing or seeing the discharge, you'll be
unpleasantly surprised to find that the voltage soon climbs back up to some
non-negligible percentage of the original value. A "discharged" HV cap
(including ones that may be integral with the CRT) can still bite you.

Tom


 

Dave,

Wow! Glad you didn't take the shock yourself.

Tom,

Good safety tip! (inscribes "Discharge CRT AT LEAST TWICE" in three inch tall red lettering on the wall behind the bench)

-- Jeff Dutky


Dave Peterson
 

Good to hear that, albeit painful, the anode charge should not be deadly. I choose to test neither.

I guess one of the aspects I wanted to put out there was: follow safe practices. They work.

My prior experiences that the anode was discharged by the time I was pulling the connector was probably leading me to complacency. I was about to grab it with pliers when I told myself, nah, put the ground on, do it one handed. Happy I did. The methodology works.

I'm still curious why this one was so significant. I've pulled a freshly hot anode before, and nothing. Did Tek change hardware slightly to provide a slow leakage path? The HV line in this one is white, where the others I've dealt with are red. Plenty of carbonized crud around this one, as I've seen on others. The SN<250K CRT is different than the SN>250K. Was there a change in CRT construction/process that makes the later less capacitive?

Fully grounded,
Dave

On Sunday, March 14, 2021, 02:57:10 PM PDT, Sean Turner <sdturne@q.com> wrote:

I got zapped once by the residual charge in a 7904 crt. I would characterize it as no worse than a very strong static shock of the type that are common on a windy days in the American southwest (I've had some painful ones). However, this jumped a *much* larger gap!! Fortunately, the current is very low. If you're following standard good practices for working around this stuff like the OP was, very unlikely to kill you unless you have certain health problems already.

Now, on the other hand, the B+ supplies in 500 series scopes we all love are low impedance and can supply significant current. Those should be treated with the utmost respect, as they will most certainly not suffer you to make the same mistake twice.

Sean

On Sun, Mar 14, 2021 at 01:54 PM, Tom Lee wrote:


Being the discharge path for a scope CRT is certainly unpleasant, but not
particularly dangerous if the unit isn't powered up (and if you're in
reasonably good health). Sure wakes you up, though, doesn't it?

If you do discharge the HV with a shorting tool, be aware that you'll need to
keep the shorting tool in place a good deal longer than you'd think. If you
remove the tool soon after hearing or seeing the discharge, you'll be
unpleasantly surprised to find that the voltage soon climbs back up to some
non-negligible percentage of the original value. A "discharged" HV cap
(including ones that may be integral with the CRT) can still bite you.

Tom


Dave Seiter
 

My father used to discharge CRTs with a very long grounded screwdriver, and he taught me the same thing (regarding doing it slowly).  Even so, I got bit a few times, including once (very mild) by a CRT that had been grounded, removed and sitting on the bench.  I'd swear it "accumulated" a charge from just sitting there.
To me, getting hit by 120AC feels like oatmeal getting pushed into your finger.  HV feels more like getting stabbed by one of those vibrating etching tools, red hot, on over drive.

-Dave (I've become much more careful as I get older...)

On Sunday, March 14, 2021, 01:55:07 PM PDT, Tom Lee <tomlee@ee.stanford.edu> wrote:

Being the discharge path for a scope CRT is certainly unpleasant, but
not particularly dangerous if the unit isn't powered up (and if you're
in reasonably good health). Sure wakes you up, though, doesn't it?

If you do discharge the HV with a shorting tool, be aware that you'll
need to keep the shorting tool in place a good deal longer than you'd
think. If you remove the tool soon after hearing or seeing the
discharge, you'll be unpleasantly surprised to find that the voltage
soon climbs back up to some non-negligible percentage of the original
value. A "discharged" HV cap (including ones that may be integral with
the CRT) can still bite you.

Tom

--
Prof. Thomas H. Lee
Allen Ctr., Rm. 205
350 Jane Stanford Way
Stanford University
Stanford, CA 94305-4070
http://www-smirc.stanford.edu

On 3/14/2021 13:40, Dave Peterson via groups.io wrote:
Thank you for saving my life!

Or at least a painful reminder.

Just experienced my first healthy CRT anode discharge arc. Thanks to advice I've read here I: grounded the chassis, kept one hand behind my back, used a pair of  insulated pliers to gently twist and pull the anode connection out, and before I could even consciously try, ZAP! To ground. Big 'ol blue nasty looking arc. But not even a mark on the chassis.

I'll have to see if my Fitbit registers a sudden spike in heart rate! :P

Been getting "comfortable" working around the HV stuff, but this is a good reminder to keep my guard up. That was a nasty looking arc. Spark seems too mild a word. But I'm fine. Scope appears fine.

Credit to those here who've espoused save HV practices.
Dave





n49ex
 

I would also suggest that you discharge the CRT and HV power supply using a resistor to ground. It can be fairly high resistance, say 50K. Take a clip lead grounded resistor and slip the other end under the rubber cap to discharge both the crt and supply filter caps together. It shouldn't be a hard short because a sudden discharge can create enough ringing or transients to possibly damage the HV multiplier.
Reinhard Metz


Brad Thompson
 

Dave Seiter wrote on 3/14/2021 7:51 PM:

My father used to discharge CRTs with a very long grounded screwdriver, and he taught me the same thing (regarding doing it slowly).  Even so, I got bit a few times, including once (very mild) by a CRT that had been grounded, removed and sitting on the bench.  I'd swear it "accumulated" a charge from just sitting there.
Hello--

IIRC, dielectric absorption is responsible for the recharge effect. I agree with OPs
who recommended using a series resistor  to limit surge current in the grounding line.

Off-topic: I recently underwent some nerve-conductivity tests which involved placing a
sensor band around my wrist and electrode needles in my bicep. In between, an instrument resembling a
multitrace oscilloscope monitored the responses evoked by electric shocks applied
to my bicep. Each shock triggered a whole-body twitch that levitated me an inch or so out of my chair.

After the first dozen or so shocks, I observed that I hadn't had this much fun since I built
my first ham-radio project<g>.

73--

Brad  AA1IP


Harvey White
 

From what I understand, the electrons orbit eccentricities store the energy.  When you discharge a capacitor, you discharge it with respect to whatever series resistance it has (ESR).  That ESR means that it takes a while for the charge to bleed down to zero.

One single zap doesn't get all the electrons in the dielectric, because they haven't been discharged yet.

One single zap therefore leaves charge behind, which then distributes itself through the dielectric.

Still charged, not quite so much.

ZAP II......

Harvey

On 3/14/2021 7:51 PM, Dave Seiter wrote:
My father used to discharge CRTs with a very long grounded screwdriver, and he taught me the same thing (regarding doing it slowly).  Even so, I got bit a few times, including once (very mild) by a CRT that had been grounded, removed and sitting on the bench.  I'd swear it "accumulated" a charge from just sitting there.
To me, getting hit by 120AC feels like oatmeal getting pushed into your finger.  HV feels more like getting stabbed by one of those vibrating etching tools, red hot, on over drive.

-Dave (I've become much more careful as I get older...)

On Sunday, March 14, 2021, 01:55:07 PM PDT, Tom Lee <tomlee@ee.stanford.edu> wrote:
Being the discharge path for a scope CRT is certainly unpleasant, but
not particularly dangerous if the unit isn't powered up (and if you're
in reasonably good health). Sure wakes you up, though, doesn't it?

If you do discharge the HV with a shorting tool, be aware that you'll
need to keep the shorting tool in place a good deal longer than you'd
think. If you remove the tool soon after hearing or seeing the
discharge, you'll be unpleasantly surprised to find that the voltage
soon climbs back up to some non-negligible percentage of the original
value. A "discharged" HV cap (including ones that may be integral with
the CRT) can still bite you.

Tom


Michael W. Lynch
 

Speaking of HV discharges, when I was an Automotive Technology Student back in the day, GM donated several cars with the then "new" HEI distributors. One thing that I knew about HEI was that it was capable of generating 40-50kV of spark energy. This system was designed to fire spark plugs under the harshest and most unfavorable conditions, HEI could deliver a reliable spark inside a combustion chamber over a 1/8 inch gap. Standard ignitions of the day would struggle at .030 - 035" with old plugs. Most people in the class did not know this, There was a classmate of mine that claimed he could hold a spark lead on "any car" and "not even feel it". He took a lead in one hand and when they started the car, the voltage from the distributor flowed through his body and found its way back to the fender of the car through certain private parts that were close enough to the car to create a spark gap that the HEI was capable of jumping. Needless to say, he never held another HEI spark lead.

--
Michael Lynch
Dardanelle, AR


greenboxmaven
 

Pranks and daring with the ignition systems on almost any vehicle made since 1970 are  truly dangerous.  I had them played on me at my father's garage, and had scrapes and cuts from flinching, fortunately I didn't lose a finger in a fan belt, many did.   All magnetos regardless of age,  can deliver dangerous currents, even if the voltage is too low to produce a spark.  Many aircraft magnetos have a built in shorting bar that will prevent any voltage from building up unless the magneto has a special connector inserted.  A careless schlock repair caused the connector to come out of a magneto on an airshow B-17 last year and contributed greatly to a horrible crash with several deaths.

     Bruce Gentry, KA2IVY

On 3/14/21 21:24, Michael W. Lynch via groups.io wrote:
Speaking of HV discharges, when I was an Automotive Technology Student back in the day, GM donated several cars with the then "new" HEI distributors. One thing that I knew about HEI was that it was capable of generating 40-50kV of spark energy. This system was designed to fire spark plugs under the harshest and most unfavorable conditions, HEI could deliver a reliable spark inside a combustion chamber over a 1/8 inch gap. Standard ignitions of the day would struggle at .030 - 035" with old plugs. Most people in the class did not know this, There was a classmate of mine that claimed he could hold a spark lead on "any car" and "not even feel it". He took a lead in one hand and when they started the car, the voltage from the distributor flowed through his body and found its way back to the fender of the car through certain private parts that were close enough to the car to create a spark gap that the HEI was capable of jumping. Needless to say, he never held another HEI spark lead.


Wayne
 

This discussion brings back memories of working in a TV repair shop in the 60s--a guy on the next bench was the macho type, and he could "handle" any discharge from a CRT anode. And, besides, in this case, "it has been turned off for several days so there's no problem at all". He reached in, attempted to remove the lead, and jerked his arm out of the back of the set with a yell. Assuming it was now completely discharged, he reached back in and did the same thing. Unfortunately, this time his arm caught the point of a sheet metal screw sticking into the case and he tore a deep gash from his elbow to his wrist--there was a LOT of blood. It isn't just the electrons you have to worry about.


Dave Peterson
 

Keep in mind this is a 465 CRT anode connector. The anode to tube is not like a TV connector where you can stick a screwdriver under the rubber cap. The actual conductor contact on the 465 anode is at the end of a ~2" insulated male end inside the corresponding female HV multiplier connector. The female lead is entirely inaccessible beneath the CRT and behind the vertical assembly within a shroud. And even if one were to disassemble the vertical assembly and remove the shroud, the HV lead is entirely insulated into the HV multiplier potting material. The only way to discharge the anode is to pull the male CRT lead.

Theoretically one could then touch the contact to a resistor to ground, but that's not likely. What happened today is that even gently rotating and pulling the male lead out of the connector resulted in a big fat arc to the chassis that is about 3/16" away. Theoretically I could just as well have been holding the anode connector in my hand and the arc would have jumped to the chassis - 3/16" to chassis vs. 2" to me. I'm still very happy to have been further isolated by a pair of insulated pliers. I'm sure the pliers insulation would actually be of little value. But the inherent design of the setup would make it difficult to get closer than the 2" of CRT connector.


See the picture of a tube I pulled sometime back. This is with a SN>250K tube with the red wire. The tip of that white connector is buried in the HV multiplier lead, and one would need to reposition your hand from the collar back by the red wire to get close to it. It'd be almost impossible to not arc it to the chassis when in-situ. Even the service manual calls for shorting the lead to the chassis after disconnection.

Still. Startled the crap out of me. Never had one pop like that. I have in the back of my mind the guy from BlueGlow Electronics (YouTube) describing making a hole in his elbow discharging a big cap with a screwdriver. But he's dealing with tubes and probably a big B+ power supply.

Following his advice I did make up a resistor discharger I use on the HV section caps. Part of taking the CRT out involves using that to discharge all the contacts on the rear CRT connector. I also check voltages before I touch any of it.

Anecdote: I used to be a "Vulcan Repairer". We worked in the same platoon as "FAAR Repairer" techs. Their transmitter case was a large roll-out drawer, and they had a big wooden pole with a metal ground loop - about 12" radius. They were to roll the drawer out and use the pole and loop to discharge the caps before working on it.

Fun stuff.
Dave

On Sunday, March 14, 2021, 05:12:37 PM PDT, n49ex via groups.io <n49ex=aol.com@groups.io> wrote:

I would also suggest that you discharge the CRT and HV power supply using a resistor to ground. It can be fairly high resistance, say 50K. Take a clip lead grounded resistor and slip the other end under the rubber cap to discharge both the crt and supply filter caps together. It shouldn't be a hard short because a sudden discharge can create enough ringing or transients to possibly damage the HV multiplier.
Reinhard Metz


Gordon Smith
 

On Sun, Mar 14, 2021 at 04:51 PM, Dave Seiter wrote:


My father used to discharge CRTs with a very long grounded screwdriver, and
he taught me the same thing (regarding doing it slowly).  Even so, I got bit
a few times, including once (very mild) by a CRT that had been grounded,
removed and sitting on the bench.  I'd swear it "accumulated" a charge from
just sitting there.

-Dave (I've become much more careful as I get older...)
Hi Dave,
From my experience, I believe it did accumulate a charge. My company creates high voltage/high amperage power supplies for use in our products. One of the things I was involved in at my work was helping test out high voltage/high capacity capacitors used in the power supplies. And I mean high capacity. From my memory, the one we tested out was approximately 2'x2'x4' high, and this was a standard size. One of the things that you did when storing them is wrap a wire around the terminals because if you didn't it could accumulate an electrical charge large enough to kill you. This was stressed multiple times by the technician I was working with who had worked with these components for years. And I was taught to always look for that shorting wire whenever I was around those capacitors. Since a CRT acts on one level as a capacitor, I can very well see it accumulating a charge.

Thank You,
Gordon


greenboxmaven
 

For a TV repair tale, I have a good one to share. I had started work at a TV shop when I as in high school.  Soon I was on a repair call with another tech, the set was an RCA 21inch round tube color set with a huge  cabinet. We were greeted at the door by a very nasty chihuahua dog nipping at my socks.  I removed the back cover and was preparing to to do the purity and convergence adjustments after replacing the 3A3 and 6BK4 to get the high voltage correct. Within a few minutes, the dog crawled behind the set and climbed inside. I could not believe what was happening. With a few seconds, it sniffed at the cap of the 3A3.  After a loud corona arc, the dog recoiled inside the cabinet a few times, I had leaned back against the wall. The dog lept right at my chest, bounded over the set, and down the hall with all four legs moving at mach 2. I got a torn shirt pocket and a few scratches, but  with resolve I have never had before or since, I remained professional and finished the repair. Once we got into the truck, I lost it and laughed so hard I couldn't breath.

      Bruce Gentry, KA2IVY

On 3/14/21 21:56, Wayne via groups.io wrote:
This discussion brings back memories of working in a TV repair shop in the 60s--a guy on the next bench was the macho type, and he could "handle" any discharge from a CRT anode. And, besides, in this case, "it has been turned off for several days so there's no problem at all". He reached in, attempted to remove the lead, and jerked his arm out of the back of the set with a yell. Assuming it was now completely discharged, he reached back in and did the same thing. Unfortunately, this time his arm caught the point of a sheet metal screw sticking into the case and he tore a deep gash from his elbow to his wrist--there was a LOT of blood. It isn't just the electrons you have to worry about.




Sean Turner
 

What might be even more amazing about that story, is that you didn't have to scrap toasty dog out of that set. Seriously, a dog that small is lucky to be alive after that!

Sean

On Sun, Mar 14, 2021 at 08:10 PM, greenboxmaven wrote:


For a TV repair tale, I have a good one to share. I had started work at a TV
shop when I as in high school.  Soon I was on a repair call with another
tech, the set was an RCA 21inch round tube color set with a huge  cabinet. We
were greeted at the door by a very nasty chihuahua dog nipping at my socks. 
I removed the back cover and was preparing to to do the purity and convergence
adjustments after replacing the 3A3 and 6BK4 to get the high voltage correct.
Within a few minutes, the dog crawled behind the set and climbed inside. I
could not believe what was happening. With a few seconds, it sniffed at the
cap of the 3A3.  After a loud corona arc, the dog recoiled inside the cabinet
a few times, I had leaned back against the wall. The dog lept right at my
chest, bounded over the set, and down the hall with all four legs moving at
mach 2. I got a torn shirt pocket and a few scratches, but  with resolve I
have never had before or since, I remained professional and finished the
repair. Once we got into the truck, I lost it and laughed so hard I couldn't
breath.

Bruce Gentry, KA2IVY


greenboxmaven
 

The dog definately survived. I think the fact the zap was 15 KC (skin effect) and the cabinet being  wood kept the internal current low.    I have never heard of dogs or cats being kiled by TVs, rats and mice are a totally different and gross issue.

             Bruce Gentry, KA2IVY

On 3/14/21 23:43, Sean Turner wrote:
What might be even more amazing about that story, is that you didn't have to scrap toasty dog out of that set. Seriously, a dog that small is lucky to be alive after that!

Sean

On Sun, Mar 14, 2021 at 08:10 PM, greenboxmaven wrote:

For a TV repair tale, I have a good one to share. I had started work at a TV
shop when I as in high school.  Soon I was on a repair call with another
tech, the set was an RCA 21inch round tube color set with a huge  cabinet. We
were greeted at the door by a very nasty chihuahua dog nipping at my socks.
I removed the back cover and was preparing to to do the purity and convergence
adjustments after replacing the 3A3 and 6BK4 to get the high voltage correct.
Within a few minutes, the dog crawled behind the set and climbed inside. I
could not believe what was happening. With a few seconds, it sniffed at the
cap of the 3A3.  After a loud corona arc, the dog recoiled inside the cabinet
a few times, I had leaned back against the wall. The dog lept right at my
chest, bounded over the set, and down the hall with all four legs moving at
mach 2. I got a torn shirt pocket and a few scratches, but  with resolve I
have never had before or since, I remained professional and finished the
repair. Once we got into the truck, I lost it and laughed so hard I couldn't
breath.

Bruce Gentry, KA2IVY


Sean Turner
 

Ewwwww, I can only imagine...

Sean

On Sun, Mar 14, 2021 at 09:04 PM, greenboxmaven wrote:


The dog definately survived. I think the fact the zap was 15 KC (skin effect)
and the cabinet being  wood kept the internal current low.    I have never
heard of dogs or cats being kiled by TVs, rats and mice are a totally
different and gross issue.

Bruce Gentry, KA2IVY


Roger M
 

Hi Gordon,
Didn't anyone you worked with, at some point, use the synonymous terms
"dielectric soak" or "dielectric absorption"? If not they sure should have.
Both are pertinent here and oh, so, Google-able. ;-)
-Roger