Author Topic: Got shocked by a CRT  (Read 2272 times)

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Offline Halcyon

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Re: Got shocked my a CRT
« Reply #25 on: January 30, 2021, 01:52:39 am »
Hm, josh132 created this thread 5 days ago. I wonder is he ok ?  :-//

According to his forum profile, he was last active today. I'd say he is fine.
 

Offline james_s

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Re: Got shocked my a CRT
« Reply #26 on: January 30, 2021, 07:30:41 pm »
I am somewhat leery of the "screwdriver" method, for the reason that the spark may induce high currents in nearby wires or PCB traces. One approach I have used is to use a high-voltage probe with banana plugs and put it into the milliamp jacks of a DMM. The approximately 1 Gohm of the probe will slow the discharge down to a very safe level and it can be simultaneously monitored.

For all the talk about CRT discharging, a very important safety issue is usually overlooked, which is that the power supply section of the monitor chassis has much more dangerous capacitors. With 250VDC and thousands of uF, they can in fact kill you. The same caution needs to be exercised as with all mains connected power supplies. If there are no bleeders or they are open circuit, you need a way to discharge these caps before working in the PSU area.

Well it's anecdotal of course, but I've been using the screwdriver with a length of plain wire method for ~30 years on hundreds of different CRT displays and never had any issues with it but YMMV. The friend who taught me how to repair monitors worked in a repair shop back when those were a thing still and that's how they did it too.

You're absolutely right about the power supply. People tend to fear the EHT and overlook the fact that the "low" voltage in the power supply is a lot more dangerous. One of the worst shocks I've ever had was from the bulk filter capacitor in an Electrohome G07 monitor. Those have a dangerous failure mode where a bad flyback transformer or HOT blows a fuse that is on the *output* of the B+ supply and the result is a large electrolytic capacitor charged to 170V with no load to discharge it, only the internal leakage of the capacitor itself.
 

Offline james_s

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Re: Got shocked my a CRT
« Reply #27 on: January 30, 2021, 07:31:55 pm »
Hm, josh132 created this thread 5 days ago. I wonder is he ok ?  :-//

Probably got the answer he came here for and left the thread.
 

Offline S. Petrukhin

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Re: Got shocked my a CRT
« Reply #28 on: February 08, 2021, 11:21:17 am »
Hey, josh132! Guy, people are worried! Let us know how you feel!
And sorry for my English.
 

Online rsjsouza

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Re: Got shocked my a CRT
« Reply #29 on: April 09, 2021, 12:54:37 am »
Well, he is apparently fine judging by his other post...

https://www.eevblog.com/forum/chat/what-did-you-buy-today-post-your-latest-purchase!/msg3545073/
Vbe - vídeo blog eletrônico http://videos.vbeletronico.com

Oh, the "whys" of the datasheets... The information is there not to be an axiomatic truth, but instead each speck of data must be slowly inhaled while carefully performing a deep search inside oneself to find the true metaphysical sense...
 

Offline josh132

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Re: Got shocked my a CRT
« Reply #30 on: April 09, 2021, 05:54:36 pm »
so for all of you it was a Tektronix scope CRT i was trying to discharge to i can work around it. It was not from an old tune TV with the removable cap. The CRT used a plug for the anode and i had to use pliers for removing it. I got shocked and f##king dropped it on the board... Which killed the sweep generator. I have since visited a doctor and had no major adverse effects. Though my hand does feel a little weak. I am ok. No need to worry.
bu du bu nu
 
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Offline shakalnokturn

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Re: Got shocked by a CRT
« Reply #31 on: April 11, 2021, 10:19:01 pm »
The last time I got a HV bite was opening the connector on a Tek 22xx scope, I have plenty experience with other CRT's and haven't been caught in decades.
I consider that Tek design a wicked F'ing fail... No bleeder resistor and a connector that's just short enough to let the bloody arc reach you!
Now when I have to open those I use gloves and wrap a grounded guard ring of aluminium foil around the connectors before tugging on them.
 

Offline Miyuki

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Re: Got shocked my a CRT
« Reply #32 on: April 12, 2021, 07:35:25 am »
You're absolutely right about the power supply. People tend to fear the EHT and overlook the fact that the "low" voltage in the power supply is a lot more dangerous. One of the worst shocks I've ever had was from the bulk filter capacitor in an Electrohome G07 monitor. Those have a dangerous failure mode where a bad flyback transformer or HOT blows a fuse that is on the *output* of the B+ supply and the result is a large electrolytic capacitor charged to 170V with no load to discharge it, only the internal leakage of the capacitor itself.
Oh, that reminds me. I once grab some small adapter board with charged 400V (230V mains so something over 300V charge), just how the board nice fit to palm. And suddenly capacitor touches the skin and left two nice marks  >:D |O

Oh and good old CRTs they can accumulate charge just on its own from the environment or hold charged for hours even days  ::)
 

Offline fordem

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Re: Got shocked my a CRT
« Reply #33 on: April 30, 2021, 01:23:31 pm »
Oh and good old CRTs they can accumulate charge just on its own from the environment or hold charged for hours even days  ::)

Back in the early '80's I worked with NCR and was sent to their training center in Dayton Ohio to learn, amongst other things to repair the CRT terminals used with the mini computers - the instructor showed us how to discharge the tube before removing it (slide a screwdriver with attached ground wire under the anode cap), and then set it aside.  He did this at 5:00 pm, at the end of the day, and when class resumed at 9:00 am the following morning, before he did anything else, he discharged the tube a second time, so we could all see the charge that had accumulated overnight.

Another one of his favorite tricks was to "walk" each student one-by-one through the power supply adjustment - he'd come over to your workstation, instruct you to put one hand in your pocket, hand you a plastic screwdriver and offer to hold the meter leads on the test points while you adjusted the voltage - the adjustment potentiometer was a few inches below the anode cap on the side of the CRT, right alongside your knuckles as you held that screwdriver, almost guaranteeing that as you twisted the pot, eyes on the meter display, that the knuckle of your pinky finger would hit the anode cap where there was just enough voltage to startle you.

The lesson there was to always be aware of exactly where you were relative to the high voltage components.
 

Offline Miyuki

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Re: Got shocked my a CRT
« Reply #34 on: April 30, 2021, 05:16:44 pm »
The lesson there was to always be aware of exactly where you were relative to the high voltage components.
And then you see pictures with train surfers jumping under wires with 22kV
They have no idea what they are playing with
 

Online BrokenYugo

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Re: Got shocked my a CRT
« Reply #35 on: April 30, 2021, 10:34:18 pm »

Another one of his favorite tricks was to "walk" each student one-by-one through the power supply adjustment - he'd come over to your workstation, instruct you to put one hand in your pocket, hand you a plastic screwdriver and offer to hold the meter leads on the test points while you adjusted the voltage - the adjustment potentiometer was a few inches below the anode cap on the side of the CRT, right alongside your knuckles as you held that screwdriver, almost guaranteeing that as you twisted the pot, eyes on the meter display, that the knuckle of your pinky finger would hit the anode cap where there was just enough voltage to startle you.

The lesson there was to always be aware of exactly where you were relative to the high voltage components.

Also that there are kinda degrees of HV insulation, and a lot of what you run into isn't really human rated. Similar problem in automotive with old pre coil on plug engines, aging ignition parts can and do arc through. You can tell who's been bit before by how close they're willing to come to the nominally insulated spark plugs wires and boots on a running engine.
 


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