Anyone have knowledge on discharging a CRT?

[kato]

As usual, the internet is full of conflicting advice, and regurgitated suggestions presented as fact. So I come to the Lab for some truth.

I've offered to recap a friend's Vectrex - an old video game console with a built in CRT screen. Even thought it's been unplugged for 10 days, I still want to discharge the CRT because there's no guarantee it's not ready to bite.  The Question: Can I short the CRT's anode to chassis without risk of damaging the tube, or is a bleeder resistor inline required? And if so, what is happening?

I'm afraid the advice of using a bleeder resistor is mixed up with stock advice for discharging electrolytic caps. (I realize the CRT is basically a creating a giant capacitor with the glass as dielectric.)

Some claim you must use:

• A high resistance part (couple MΩs.)
  A high wattage part (but only to keep the charge from arcing over a 1/4w) Some suggest 25 watts.
  A string of 100k resistors.

I'm thinking a 20kV jolt will zip through a 2MΩ resistor just about as fast as it will a 2k.
Also, I'm happy to try a bleeder resistor if it will prevent damage but all my high wattage resistors are low ohmage. <100Ω
Although I do have a 7w 2k resistor which may be a happy medium. But with a resistor, I won't hear the spark telling me I've made contact.

Any thoughts?

 

[bitman]

We used to do it all the time as young lads in a monitor repair shop.

DO BE CAREFUL!!

I was taught this by who came before me there.

We would take two long handled screwdrivers. One, a blade type.

UNPLUG THE MONITOR FROM THE MAINS

Slip the blade end under the flyback rubber thing attached to the CRT so as to touch the contacts underneath it.
Touch the other screw driver to chassis ground, cross the two screwdriver blades making contact at the shafts. You will get a good snap and spark where the two screwdriver shafts meet.

Do this a couple more times to be sure you have discharged the flyback (no more sparking).
Then it's ok to remove the flyback cable thing from the CRT.

I must have replaced 100 flybacks in dead wyse terminals this way without incident.

 

[pucho812]

I've done that before, the first is the worst as your not expecting such a sound..

 

[s2udio]

We used to do it all the time as young lads in a monitor repair shop.

DO BE CAREFUL!!

I was taught this by who came before me there.

We would take two long handled screwdrivers. One, a blade type.

UNPLUG THE MONITOR FROM THE MAINS

Slip the blade end under the flyback rubber thing attached to the CRT so as to touch the contacts underneath it.
Touch the other screw driver to chassis ground, cross the two screwdriver blades making contact at the shafts. You will get a good snap and spark where the two screwdriver shafts meet.

Do this a couple more times to be sure you have discharged the flyback (no more sparking).
Then it's ok to remove the flyback cable thing from the CRT.

Absolutely correct....also make sure you discharge the main PSU cap as these can bite too !
Some tubes and LOPT's will also recharge themselves with static...beware
A screwdriver with a piece of insulated wire twisted around it to chassis ground, is also easier to use,there is usally a ground strap on the back of the crt.
Removed thousands .....back in my TV repair days...ahh nostalgia ain't what it used to be
;D

 

[JohnRoberts]

Only thing to add is discharge it twice, since there may be some DA (distributed charge) so after the first discharge it may rebuild again to some intermediate voltage level. Even if the lower voltage may not hurt you, it could surprise you while lifting it and you don't want to drop an irreplaceable CRT.

JR

 

[kato]

Thank you friends for the good advice. I really appreciate it.

Only thing to add is discharge it twice...
    ...it could surprise you while lifting it and you don't want to drop an irreplaceable CRT.

Good call, thanks John. Finding a 9" XY monitor tube would be all but impossible short of buying another Vectrex.

Incidentally, I could not ferret out a charge anywhere. Even the PSU caps are showing 0v so I'm calling this "safe" until I find out otherwise. (the hard way.) Should be a straightforward job. A couple curious things:

1.) There's a braided ground strap between the power board and the logic board. I'm wondering why it would be braided. You can see it in the top left corner. (Cut, just now by me.)

2.) The traces look wrinkled. Ever seen that before? I don't know how it's been stored re: humidity and temperature. That seems like a bad sign. I'm assuming it's the masking that's wrinkled and not the copper. I guess I'll scrape a little bit off and find out...

 

[JohnRoberts]

Braided cable has lots of surface area so like poor man's Litz wire for low Z at HF and LF.

JR

 

[jackies]

Hey kato, good to see that you are keeping busy!
I've seen this type of braid (shield?) used for ground wiring on a number of devices, my assumption was it's just for the convenience of having a thick yet flexible ground connection.
I have also seen this "wrinkled copper" on old boards many times - I attributed it to low quality of the PCB material in combination with dated wave soldering technology, don't think it's anything to worry about...

 

[Jidis]

I have also seen this "wrinkled copper" on old boards many times

So it's not in fact solder mask as Kato (& I) figured? I've seen it a bunch too, but something I actually like here (an old GLi mixer) had it pretty bad from what I remember.

 

[jackies]

Well, I didn't go as far as to actually investigate whether it's the copper that's wrinkled, or the solder mask - although I think it is the copper - I was just saying that it's pretty common and shouldn't cause a problem...

 

[kato]

I've come to the conclusion that the wrinkly mask was applied after soldering - based on a few spots where it rides up on the bigger lumps of solder. (The solder is actually underneath the mask in spots.)

The board is also unusual (at least in my limited experience with modern PCBs) in that there are no plated vias on several of the caps with connecting traces on both sides. It actually requires me to solder both the top and bottom pads for the circuit to function. It's kind of a pain, and nothing I've ever seen before. Also, the pads lift really easily. At least the isolated pads do. 2 seconds at 300ºC and they float away... But then they re-adhere when the temp cools down. So weird.

Good to hear from you too Jackies. My friend who went with us to lunch recently remarked how cool he thought you were.