How to properly discharge my scope's CRT? - Page 1
Forum section: electronics / testgear

author: parabuzzle
author_rank: Contributor
timestamp: October 23, 2013, 02:39:04 am
content: Hello,I want to replace the caps in my old school scope's power supply but before I work in that section, I think it best to discharge the CRT... I found this video showing someone doing it in an extremely unsafe looking way ()How is one supposed to render the CRT safe to work on the power supply?btw, I have the Philips PM3216 scope if that makes any difference.-Mike

author: tautech
author_rank: Super Contributor
timestamp: October 23, 2013, 03:01:13 am
content: MikeThe UHT side of quintupler (PDA(post deflection acceleration)) is where you need to go.A short jumper lead grounded and connected to a screwdriver and gently slide under grommet on CRT where lead ends.Be gentle with grommet and CRTPlus any big electrolytic you feel threatened byRob

author: Stonent
author_rank: Super Contributor
timestamp: October 23, 2013, 03:40:37 am
content: I've seen TV repairmen use a long screwdriver wrapped in tape where just the tip of the metal is exposed. Underneath the tape is a piece of lamp cord wrapped around the screwdriver that is grounded to the chassis or ground point on the tv. They just slip it under the cup and wait for the snap sound. Then repeat until there are no more sounds.

author: c4757p
author_rank: Super Contributor
timestamp: October 23, 2013, 03:47:53 am
content: I clip one end of a wire with alligator clips to the chassis, then slip the other clip under the anode cup. Once I'm sure it's made contact a couple times, I bend the cap back a bit and attach the clip to the connector, just to make sure it stays discharged.It's just a scope CRT, it's charged to a somewhat lower voltage than a bigass television tube and has less capacitance. You still don't want to go sticking your tongue in the anode port, but other than that there's not much to it. Ground it for a minute and you're done.

author: ddavidebor
author_rank: Super Contributor
timestamp: October 23, 2013, 05:23:58 am
content: Well, clips are not exactly safe. If you use the crappy one and it doesn't hold on the chassis and felt down, you get zapped.

author: amyk
author_rank: Super Contributor
timestamp: October 23, 2013, 05:59:18 am
content: The CRT won't bite unless you touch the HV connector. I wouldn't discharge it unnecessarily if I'm not working on it. If you are, be careful as it may charge up again on its own (due to dielectric absorption) if not constantly shorted.

author: BravoV
author_rank: Super Contributor
timestamp: October 23, 2013, 06:53:36 am
content: Quote from: amyk on October 23, 2013, 05:59:18 am+1, the devil is in the detail, learned this too from my mentor which was a TV repairman.

author: tautech
author_rank: Super Contributor
timestamp: October 23, 2013, 07:02:46 am
content: Quote from: amyk on October 23, 2013, 05:59:18 amYou are quite right, however often the cable winds its way through the chassis and needs to be disconnected to access PCB's for removal/repair, especially the more upright types.But if you are not working on the tube area leave it be.

author: parabuzzle
author_rank: Contributor
timestamp: October 23, 2013, 02:05:55 pm
content: Quote from: BravoV on October 23, 2013, 06:53:36 amWow.. I would hate to learn that CRT's can recharge on their own the hard way

author: parabuzzle
author_rank: Contributor
timestamp: October 23, 2013, 02:06:50 pm
content: Quote from: tautech on October 23, 2013, 07:02:46 amBingo... it winds right over one of the caps I want to replace :/

author: Neilm
author_rank: Super Contributor
timestamp: October 23, 2013, 05:41:23 pm
content: Quote from: parabuzzle on October 23, 2013, 02:05:55 pmAll capacitors will exhibit this affect. It is just more apparent with high voltage ones. (i.e. it hurts). Quite often you will see people leaving shorting links across capacitor pins to prevent this.