Discharging a oscilloscope CRT  - Page 1
Forum section: electronics / repair

author: gkmaia
author_rank: Frequent Contributor
timestamp: January 29, 2019, 04:48:44 am
content: I just got this beauty and I am planning to work on it. First thing I was going to do was to discharge the CRT but could not find the usual plug at the tube's side.The only outputs I can see are the ones on the back.Can anyone put some light on how to discharge this guy without killing myself?

author: gkmaia
author_rank: Frequent Contributor
timestamp: January 29, 2019, 05:59:56 am
content: Thanks!But basically if I ground all the connectors coming out of the crt I should be fine?

author: edpalmer42
author_rank: Super Contributor
timestamp: January 29, 2019, 06:11:43 pm
content: Did you look on the bottom side of the CRT? I've never seen a CRT that didn't use one of those big connectors. I have seen oscilloscopes where the connector was hidden underneath and required further disassembly to access.Ed

author: cvanc
author_rank: Frequent Contributor
timestamp: January 29, 2019, 07:10:16 pm
content: Quote from: edpalmer42 on January 29, 2019, 06:11:43 pmYeah, this.It's odd that the schematic doesn't explicitly show it, but I too have never seen a scope CRT that did not have the the big side connection for HV. Be safe and good luck with your work.

author: Squiddaddy
author_rank: Regular Contributor
timestamp: January 29, 2019, 08:16:33 pm
content: This one could have the high negative voltage at the cathode, and the anode is grounded.Need to trace the schematic more to know for sure.

author: Chris56000
author_rank: Frequent Contributor
timestamp: January 29, 2019, 08:58:10 pm
content: Hi!Oscilloscopes with a Y (vertical) bandwidth of typically 20 MHz or greater nearly always use a tube with p.d.a. – post deflection acceleration requiring typically +8 to +14 kV and this is always usually supplied by a TV tube type cavity anode connector, but many lower spec Hameg 'scopes, along with the CRT shown in the circuit–extract the OP provided, do NOT need post–deflection acceleration, and therefore there is no multi–kV to discharge on the OP's tube!However, the heaters, cathode, grid and the first/focus anodes of the CRT in the OP's extract will still be below earth potential at low –E.H.T., typical. –1200 to –2000V obtained either from a secondary on the mains transformer or a h.f. oscillator, either being fairly dangerous to handle!However, if you leave the 'scope disconnected from the mains for at least 30–45 minutes, the resistors and potentiometers in the brightness/focus bleeder chain will discharge this voltage enough for you to be able to measure it safely with the 1kV range of a dvm, which you can leave connected until the voltage decays away to zero, when the CRT's base will be safe to handle – taking regard to implosion safety–precautions of course!Chris Williams

author: David Hess
author_rank: Super Contributor
timestamp: January 29, 2019, 09:10:58 pm
content: I have tested a few modern oscilloscope CRTs and their PDA discharged within 10s of seconds. Older and lower frequency CRTs up to maybe 20 MHz tend not to even have PDA so there is nothing to discharge. In this particular case, I doubt there is any PDA.

author: bd139
author_rank: Super Contributor
timestamp: January 29, 2019, 09:13:14 pm
content: I dunno about seconds. I got the shit scared out of me on my 2235 a while back after about 20 minutes.

author: David Hess
author_rank: Super Contributor
timestamp: January 29, 2019, 09:25:26 pm
content: Quote from: bd139 on January 29, 2019, 09:13:14 pmThe 2235 CRT was one of the ones I tested. These CRTs have an internal shunt resistance from the PDA to ground in the form of a helical thick film resistor which controls the electrostatic gradient and the vacuum dielectric yields low capacitance so they discharge pretty quickly.Now the high voltage multiplier is an entirely different matter but it is on the shielded end of the PDA connector.