High Voltage  - Page 1
Forum section: electronics / beginners

author: TorqueRanger
author_rank: Frequent Contributor
timestamp: January 15, 2016, 02:38:11 am
content: I have been working on repairing a scope for me to use .. I checked most of the voltages but how can I check the high voltage side of the crt without blowing up my fluke

author: kc8apf
author_rank: Regular Contributor
timestamp: January 15, 2016, 02:45:47 am
content: Which Fluke? I know the 87 is rated up to 1000V and I've used to 990V personally.

author: c4757p
author_rank: Super Contributor
timestamp: January 15, 2016, 03:06:03 am
content: Quote from: kc8apf on January 15, 2016, 02:45:47 am...which is nowherenearhigh enough for the "high voltage side of the crt".You need a proper high voltage probe, ideally one rated to at least double the maximum voltage you expect to find (so around 25kV or so). This will have a series resistor in it, forming a voltage divider with the input resistance of your multimeter, so that the meter reads 1/100 or 1/1000 of the voltage the probe sees.

author: KD0CAC John
author_rank: Frequent Contributor
timestamp: January 15, 2016, 03:10:09 am
content: There is a high voltage probe about 4,000 volts as I remember , that plugs into most fluke meters .If you take your time & wait , you can find a good deal , I think I paid about $25 new on ebay .

author: jh15
author_rank: Frequent Contributor
timestamp: January 15, 2016, 03:14:26 am
content: Do you need at this time to measure the crt HV? Is it not showing any spot or trace at all?While waiting for your HV probe, be sure it is not an intensity, blanking or sweep problem, or beam pulled out of sight etc.

author: kc8apf
author_rank: Regular Contributor
timestamp: January 15, 2016, 04:37:32 am
content: Quote from: c4757p on January 15, 2016, 03:06:03 amBased on a friend's experience building drivers for a variety of CRTs, the voltage used vary a lot. Certainly having a proper HV probe is a good way to go. Knowing the specifics of the CRT TorqueRanger is working with might open other options.

author: tautech
author_rank: Super Contributor
timestamp: January 15, 2016, 06:07:38 am
content: Quote from: TorqueRanger on January 15, 2016, 02:38:11 amWhich CRO?Which HV?What is the expected test voltage?PDA voltages are not normally tested, they require specialist HV probes and voltages can range between 8 and 15 kV.Cathode voltages in some CRO's can be as high as minus 2800V, some as low as -1 kV, some are adjustable some are not.

author: TorqueRanger
author_rank: Frequent Contributor
timestamp: January 15, 2016, 08:19:21 pm
content: Its a bk precision 20mhz model 2120 scope. The max voltage is 2k volts on the crt that I am trying to measure ..

author: tautech
author_rank: Super Contributor
timestamp: January 15, 2016, 09:24:55 pm
content: Quote from: TorqueRanger on January 15, 2016, 08:19:21 pmKeep your eyes out for an old AVO, they have a 2.5 kV range and it may be your cheapest option.

author: dom0
author_rank: Super Contributor
timestamp: January 15, 2016, 09:49:06 pm
content: If the HV is regulated I'd just checka) has the regulator a stable, linear working point?b) is the feedback resistor network okay (can measure off-state)

author: rdl
author_rank: Super Contributor
timestamp: January 15, 2016, 11:49:39 pm
content: You can sometimes find the Fluke 27/FM military surplus on ebay for under $100 with the carry case and 6000v high voltage probe. I've been meaning to buy one for years. I probably shouldn't wait much longer, I remember when you could get them for under $50.

author: Macbeth
author_rank: Super Contributor
timestamp: January 16, 2016, 12:07:24 am
content: Quote from: tautech on January 15, 2016, 09:24:55 pm3kV range for the AVO 9 and military Test Set No.1 - but that is just how the scale is calibrated.The long wirewound HV resistors can be taken out of a broken AVO meter that you get for nothing and turned into your own HT probe for use on more modern devices

author: GreyWoolfe
author_rank: Unknown
timestamp: January 16, 2016, 12:34:41 am
content: Quote from: rdl on January 15, 2016, 11:49:39 pmI got one about a year ago. The high voltage probe looked brand new. $75 USD shipped.

author: TorqueRanger
author_rank: Frequent Contributor
timestamp: January 16, 2016, 12:43:53 am
content: I might try to see if I can catch one on sale or something.. Is there a way to make one that would be safe to use and not kill someone???

author: vk6zgo
author_rank: Super Contributor
timestamp: January 16, 2016, 05:19:54 am
content: Some older Oscilloscopes had a built in voltage divider off the EHT so that you could read it with any Multimeter.The HV probes you can buy for use with DMMs will just be "loafing along" looking at 'scope EHT.They were commonly used for Colour TV EHT which can be up around 25kV or more.You occasionally find HV probes designed for VTVMs or Electrostatic voltmeters.Although they are usually beautifully engineered,either would be problematical for use with DMMs,The VTVMs have similar input impedance to a DMM but,the Electrostatic meters may not.In any case both have leads with old style screw up coax connectors.

author: rqsall
author_rank: Regular Contributor
timestamp: January 16, 2016, 09:41:59 am
content: Quote from: Macbeth on January 16, 2016, 12:07:24 amAn AVO is completely useless as it's way too low impedance and will load the HV CRT circuit down. You need a proper high impedance HV (Giga Ohms range) probe that you can plug into your fluke. Such as this one:http://www.ebay.com/itm/FLUKE-80K-6-HV-PROBE-6KV-PK-MAX-/390956647807?hash=item5b06d5057f:g:HrsAAOSwQJ5USpa7Good luck.

author: HighVoltage
author_rank: Super Contributor
timestamp: January 16, 2016, 09:46:27 am
content: The Fluke 80K-6 goes only up to 6 kVDepending on high the voltage level you are measuring, you might want to use theFluke 80K-40High Voltage Probe for up to 40 kV

author: HighVoltage
author_rank: Super Contributor
timestamp: January 16, 2016, 01:34:38 pm
content: There are also high voltage probes made by other companies.This one here I got really cheap, almost free on ebay a while back and it works great as well.Or if you want to go really professional, you get a Tektronix P6015A, one of the best high voltage probes for a scope.