Help! How to measure 2450 volts with 1000 volt max meter.

[ichiban]

I'm finally attempting to fix my old 465B HP oscilloscope and need to
measure the 2450 volt power supply.
My multimeter max test voltage is 1000 dcV. Would I use an inline resistance to drop the voltage to a safe level? What values,wattage?
Problem with the scope is there is no visible trace. I suspect the crt high voltage supply. Probably the power transistor in the supply.
Any help/ideas would be appreciated.

Many thanks in advance.

 

[zigzagflux]

Might want to try an approach similar to this:

http://www.cromwell-intl.com/radio/probes.html

Except you will want to pursue a 100X or 1000X design. Not that you need to actually build a probe, all you are interested in is the voltage divider. Difficulty is selecting resistances that are capable of the voltage, small enough in value such that your scope does not load down the divider, and of appropriate wattage.

 

[ichiban]

Might want to try an approach similar to this:
http://www.cromwell-intl.com/radio/probes.html

Thank you for the reply , I will look into this.

I'm guessing that this cable approach would apply to a multimeter as the input device, as the article applies to a cable into an o-scope.

Note: The o-scope is a Tektronix not HP

 

[star882]

If it's based on a 60Hz transformer and unregulated, apply 12v to the input and measure the output. Then multiply by 10 to calculate the output voltage at normal 120v input.

Or take about 10 220k-470k resistors and connect them in series with a common neon bulb (be sure to insulate properly!). Should make a good go/no go tester.

Or if it's a switcher, tape a neon bulb on the end of a nonconductive stick and put it near the transformer. The electric fields will light up the bulb.

 

[ichiban]

Thanks for the info star882,

what I did is hook the scope to a variac, connect the probes to the test points, then slowly turn the power up on the variac while watching the volt meter.
I got all the way up to line voltage on the variac and no voltage on the -2450v test point. Some of the low voltage test points are out of spec also, possibly one of those controls the -2450v supply. So, the debugging continues.

 

[keantoken]

Don't forget about your meter's input impedance.

My 'scope has an input impedance of 1megOhm. So if I put a 9megOhm resistor in series with the probe, I would get the measured voltage divided by 10.

To divide by 100, I would use a 99megOhm resistor.

However my scope's input capacitance is 47pF. This means that I can kiss goodbye any HF work.

If I simply put a 9megOhm resistor in series with a 1megOhm to use as a divider, then the reading would be off because my scope's resistance paralleled with the 1Megohm would become 500k.

And make sure the resistors are the appropriate wattage.

BTW, there is a Tektronix scope Yahoo group, and people post constantly asking how to fix their scopes. Here is the link:

http://tech.groups.yahoo.com/group/TekScopes/

I guess we're scope buddies because I have a Tek 561B (with a 3A9 diff. Amp and 3B4 timebase).

Good luck
- keantoken

 

[ichiban]

Don't forget about your meter's input impedance.
- keantoken

Good info , thanks.

 

[keantoken]

Your welcome.

- keantoken

 

[jackinnj]

There's always a bunch of Eico or Heath HV probes on the 'Bay -- fwiw, I use an HV probe AND my old Eico VTVM -- the tube in the VTVM is very difficult to fry.

It took me a whole summer to save up the money to buy that EICO VTVM!!!!