Tektronix 434 s/n below 500000 faulty ps.

[Craig]

Just checked three different Fluke DVM's against each other (ohms range on one to measure the input R of the voltage range of another. They were all between 10.12M and 11.22M.

[johnmcgivern]

It's a Fluke 85 bought originally from RS components mint condition checked ohms value thru mA read 1002 ohms seems good.

[Craig]

Well, the spec of the 85 is 10M in parallel with <100pF. So I suggest that if you are measuring significantly different to 10M (you are) I suggest that there is a problem with your method of measurement. Or something is very wrong with your DVM.

A good method to verify is to put a 10M resistor in series with the meter, and use a battery. First measure the voltage of battery using DVM. Now connect the 10M in series with the DVM and see what the voltage reading is. If it is half the battery voltage the input resistance is 10M. Anyway, you get the idea.

[ppppenguin]

Just checked three different Fluke DVM's against each other (ohms range on one to measure the input R of the voltage range of another. They were all between 10.12M and 11.22M.

This suggests that you shouldn't rely on the 10M input resistance of a DMM as part of a voltage divider. My own EHT probe was designed to work with the 50uA range of an analogue multimeter, giving 25kV FSD. It works equally well with a DMM though I have to divide the uA reading by 2 to get the voltage in kV. If I was starting again I'd make a 1000M probe rather than 500M.

Some inexpensive DMMs have a 1M input resistance. Best avoided altogether.

[johnmcgivern]

The 2x200meg and the 100k are 1% tolerance so that will alter the 4000:1 ratio + the 8.4meg of the meter I have another meter not fluke that measures 10meg will try that tomorrow.
Also try using the 10 meg resistor in series if I have one which I doubt.

[Craig]

I use a Fluke 8K40, which is a 1000x probe designed for use with a 10M+/-10% DVM. It has a 1G input R and works up to 40kV. The internal resistors have a voltage coefficient (of course) which introduces an error. It is set for nominal 1000x at 25kV with 1% error with a 10M DVM, which means that at lower and higher voltage the errors become 4% at 1kV and 40kV. And it really is a DC probe, and has an error of +/-5% at 60Hz.

They say that if the DVM has much greater than 10M input R, add a shunt, and if much less apply a calculated correction factor.

Basically it is 1G between the tip and the DVM positive and a 1.11M to DVM negative. So a 10M input R DVM shunts the 1.11M to give 1M and hence the 1000x attenuation. And that shunting action means that a 10% error in DVM R translates to 1% error in attenuation.

They can be picked up on eBay from time to time for quite reasonable sums.

It is one of Fluke's longest living products, introduced in the late '70's and is still a current product.

[Craig]

It actually brings to mind voltage coefficient. The Ohmite Slim MOX I suggested have a very low voltage coefficient - <2ppm per volt, at least for the values that John has used. That means that the resistance will change by less than <0.4% (2ppm x 2000V). Which is actually very good indeed.

The temperature coefficient is also low at 25ppm/C. So even a 20C change in temperature will only change the resistance value by 0.05%

Both these changes are insignificant as compared to the component tolerance of 1%, although my measurements of these indicate that the tolerance is considerably tighter than that.

[johnmcgivern]

Well lads used HV probe today on my faulty Hameg.
-1900v read -1892v, -1877v read -1869v, -1962 read -1960 seems ok 4000:1 ratio. Within a few volts.
I would like to thank everyone for there help.
Unfortunately my replacement parts never arrived yesterday I phoned the supplier who said there was a delayed payment from PayPal that's why it was posted late .
Strange when the money was deducted from my bank last Saturday.
So as soon as I get diodes and zeners I am on hold.

[Craig]

Sounds great John. Look forward to hearing how things develop!

Craig

[johnmcgivern]

           
Hi Gents, well Craig good news parts came today and I did the job tonight.
See photo's for result. ZOOM in.
Trace doesn't cover 10x graticules only 9 1/2.
See bright dot at start of trace never noticed this before.
Signal is with a 1x probe using the scopes calibration which is 0.6v @ 1Khz.
Apart from that all seems ok. Many thanks.