20 kOhms/V meter versus Digital meter in valve circuits

[MalcolmB]

Hi All...
I'm in the process of restoring an old Eddystone 730-1a receiver (all valves for you younger folk) and the service manual gives the test point voltages based on a 20k/Volt meter. I realize that some, if not all of these voltages are relative to a high impedance source and the 20k/V meter would drag the voltage down to the correct value (according to the service manual).
As my old AVO 8 has long since gone, are there any suggestions as to how I can read meaningful values with my Amecal digital meter?
My gut reaction is to insert a suitable resistor in parallel with the meter probes to load up the test points in order to get a valid reading. Obviously the resistor value would have to be calculated based on the voltage range that I'm trying to measure.

Best wishes

Malcolm M6MYB

PS Wish I'd kept that old AVO....

PPS Thanks for a brilliant forum.

 

[Merlin3189]

Sounds the simplest solution to me.
The resistor to use will depend on the range that the (old 20k/V) meter was switched to. So you may have to make an educated guess as to what this would be.
Eg. Say you measured 200V on a 0-300V range of a 20k/V meter, then the resistance of the meter would be 300 x 20k = 6M. So when using an electronic meter with a 10M input resistance (usually just 10M, not 10M/V) then you would use a 15M in parallel with the 10M to give 6M.
But if you measured the 200V on a 0-500V range, the old meter resistance is 20k x 500 = 10M so no parallel resistance is needed.

 

[MalcolmB]

HI Merlin

Thanks for confirming what I thought. The highest test point voltage is 275V. Incidentally, the service manual quotes values for two meters common at the time (Weston and AVO Model 40).
My meter's auto ranging so I'll check the spec to see if the resistance changes with different ranges.
Thanks again/

 

[Baluncore]

Auto-ranging digital multimeters usually employ a resistive ladder as an input attenuator. That presents a fixed input resistance, usually 10MΩ. The ladder is tapped at different points for different ranges.

 

[MalcolmB]

Thanks for the info - most useful.

 

[jim hardy]

You'll soon figure out that it only makes a noticeable difference when the point you're measuring is intensely current limited , as when surrounded by high value resistors. Your 20kΩ per volt meter only draws 50μamps (60 if it's a Triplett 630) .
If a few tens of microamps doesn't drag the voltage down then your DMM and the analog meter will agree.

old jim

 

[Merlin3189]

My post was a quick off the cuff response, but, as often with these questions, it may not be as obvious as I first thought.
As I found when I did those simple example calculations that a 10M electronic meter would not be far different from a 20k/V meter when used on the higher ranges. Given that my analogue 20k/V meter claims 5% accuracy and the electronic one 2%, I guess they might agree within the limits of error without bothering to correct for resistance. So sometimes maybe the "simplest" solution is to just use it as is! (When you get to measuring at points where the)Edit: an incomplete comment!
In fact, although you raised this in the context of valve equipment and 100+ Volts, the problem is most significant with low voltage measurements. On a 10V range the 20k/V meter has a resistance of only 200k, one fiftieth of the electronic meter's 10M.

The concern on my mind when making my first reply was, how do I know what range the 20k/V meter was on when the measurements were taken? So I tried to look up some meters to see what the typical ranges were. I didn't find a Weston 20k/V meter, but this site had lots of info on AVO meters. The model 40 seems to have a 3mA fsd meter implying only 333 Ω/V. It's hardly surprising that there are some big discrepancies between the readings for it and for the 20k/V meter.

Knowing the ranges and resistance of the AVO 40, I did have a look at the circuit and datasheet to see if I could calculate what the resistance of the Weston meter must be. It's harder than I had expected! First, I can't be sure the AVO was used on the lowest range for the voltage measured. Maybe they kept it on the 600V range even for the lower readings? (The advice in the manual for my analogue meter was to start on the highest range, then switch to lower ranges to reach the best scale for reading. If there is a noticeable change in reading as you go to lower ranges, then you know you are loading the circuit significantly and the best reading may be from the highest range, even though you can't read the scale as accurately.)

Secondly, since the meters seem to be making significant disturbance to the circuit, it's hard to find reference points where you know the true voltage. Even in places where the voltage is given at both ends of a resistor, like U & S, T & Y say, the calculated currents don't work. Since they (I assume) measure only one voltage at a time, you know the value at the end the meter is attached, but not what the value is at the other end. When the other end is measured, now you don't know the value at the first end. Maybe it is calculable with enough analysis (though I gave up on that when I discovered X & Z having different voltages even though they seem to have a short circuit between them!), but I think the measured voltages are to be used just as diagnostic aids, rather than to calculate true quiescent values. In that case you are probably best to try to mimic one of the meters by adding parallel resistance, so that your readings should match the ones given.

Anyhow, I wish you joy with your restoration. You've given me a few hours brain stimulation and nostalgic pleasure looking over these old circuits. It really took me back when I read the fault finding instruction, to start by checking the valves had a red heater glow and that V14 normally exhibits a violet glow!

 

[MalcolmB]

Thanks for all your replies guys - it's really appreciated. The 730-1a is my first real restoration although I've done other small stuff before.
I ran it up slowly with a variac and all the valves light up with V14 producing a nice violet glow. There's some kind of life in it because I can get a signal all the way from the aerial to the output. I've got about 30 paper capacitors to change out plus half a dozen electrolytics, probably some resistors and I hope the mica capacitors are OK. The set itself is cosmetically perfect apart from some wear around the tuning knob and there's no dust anywhere inside. It's been in dry storage for the last 25 years or so.

The next project is a R1475 which is in a horrible state but at least I have the PSU that goes with it.

Thanks again for all the help.