Help with Calibration: Hickok Western Electric KS-15750-L1 Tubes Tester

[HP8903B]

I recently got a Hickok KS-15750-L1
I am trying to calibrate it. I attached the operation manual and calibration process and schematic.

The AC voltages are fine.
The Plate DC voltage (#2b) is 214 VDC. It is suppose to be 148 - 156 VDC.
The Screen DC voltage is fine.

Does anyone have any idea what is the problem ?

Could it be a bad 83 tube ?
I tried with 2 different 83 tubes and get the same result.

 

[TheGimp]

Are you using a variac and setting the filament voltage to nominal?

Line voltage is higher now days and could account for at least half of the difference.

 

[astouffer]

One thing to remember, those old manuals specified an analog meter with maybe 10K or 20K ohms per volt. Digital meters barely load the circuit at all.

 

[HP8903B]

I am not using a variac.
This tube tester has an adjustment for the AC voltage coming in.

 

[HP8903B]

One thing to remember, those old manuals specified an analog meter with maybe 10K or 20K ohms per volt. Digital meters barely load the circuit at all.

That is something I am unclear about.
The manual specifies a 1000 Ohm / volt DC meter.
I have a Fluke 87 V.

Does this mean, the DC voltage readings on the Fluke will be higher ?
If so, by what percentage ?

The manual also specifies to use a 1000 ohm / volt DC meter to measure the Screen voltage.
I use the Fluke and I get the correct reading.

 

[Bill_P]

I am not using a variac.
This tube tester has an adjustment for the AC voltage coming in.

The meter input impedance is causing the problem. The calibration procedure specifies a 1000 Ohm/Volt meter and yours is probably 10 Megohms. Connect a 200K resistor from the meter plus to minus terminals and try the measurement again.

 

[HP8903B]

Could you explain the purpose of the resistor ?
Also, how did you come up with 200K ?

Just to make sure I understand you correctly, I put the 200k resistor in between the + and - probes of my Fluke meter ?

 

[HP8903B]

The meter input impedance is causing the problem. The calibration procedure specifies a 1000 Ohm/Volt meter and yours is probably 10 Megohms. Connect a 200K resistor from the meter plus to minus terminals and try the measurement again.

I tried that. I had to put 2 resistors in series to get 200k.
I get a reading of 150 VDC.
The manual specifies 148 - 156VDC.

 

[HP8903B]

I also measured the Screen voltage with the 200k resistor.

The voltage is within specs from the manual. I am surprised.
The 200k resistor only drop the voltage of the plate, not the screen.

With the 200k resistor, both Plate and Screen voltage are in specs.

 

[astouffer]

I also measured the Screen voltage with the 200k resistor.

The voltage is within specs from the manual. I am surprised.
The 200k resistor only drop the voltage of the plate, not the screen.

With the 200k resistor, both Plate and Screen voltage are in specs.

The screen current is very small so you probably won't see much drop using the resistor.

 

[Bill_P]

Could you explain the purpose of the resistor ?
Also, how did you come up with 200K ?

Just to make sure I understand you correctly, I put the 200k resistor in between the + and - probes of my Fluke meter ?

The resistor makes your meter input impedance match the test instrument specified in the calibration procedure.

200K = 1000 Ohms/volt multiplied by 200 volts. The specified test meter has a 200 volt scale. For lower voltages the scale might be 50 volts or 10 volts. the resistor should be adjusted according to which meter scale is used.

As you found, yes the resistor is connected across the Fluke meter test leads.

Since the Fluke meter is 10 Meg, the actual resistor value should be 204K but considering the approximate nature of the tube tester, 200K is close enough.

 

[HP8903B]

Thank you very much.

The manual specifies 3 ranges of DC Voltmeter: 200V, 50V, and 10V.

What resistor value should I use for 50V and 10V range ?

 

[HP8903B]

50k resistor for 50V range and 10k for 10V range ?

 

[HP8903B]

When using the Fluke as a DC mA meter, do I need to connect any resistor ?

 

[Bill_P]

50k resistor for 50V range and 10k for 10V range ?

Yes

 

[Bill_P]

When using the Fluke as a DC mA meter, do I need to connect any resistor ?

No, you want all the current to go through the meter.

 

[HP8903B]

Thank you

 

[tigerbalm]

Could you explain the purpose of the resistor ?
Also, how did you come up with 200K ?

Just to make sure I understand you correctly, I put the 200k resistor in between the + and - probes of my Fluke meter ?

The resistor is used to simulate the 1000 ohms per volt meter. When measuring the plate and green voltage, you support to set the meter at 250V range, therefore the shunt resistor should be 240 to 250K ohms.