Waveforms 520a AC VTVM Calibration

[Gold]

I have a Waveforms 520a AC VTVM. At high gains there is so much RF it pins the meter. If the Input is shorted it goes away. I tried putting a few different value caps across the input as a LPF. It helps but by the time the meter is useable at high gains (40-60dB) the response is almost eating into the audio band. The only trimmers I see are in the first stage. Is there any obvious way to reduce high frequency gain or filter the RF out?

 

[radardoug]

Where is the rf coming from? What band is it?

 

[radardoug]

Looks like its a 2 wire power cord. Ground the chassis. Is it a shielded metal box?


































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[Gold]

Where is the rf coming from? What band is it?

I don't know. I'm assuming it is RF. I can't measure it. I do know that the level decreases as I increased the value of the capacitor across the input.

Looks like its a 2 wire power cord. Ground the chassis. Is it a shielded metal box?

The two wire power cable was replaced with a grounded three wire power cable. The negative input terminal is tied to the steel case and the case is tied to the ground pin on the power cable.

 

[moamps]

Do you really need a very high input impedance (10Mohm)?
If you don't need it, try connecting a resistor of say 100kohm across the input and check it.

 

[Gold]

Do you really need a very high input impedance (10Mohm)?
If you don't need it, try connecting a resistor of say 100kohm across the input and check it.

I tried putting a 100KΩ across the input terminals. with 60dB of gain it reduced the level on the meter by about 3dB.

 

[moamps]

Does this mean that you connected some kind of generator to the input of the meter, and that connecting 100kohm resistors over the input resulted in a reduction of 3dB in the measurement? What is the output impedance of the generator?

 

[Gold]

Does this mean that you connected some kind of generator to the input of the meter, and that connecting 100kohm resistors over the input resulted in a reduction of 3dB in the measurement? What is the output impedance of the generator?

No, this is with an unterminated input. No test leads connected to the input terminals. The RF pins the meter at 60dB gain. If I input a signal from the AP Portable One at -60dBu it doesn’t move the needle from pinned against the end stop. Or reduced by 3dB with the 100K R.

 

[moamps]

It seems to me that you have a problem with parasitic oscillations in the meter or with noise in the vacuum tubes.

 

[Gold]

It seems to me that you have a problem with parasitic oscillations in the meter or with noise in the vacuum tubes.

How would you go about fixing it?

 

[moamps]

Connect signal 10mV/1kHz from AP P1 to the input of the meter, set it to 50 or 60dB, and check with an oscilloscope the waveforms at the outputs of both the input and output stage.

 

[Gold]

Connect signal 10mV/1kHz from AP P1 to the input of the meter, set it to 50 or 60dB, and check with an oscilloscope the waveforms at the outputs of both the input and output stage.

With the AP P1 if I physically connect generator output to analyzer input at -40dBu the analyzer displays the correct level and displays the correct frequency input. At -50dBu the AP P1 analyzer displays the correct level but doesn’t display the correct frequency. It reads “low”. Too much noise? I’m wondering if the P1needs to be looked at?

When I look at the Generator Output on the scope at 0dBu it looks clean. When I get down to -40dBu the sine wave is starting to look like a triangle wav. With the generator at -60dBu the Oscope display shows mostly noise. The scope is on 2mV/div which is maximum gain.

 

[Gold]

Okay, I fixed it. C5 0.047 had a cold solder joint to ground.