I'm working with my Boonton 260A, it is a older device with tubes. I don't understand how a D'arsonval movement is used where I 'think' there is an AC signal. What am I missing? Here is a schematic.
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Thanks, Mikek
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OK great, I have a legible schematic that you can zoom in on to read. How are they using a DC meter in a circuit I think would be delivering AC. It is obviously working, so, what don't I understand about the circuit?
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Thank you Tim, So now as I understand it, "the device (tube) is biased almost completely off" only one side of the AC signal on the grid pushes the tube into conduction. The half wave pulsed output is smoothed by C301 a
0.1uf capacitor. The end result is I have DC on my meter.
You sir are a gentleman and a scholar, you have my undying gratitude.
Mikek
PS. My plan was to double the FS voltage of the meter, in my quest to measure Q's over 625, the limit of the Boonton 260A. I'm hoping to measure up to 1250. The meter has an internal resistance of 1,210 ohms. I paralleled it with a 1,210 ohm resistor and then added a series 605 ohm resistor to keep the same resistance in the circuit hopefully keeping the circuit aligned. This messed up the linearity when adjusting the input drive. A drive voltage of 13.33mv gives a Q of 325. A drive voltage of 8mv gives a Q of 388. Without the meter modification, the Q's read the same. Seems odd. Removing the series resistor didn't solve the problem. I did this with 4-9" wires run out to a switch, so I could have a normal and a times two setting. I don't see how the wires could affect linearity with a dc circuit. Maybe the filtering is not as good as I thought.
The tube is a peak detector at high amplitudes. At low level, the grid nonlinearity makes some DC output, sort of a diode square-law detector. The advantage of the cathode-follower detector is that it minimally loads the series LC, which loading would kill the Q.
The voltage at the grid is the voltage across R202 times the Q, so it's likely to be substantial at higher Qs.
I made my own Q-meter: a signal generator drives a low value resistor, then the series LC, then a super-low-capacitance sampling scope probe.
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John Larkin Highland Technology, Inc
lunatic fringe electronics
This unit test Q with a series circuit. Tt is my understanding that the maximum voltage across R202 is 20mv. That is one amp of current. There are several warnings to not exceed that. 20mv is 1 on the multiplier meter, to measure higher Q's, you reduce the voltage on R202 to 10mv for a 2 times multiplier and 8mv for a 2.5 times multiplier.
I may end up using my scope and 3db points, but certainly would like to get my Boonton configured to measure much higher Qs. My 6-5/8" dia x 6 " inductors would not fit very well on your circuit board. I need to stay 2ft away just to not affect the Q.
M201 is measuring the voltage on a thermocouple, which is heated proportionally to the RMS voltage on R202 -- it probably IS heated by R202, but it could have an internal resistor.
M301 is measuring the rectified voltage on the cathode of V301, per John Larkin's comments.
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Tim Wescott
Wescott Design Services
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Yes, I didn't understand the rectification, but it has been explained. Although if you read my post where I got understanding, the bottom part explains another problem I don't understand. I have a feeling any gurus would need a little more understanding of how operate a Boonton 260A in order to give insight on my question.
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