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Old 25th Mar 2018, 3:06 pm   #1
IsquaredR
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Join Date: Apr 2017
Location: Minocqua, Wisconsin, USA.
Posts: 86
Default B&K 801 Capacitor Analyst Odd Behavior

I am restoring a B&K Model 801 Capacitor Analyst. After obtaining 1% parts for calibration standards, re-capping the electrolytics and performing the simple calibration procedure, all went smoothly until attempting the OPEN calibration.

When performing the calibration for the OPEN test, all adjustments go well until I parallel the 25 pF capacitor to the 50 ohm resistor on the blue and black test lead. When doing so the needle swings clearly into the middle of the BAD arc and NOT "somewhere in the GOOD arc" as the manual states.

I decided to check the test lead frequency resonance with my grid dipper because the manual said the lead (with the two capacitors and coil) should resonate at 20 mc. When checking the test lead it was resonant at 31.5 MC and NOT "approximately 20 mc" as the manual said. I further checked the oscillator and it was generating a stable 19.5 mc signal. I thought perhaps the discrepancy between the two was my problem.

My first knee-jerk attempt at a fix was to extend the length of the test lead. It was 44-inches long. I incrementally added length and after adding 22 more inches my resonant frequency only had dropped by two hundred kc. I figured it would be absurd to add a test lead length that would be necessary to change resonance from 31.5 mc all the way down to 20 mc, so I changed my tack to considering either reduce the capacitor values or increase the inductance of the coil. I experimented with differing values in place of the 200 and 300 pF capacitors but none had any effect on test lead resonance. To lower the resonant frequency so the oscillator and lest lead resonance would match the only thing left was to add inductance to coil L101, so I increased the coil's inductance by 2 microhenries (the coil secondary itself measured 5 microhenries to start with). My test lead resonance dipped properly at 20.5 mc. Hurray! . This was purely accidental because my choice of adding 2 microhenries was just a stab in the dark. Using a frequency counter I then checked the oscillator frequency and correspondingly adjusted the oscillator to 20.5 mc. I figured this surely will solve my problem, right? Wrong. So I persevered and followed their OPEN calibration procedure and indeed the test lead null on the grid dipper and resonant frequency of the 801's oscillator dipped at the same frequency, 20.5 mc. I likened the effect to what one might expect when neutralizing a transmitter in which you want the plate current dip to occur at the same point as reaching maximum RF output. Only this time I was dipping where and as I should be by artificially changing the inductance. I thought wrongly that surely by adding the 25 pF cap the OPEN test would shove that needle over into the GOOD range. Wrong again. With the test lead and oscillator now matching and tuned to 20.5 mc the darned thing still dropped into the mid-range of the BAD arc when paralleling that 25 pF capacitor. I tried another 25 pF cap just in case the first one was wonky but there was no change.

I am left wondering if there is any chance the manual is incorrect and they meant to say adding the 25 pF capacitor should move the needle into the BAD arc instead of the "GOOD" arc.

I would be most appreciative if anyone might be able to help me see what I am missing.
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Old 28th Mar 2018, 4:37 pm   #2
John_BS
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Default Re: B&K 801 Capacitor Analyst Odd Behavior

Is the test lead some kind of "figure of 8" configuration, i.e. a pair of conductors held at a certain spacing?

On the schematic I found, L101 is marked up at 42uH? I'm not sure that the test lead is intended to actually resonate: operation appears to rely on forming a quarter-wave transformer: a mix of the lead itself, which has an electrical length of c. 30 degrees, (assuming largely air dielectric) and the lumped elements L101 / C101/102. If L101 really is 42uH, the Zo of the pseudo-line will be very high (300 ohms ish?). Can you measure the capacitance of the lead on its own?

John
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