Schematic for 77-2A range extender

My Boonton 72B (I think) capacitance meter has all the required operating instructions and calibration printed on the inside top cover.

I couldn't find any information on it until I took it apart!!

Also the schematic of the circuit is available in a couple of my university text books from the early 70s.

I hope that's of some help to you?

John.

Hi Electroman,

All I want is the schematic of the 77-2A range extender. The operating manual for the 75A-S8 claims it's a 1000X multiplier, with a practical limit of 0.1uF. That would only be 100X. I have been experimenting with a transformer with limited success. I got it to measure 0.01uF, but it is non-linear.

regards,

S

You've got me fascinated as to what this Boonton 75 / 77 capacitor bridge is now...

I spent a while searching the internet yesterday, but like you, drew and absolute blank!!

I would really love to see a picture of it and how you use it, specs etc...?

John.

Hi Electroman,

I found this old picture. This is not a good picture, but it's the one on the bottom.

It's Boonton green, has a capacitance range of 0.0002 pF to 1000 pF, conductance range of 0.01 uMho to 1000 uMho, resistance range of 1 Kohm to 100 Megohm (full-scale ranges of 1,10,100,1000pF and 1,10,100,1000 uMho). It has 2-terminal mode (GND position) and 3-terminal mode (DIR pos.). You can read 4 digits on these precision dials. Capacitance ranges are adjusted to 0.1% or better.

I don't have the 77-2A, but have just put a 1000 pF silver mica capacitor in a box in series with the input. This will allow me to measure up to at least 0.1uF with reasonable accuracy for both capacitance and resistance/conductance. I have been writing a program to print a look-up table for that use (the scale is non-linear for my range extender). I have other ways to measure capacitance, but no other way for resistance/conductance, which is basically a measure of the quality of the component.

regards,

S

It looks a fascinating bit of kit...

I'm wondering what principle it uses to measure these low capacitance, is it an rf bridge? or perhaps a ratio transformer balance?

I know with my General Radio 1615A ratio transformer bridge it can measure / resolve down to 10 atto Farads or 0.00001 pF and it can compare capacitors to within a few ppm...

A series capacitor to multiply the range sounds an excellent idea, although I guess it must make loss (D) measurement very awkward??

John.

Hi Electroman,

My reply before went into the bit bucket!

"I'm wondering what principle it uses to measure these low capacitance, is it an rf bridge? or perhaps a ratio transformer balance?

It's a modified Young bridge operating at 1MHz.

"A series capacitor to multiply the range sounds an excellent idea, although I guess it must make loss (D) measurement very awkward??"

You're right. I though that it would just read a bit high, but not so. Another 1000pF silver mica that reads a G of 4uMho normally, but reads 1uMho when using the range extender. An orange pigtail 1000pF reads 26 uMho with the R.E., and 97 without it. How do I calculate what the R.E. reading means for conductance or resistance?

Regards,

S

Standardsguy, while I digest your post above... maybe you can advise me on a problem...

Using my Genral Radio ration transformer bridge for capacitance calibration at the lower values everything is fine. but when you add the range extension for the 1 to 10 uF range you need to calaculate the effects of the internal impedances, as the bridge is only 3 terminal the inductance of the leads and internal impedances aren't nulled out...

The only way I can think of calculating the correction factor of these impedances is by using a 10uF capacitor of known inductance and ESR etc... and they're not available to my knowledge!!

As the impedances change with the set range it is not as easy as just setting lower ranges to measure the internal residuals...

So have you come across any method that allows you to work out the compensation factor when measuring very low impedance capacitors of upt o 10uF on a ration transformer bridge???

Sorry to drift off topic, but its been a problem for some time now and talking to other calibration companies most don't know what I'm talking about and the others are like me and apply rough compensation figures!!!

John.

At work we have a Genrad 1620-A capacitance measurement system (1615 bridge?). It has a transformer, but is no doubt different from the one you have. I have heard it called an NBS type 2 bridge. It makes measurements to 0.01% or better at 1kHz. We have a range extender that allows 10uF measurements. The R.E. has an adjustment to make it read the same as without it. I don't remember any loss difficulties, but we were mostly interested in capacitance. Sorry, I can't help you with your problem.

S

Thanks anyway and yes the 1620 uses the 1615A bridge, its just the name for the bridge plus oscillator and tuned detector.

The range extension capacitor was the first thing I made for it !!

John

Hi Electroman and Standards Guy,

I have a bunch of old mica capacitors from a military (1962?) carrier frequency telephone system. These were used with ferrite inductors to create the filters for channel separation.

Also existing some very unusual quartz filters (bending plate mode) , frequencies from 5 to 45 KHz.

If you are interested I can take some photos.

RHABE

I would be interested in seeing them RHABE.

Would they be the glass envelope type of low frequency crystals?

We used some of them in the Marconi spectrum analyser to give a bandwidth of 5 Hz even at 100 MHz!! they were superbly stable...

John.

Hi John, these quartz filters are not the glass encapsulated ones, and low frequency bending mode type. I will take a photo and post ist here. RHABE