The repair from He!! - HP 4275A LCR meter

[vtp]

How did you test the JFETs?

They are depletion mode so if you just take one off the board and measure between drain and source you will see ohmic reading, maybe in few tens of ohms. Depletion mode (they all are depletion mode) N-JFET needs negative gate to source voltage to turn it off.

You may want to check the capacitors for shorts, especially if they are ceramics.

Q10 is also depletion mode but a p-channel MOSFET - a quite rare thing today, especially with body (substrate) brought out. If you take a look at its characteristics it should give an idea how the control of the phase shift at the MOSFET gate works.

Just a note on testing semiconductors: If you want to be absolutely certain that a device works or does not, do not use anything but a curve tracer to check it. Multimeter diode tester may show that bipolars or any other PN-junctions are ok even though device gain may be almost non-existent. Has happened to me and have learned the lesson.

For FET information, I would highly recommend Siliconix book Designing with field-effect transistors, 2nd ed.

[vtp]

Just out of curiosity I made a LT-Spice model of the phase shifter.

It looks like the controlling resistance would need to be very low compared to the fixed resistance in the upper transformer leg path in order to get any phase shift out of the circuit.

So, I would concentrate my efforts to find out why Q10 is not controlled properly. It is a depletion mode p-channel MOSFET so it is normally on (hint is a continuous line in the symbol which denotes depletion mode FET - straight through). To turn it off gate to source voltage needs to be positive as it is p-channel. If you measure the gate voltage I would guess you get about 12 volts so the FET is totally off. In that case the simulation model shows what you seen on your scope screen, a 180 degree shift.

Measurements in the model are taken from R7 and V1.

[precaud]

How did you test the JFETs?

I used one of the ATmega-based transistor testers, which has correctly identified everything I've thrown at it so far. Of the 10 I tested, it identified most of them as two diodes, a couple as a diode and a resistor (20-ish ohms), and two as JFETs. It did cross my mind that maybe it has trouble with low-Rds-on JFETs.

After installing them in circuit, I verified the switches were acting correctly by measuring ACV across each JFET while turning them on and off. No-voltage condition = JFET on.

They are depletion mode so if you just take one off the board and measure between drain and source you will see ohmic reading, maybe in few tens of ohms. Depletion mode (they all are depletion mode) N-JFET needs negative gate to source voltage to turn it off.

Understood. That is what the circuit does, Vgs ~ 0V for on, -11.5V for off.

You may want to check the capacitors for shorts, especially if they are ceramics.

While I had the FETs out, I checked the two that are switched, one a mica, one a film, they measured good. The 39pF is also mica, I didn't bother with it, perhaps I should have. But I've never seen a mica cap go bad. I'll pull it today and check it.

Q10 is also depletion mode but a p-channel MOSFET - a quite rare thing today, especially with body (substrate) brought out. If you take a look at its characteristics it should give an idea how the control of the phase shift at the MOSFET gate works.

Now that I have confidence that the capacitor switching is working (which is where most of the phase shift occurs), Q10 is once again the "prime suspect". Each capacitor covers approx a decade freq range. Ex: the 5.6nF covers 10kHz-100kHz. At 100kHz, the impedance of the cap + FET is about 300 Ohms. Same for the 560pF at 1MHz. So I think mosfet Q10 resistance will be low (~1kOhms and less) to affect phase. Your model agrees with this.

I removed U1 (again) and installed a socket to facilitate testing Q10 with the known-good cap switches. Using DCV as high as +2V (I don't think the 1496 + U1 circuit produces more than that) on Q10, it is not changing the phase. It is acting "open". If it were shorted, the signal at TP3 would be in-phase, not inverted.

Just a note on testing semiconductors: If you want to be absolutely certain that a device works or does not, do not use anything but a curve tracer to check it. Multimeter diode tester may show that bipolars or any other PN-junctions are ok even though device gain may be almost non-existent. Has happened to me and have learned the lesson.
For FET information, I would highly recommend Siliconix book Designing with field-effect transistors, 2nd ed.

Thanks, yes, this experience has made me suspect of FET testing. A dedicated tester is probably needed.

I am thinking that Q10 may also need to be tested out of circuit.

It is still not clear to me: what is the mechanism that causes this circuit to seek the 90º phase offset condition?

Thanks for your help, vtp.

[precaud]

The 39pF mica tests fine.

FWIW, the block diagram of the modulator board shows the same general functioning that we've sussed out...

[Kleinstein]

The part around U1, U2 is a phase detector and loop filter.  So the circuit is a kind of PLL with the phase shifter instead of the more normal VCO. It looks for the signals send to the MC1496 to be 90 phase shifted.

The control voltage at the gate of Q10 might go rather negative in some cases, at least for the new sony CX07500M part.
The circuit also allows for quite a negative voltage. So I would expect something like -10 to 0 V at the gate of Q10.

Just for curiosity: how large is the amplitude at the phase shifter ? 

[precaud]

The part around U1, U2 is a phase detector and loop filter.  So the circuit is a kind of PLL with the phase shifter instead of the more normal VCO. It looks for the signals send to the MC1496 to be 90 phase shifted.

I understand that, with same-freq carrier and signal, the 1496 can be used as phase detector. I just don't grok the mechanism that causes this circuit to "balance" at -90º offset.

The control voltage at the gate of Q10 might go rather negative in some cases, at least for the new sony CX07500M part.
The circuit also allows for quite a negative voltage. So I would expect something like -10 to 0 V at the gate of Q10.

I assumed CR1 oriented as it is across C18 would result in only positive voltage output, no?

Just for curiosity: how large is the amplitude at the phase shifter ?

Not very high, only 1.3Vp-p at the input (C38) and about half that at TP2 and TP3. So the voltage swing at U1 can not be very large...

[Kleinstein]

I would consider CR1 a limiter for the control voltage to prevent it to go too positive (e.g. more than about +0.7 V). The normal working range is likely in the negative range, maybe from -8 V to -2 V. There likely is some note in the service manual about the voltage too, as it is a test-point. The circuit could vary the voltage from about -10 V to +0.6 V. So for a manual test this range could be checked.

The circuit around U1 is a difference amplifier with a little gain (e.g. 1.5) first and than followed by an integrator use to regulate the phase. So depending on the phase measured the control voltage is slowly adjusted up or down.

Something like 0.6 V voltage swing is still too much to replace Q10 with something like a photo-fet.

[vtp]

I assumed CR1 oriented as it is across C18 would result in only positive voltage output, no?

No, Kleinstein is right. The U1 circuit only allows highly negative or slightly positive (about diode drop) control voltage range. It goes very fast negative even with very small positive input.

Q10 seem to be quite weird thing indeed. I think it can be assumed that it has negative treshold voltage and that the channel is normally off (which is contrary to normal depletion mode operation and how it has been drawn) if it is anything like the newer part.

For out of circuit measurements I would like to recommend Tektronix 577 with storage display. They can be had for reasonable money and are easy to recondition and adjust to mfg specifications - in case you need another project in future.

[precaud]

Interesting. Unfortunately the manual gives no clues about what to expect for the control voltage.

Yes, diff amp with a little gain, followed by integrator. As it is now, with phase near 180º, voltage at TP1 is around +0.5V and varies very little (<0.05V) as freq is changed. I think I would need to study the action of the 1496 to understand it better. The waveform I see now (at diff amp output) is a triangle wave at same freq as input.

This is 100kHz input waveform (sine, 500mV/div) and output of diff amp (triangle, 1V/div) with the phase shifter output as it is now, inverted. There is not much DC offset, but it is a little negative. Triangle increases to ~9Vp-p at 10kHz and decreases rapidly with freq, as you'd expect with a 1458 and 470pF in the feedback loop. This results in about +0.5V gate voltage. Even at 10kHz, with 9Vp-p at diff amp out, gate voltage is still ~0.5V. It never goes negative.

[precaud]

No, Kleinstein is right. The U1 circuit only allows highly negative or slightly positive (about diode drop) control voltage range. It goes very fast negative even with very small positive input.

OK, thanks for confirming.

Q10 seem to be quite weird thing indeed. I think it can be assumed that it has negative treshold voltage and that the channel is normally off (which is contrary to normal depletion mode operation and how it has been drawn) if it is anything like the newer part.

The only mention I see in CX07500M data sheet is on p.4, the attenuation test uses threshold voltage of -1V.

For out of circuit measurements I would like to recommend Tektronix 577 with storage display. They can be had for reasonable money and are easy to recondition and adjust to mfg specifications - in case you need another project in future.

Thanks for making another future project for me 

[precaud]

I decided to try a negative gate voltage for Q10. You guys were right, that's what it needs to be.

This is a 10kHz signal with U1 removed and a negative DC supply at TP1. Threshold voltage is somewhere around -1V. The -90º phase offset shown in the photo was at around Vg of -1.2V.  Changing the freq to 40kHz, -2.8V was required for -90º.

So, the good news is, Q10 works. The question is, why is there no negative voltage coming out of U1?

[rf+tech]

precaud,

Consider the mixer equations:

With two different frequencies, there will be two IF outputs

Code: [Select]

  A1 = cos [(ωLO – ωRF)t – (ØLO – ØRF)]and

Code: [Select]

  A2 =  cos [(ωLO + ωRF)t – (ØLO + ØRF)]plus higher order products. In down-conversion applications, low-pass filtering removes A2 and higher products.

With two equal frequencies, ωLO – ωRF is zero, leaving A1 = cos (ØRF – ØLO). Again, the A2 summation products (2f) are removed by filtering.

With two equal frequencies applied to a balanced modulator (used as a phase detector), the IF port difference term is DC and related to the phase angle between the two inputs. The IF output will be maximum positive at 0 degrees (cos 0 = 1), maximum negative at 180 degrees (cos 180 = -1), and zero at 90 degrees (cos 90 = 0). The natural tendency of the IF DC output is to drive the controlled element in the direction that will satisfy the A1 equation.

Code: [Select]

Ex: ØRF is  +55 degress relative to ØLO    cos(+55) = 0.57
Ex: ØRF is +150 degrees relative to ØLO   cos(+150) = -0.87
Ex: ØRF is  -20 degrees relative to ØLO    cos(-20) = 0.94
Ex: ØRF is -100 degrees relative to ØLO   cos(-100) = -0.17

Notice how the sign changes when the difference exceeds 90 degrees. This process is similar to negative feedback around an opamp, driving the inputs to equality.

These values are scalar, multiplied by the loop gain factor. It's not just the apparent DC gain based on U1 and its feedback resistors - there's also the non-obvious "sensitivity" of the controlled element - expressed in degrees per volt (or MHz/volt as with a VCO). What may appear to be a small DC swing can have a much larger effect.

As to exactly why 90 degrees requires an understanding of vector rotation and summation, something that could best be explained by Alan wa2aew, if he has not already done so.

Now, apply this knowledge to the scope photo. When both signals are present, the differential amp output should be DC. But there's a triangle wave at the input frequency. This implies one of the two inputs to U2 is missing, insufficient amplitude or U2 is defective.

RF+ Tech

[Kleinstein]

A working Q10 is good news. 

The wrong control voltage could be due a problem with the supply of U1 or maybe U2 not working right (either U2 itself or some resistors). Pin 9? and 5   (output to differential amplifier) of U2 should already show the result of the phase comparison. So one should see about the right currents and a change with phase shift.

[precaud]

Thanks for the detailed explanation, rf.

Code: [Select]

  A1 = cos [(ωLO – ωRF)t – (ØLO – ØRF)]
With two equal frequencies applied to a balanced modulator (used as a phase detector), the IF port difference term is DC and related to the phase angle between the two inputs.

That part makes sense.

The IF output will be maximum positive at 0 degrees (cos 0 = 1), maximum negative at 180 degrees (cos 180 = -1), and zero at 90 degrees (cos 90 = 0). The natural tendency of the IF DC output is to drive the controlled element in the direction that will satisfy the A1 equation.

That will take further study.

Now, apply this knowledge to the scope photo. When both signals are present, the differential amp output should be DC. But there's a triangle wave at the input frequency. This implies one of the two inputs to U2 is missing, insufficient amplitude or U2 is defective.

Agreed. I'm pulling U2 and will check all components around it and the diff amp for added confidence.

Thanks again for the details. I've never looked into mixer theory, its interesting stuff.

[precaud]

I removed one leg of enough parts to be able to test all the passives around U1 and U2. Everything measured well within tolerances.

That pretty much left the 1496 as being the problem. So I installed yet another one. Hooked up the probes to watch the waveforms in and out of the phase shifter and monitor the DC at the mosfet gate, and expected to see it working properly on powerup. But no. Same old story. Nothing had changed. Moments like this deserve at least a few cuss words.

After which I probed around the 1496 to see wtf was causing the triangle wave output. And I find over 5VDC on one of the inputs. None to speak of on the other one. The schematic says, C21 should be blocking this. Measured the DC on the other side of C21, and it was the same. C21 has measured fine with no DC on it, and had basically gone short with 5V on one side.

See pic below for the cap and the schematic, including a correction. (TP3 is actually right at the 1496 input and not on the trace to the modulator stage).

C21 is your basic junk ceramic 100nF 50V cap. The manual doesn't say, but given its size, I'd guess X7R (or worse). There are nearly 20 of these things on this board, most of them for local power supply decoupling. I'm tempted to replace them all. Why are they using such a poor-quality cap at such a critical place in the signal path? Wouldn't a film type be better for interstage?

I have some 100nF Y5S which have proven to be stable performers over the years so I replaced C21 and C4 with one of them. Powered the unit up, and saw two waveforms 90º apart on the scope, and -7V or so on the gate of Q10. Finally!

After a couple minutes warmup, it passed self-tests in both open and short modes. It successfully did open and short compensation with the test fixture installed. I then tested a number of hi- and lo-Z components over the entire freq range. It's working!!

It is quite possible that this bad ceramic cap was the major problem with this board all along, though the jury is out on the JFETs.

Thank you guys for your help sorting through this.   

Next I'll install the GPIB and DC offset boards and make sure they work. And then go through the adjustment procedure and do as much as I can without the standards. Making a set of 4-terminal standards might be a good project at some point.

[vtp]

Excellent, glad you finally found the reason.

That Murata blue ceramic is not exactly the worst choice for that position, X7R is fairly temperature stable whereas worse ceramics like Y5 may lead to temperature related issues. There is also R19 to consider so you may want to have the DC-blocking capacitor there to be a good one.

I have seen that same Murata blue go short before, ahem, I did tell you to check the ceramics, did'n I...   Mine was in a 3577A converted B and it was pulling one rail down. I had to remove quite a lot of stuff to get to that one, just like you did now. Those caps have been made gazillion a year and even with sub-ppm level failure rates one pops out every now and then.

If you grind that cap open you will find inside a today's normal SMD cap with leads welded to it and then dipped in epoxy. SMD is actually a cost "improvement" where those flexible leads and protective dipping have been left out.

Making standards for calibration is something I have been considering too. The difficult one would be capacitive standards, others are fairly easy and have good documentation available, see 16074A.

[precaud]

Excellent, glad you finally found the reason.

It was bound to happen sooner or later... we had the bear in a corner...

That Murata blue ceramic is not exactly the worst choice for that position, X7R is fairly temperature stable whereas worse ceramics like Y5 may lead to temperature related issues. There is also R19 to consider so you may want to have the DC-blocking capacitor there to be a good one.

What would you suggest to be a good one? Isn't a high-quality film a better choice for DC blocking? I have never liked using ceramics in the signal path, with capacitance and ESR changing as DC bias changes. And this thing has quite a few of them, even 470nF and 1uF.

BTW, the Y5S temp stability is only slightly worse than X7R:
http://iopscience.iop.org/article/10.7567/JJAPS.24S2.1027

But C and ESR are both more stable with delta DC.

BTW2, the HP parts list describe this 100nF cap as " +80 -20% " which is more like a Y5V...

I have seen that same Murata blue go short before, ahem, I did tell you to check the ceramics, did'n I...

Yes, and I DID check them, just not with DC bias...

Mine was in a 3577A converted B and it was pulling one rail down. I had to remove quite a lot of stuff to get to that one, just like you did now.

Interesting. That will be one of my future projects, reviving a 3577A. I know who to call now 

Those caps have been made gazillion a year and even with sub-ppm level failure rates one pops out every now and then.

OK, I won't do wholesale replacement of them, then.

If you grind that cap open you will find inside a today's normal SMD cap with leads welded to it and then dipped in epoxy. SMD is actually a cost "improvement" where those flexible leads and protective dipping have been left out.

That makes sense.

Making standards for calibration is something I have been considering too. The difficult one would be capacitive standards, others are fairly easy and have good documentation available, see 16074A.

Yes, the cap standards are the hard ones. For my needs a full set of R standards is not necessary, I'm mostly interested in the lower-Z end of things. And the HP LCR cal procedures I've seen only use open, short, and 2 or 3 others at most. The open you can sub short BNC cables for.

Getting back to 4275A territory... an odd idiosyncrasy of it (and the 4274A) is, the DC bias option can not be operated from the front panel, and is only controllable via GPIB. I realized this morning why that is. The code to support it is on the GPIB option ROM! There wasn't room for it in the 16kB address space allocated for program code in the memory map. (Can you imagine, the entire operating system, complex math package, everything, in 16kB?) So DC bias is not for bench use, only an ATE function. Too bad...

This instrument was really an amazing piece of engineering in its day. Kudos to the Yokogawa engineers.

[GerryBags]

Thoroughly enjoying this saga, Precaud. 

Isn't it funny how far engineers used to be able to make 16kb stretch? When I started out on using computers for design work, quite a while after this beast was made, 16kb was the standard size for a true-type font file. Now I think they're all ten times that at least. Flabby code caused by the use of building blocks, with the creation of new basic elements becoming less frequent and combinations of basic structures becoming building blocks themselves. Gradually, knowledge of how to construct the most basic elements becomes more scarce and, before you know it, half of the gains of forty years of Moore's Law are flushed down the loo.

[precaud]

Thoroughly enjoying this saga, Precaud. 

Thanks Gerry, better drama than a TV soap, I'd say 

Isn't it funny how far engineers used to be able to make 16kb stretch? When I started out on using computers for design work, quite a while after this beast was made, 16kb was the standard size for a true-type font file. Now I think they're all ten times that at least. Flabby code caused by the use of building blocks, with the creation of new basic elements becoming less frequent and combinations of basic structures becoming building blocks themselves. Gradually, knowledge of how to construct the most basic elements becomes more scarce and, before you know it, half of the gains of forty years of Moore's Law are flushed down the loo.

It's true. Cheap memory, GHz cpu's, and 64-bit address busses and size no longer matters. My son is a CS guy working with large data structures, and when he looks at my past coding (much of it of Z80), and the OS's of instruments of this era, its almost unreal to him. Concise and compact mean nothing in his world. Different times.

[vtp]

Sorry, I missed your point about the cap going short (ohmic?) under DC-bias. What does your multimeter show in the ohms range if you measure it?

To find out all simple issues I typically first visually inspect boards for any obvious faults, then go over each component with multimeter, fuses, resistors, ceramics and tantalums in ohms range and then diodes and transistor PN-junctions in diode tester setting. Most of the simple stuff gets caught this way. It is a kind of challenge to fix a test equipment only with Fluke 87. Last one I did completely that way was a 3458A and that even included debugging serial communications (shift registers).

Getting back to 4275A territory... an odd idiosyncrasy of it (and the 4274A) is, the DC bias option can not be operated from the front panel, and is only controllable via GPIB. I realized this morning why that is. The code to support it is on the GPIB option ROM! There wasn't room for it in the 16kB address space allocated for program code in the memory map. (Can you imagine, the entire operating system, complex math package, everything, in 16kB?) So DC bias is not for bench use, only an ATE function. Too bad...

Manual DC-bias control is a separate device, 16023B. It connects to amphenol connector at the back of the instrument where the connector has been brought out with a kind of extender board from the bias supply. At least that's how it is in my 4274A. I think the DC-bias option is just one board + the extender for rear panel connector. In 4284A it is two different boards compared to standard unit. 4274A is in any case more usable than 4284A in my opinion but that's a different story.

[precaud]

Sorry, I missed your point about the cap going short (ohmic?) under DC-bias. What does your multimeter show in the ohms range if you measure it?

Out-of-circuit with no DC bias, 55 Ohms.

To find out all simple issues I typically first visually inspect boards for any obvious faults, then go over each component with multimeter, fuses, resistors, ceramics and tantalums in ohms range and then diodes and transistor PN-junctions in diode tester setting. Most of the simple stuff gets caught this way. It is a kind of challenge to fix a test equipment only with Fluke 87. Last one I did completely that way was a 3458A and that even included debugging serial communications (shift registers).

Challenge, indeed!  I'd say that is close to my typical procedure... unless I'm navigating in completely foreign waters, then I follow the manual.

Manual DC-bias control is a separate device, 16023B. It connects to amphenol connector at the back of the instrument where the connector has been brought out with a kind of extender board from the bias supply. At least that's how it is in my 4274A. I think the DC-bias option is just one board + the extender for rear panel connector.

Yes, but that misses my point. Why is a separate device (16023B) even necessary? Why aren't the controls on the front panel? Because of 1. the way they allocated (and addressed) memory, and 2. memory-mapped I/O.

Attached is the memory map of the 4274A and 4275A. As you can see, 16kB is allocated each for ROM, I/O ports, option ROM, and RAM. The upper two bits of the address determine which "device" is read from or written to.
Program code and all fixed data constants get the top 16kB. That's all.
Next 16kB goes to IO devices. How many actual addressable IO ports are needed? Maybe 2-3 dozen? That's over 16,050 addresses wasted.
Then the option ROMs. How big are they? 1kB each for GPIB and special freq option. Another 14kB wasted.
Last 16kB is RAM. How much RAM is there? 3kB if the battery-backed option is used. That's over 13kB of addressable space wasted.

You see, so much wasted memory space, and no extra room IN PROGRAM ROM SPACE for the code to service a couple more buttons and control the DC bias board.

The 4276A came a couple years later. They learned the lesson. Went with a Z-80 with register-based I/O. Instantly you actually have the entire 64kB to work with.

In 4284A it is two different boards compared to standard unit. 4274A is in any case more usable than 4284A in my opinion but that's a different story.

I've never compared the two, but I wouldn't own a 4284A, because there is no service manual / schematics for it.

PS - I was looking forward to your answer to the ?? about the DC-blocking caps... (or anyone else's thoughts). The more I think about it, the more I'm inclined to go with a polyprop film.

[vtp]

Yes, but that misses my point. Why is a separate device (16023B) even necessary? Why aren't the controls on the front panel? Because of 1. the way they allocated (and addressed) memory, and 2. memory-mapped I/O.

Yes, perhaps they first did the meter and then got the idea that it would need DC-bias too... Too late to change the front panel or other boards. In 8662A they did SMPS transformer pin out in mirror and had to fix it in layout - it's quite obvious to see what happened by looking at the layout.

I've never compared the two, but I wouldn't own a 4284A, because there is no service manual / schematics for it.

Ahh, that's kind of saying "I give up!" After fixing equipment using service manuals it gets boring. The next obvious challenge is to get equipment without service manuals or block diagram only. That is why I got the 4284A, but it has been a disappointment, I found the failure (although I had to resort to an oscilloscope) and its construction is substandard IMHO.

PS - I was looking forward to your answer to the ?? about the DC-blocking caps... (or anyone else's thoughts). The more I think about it, the more I'm inclined to go with a polyprop film.

I would have dropped in an X7R 100nF ceramic and called it a day. Too much thinking is dangerous and leads to gold deposited teflon film in hermetic can being in there. But film capacitor probably is fine as long as it works ok at 10 MHz too which it probably does. Use your now functioning 4275A to compare ceramic and film cap properties up to 10 MHz.

[precaud]

Ahh, that's kind of saying "I give up!" After fixing equipment using service manuals it gets boring.

A little eevblog bravado, eh?

That is why I got the 4284A... its construction is substandard IMHO.

I wasn't impressed with that aspect either.

Use your now functioning 4275A to compare ceramic and film cap properties up to 10 MHz.

My thought exactly. 

Now Witness the power of this fully armed and operational battle station! Loafing along at 2MHz...

[precaud]

For anyone else repairing the cpu board of their 4275A, here are the ROM images.

As explained in the .TXT file, these images are for cpu boards with part number 04275-66519 and lower.
The U10 code is used with A11 (HPIB) boards with part number 04274-66522.

For cpu boards with suffix -66529 and later, or for U10 code for use with HPIB boards with suffix -66551, use the images available for download in the files section of the Yahoo HP/Agilent Users Group.

[precaud]

I went through the adjustment procedures today, did everything except the range resistors (which requires 4-terminal R standards). The cal procedures are pretty straightforward. A few adjustments were a ways out but I encountered no problems bringing everything to within specs. The power amp board was out of spec and couldn't be adjusted in. I found a 33uF lytic going open, it was sitting right above a heat sink and cooking. Replacing it brought that whole board into adjustment.

I then took a number of different components and compared them between the 4275A and a Wayne Kerr 6425. The highest freq they have in common is 200kHz so I compared them at 10kHz, 100kHz, and 200kHz.

The differences were insignificant, mostly the result of the small lead length differences imposed by their respective fixtures.

The only head-scratcher I encountered was a difference in the way they computed the capacitance value of 68uF 'lytic at 100kHz. Both instruments measured the impedance and phase the same, yet computed quite different C values from it. No biggy for me, I work mostly with raw Z/phase values anyway.

You may recall I mentioned early on how the HP 4192A has low-impedance limits, below which it simply blanks the phase display when the measured value is below it (50 mOhms). The 4275A has a similar "feature". It blanks the phase display for all measurements below 10mOhms. You can measure phase at 10.2mOhms but not at 9.8mOhms. Not a big deal for me but worth mentioning.

This weekend I'll compare its results at higher freqs to those using a VNA. So far I'm very pleased with it. It's a very nice instrument.