Electronics > Repair

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

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precaud:

--- Quote from: vtp on April 22, 2018, 03:48:52 pm ---Use your now functioning 4275A to compare ceramic and film cap properties up to 10 MHz.
--- End quote ---

After looking at the 100nF films caps I have here, I decided to skip them. Their larger package size and hence longer leads puts their self-resonant frequency well below 10MHz, which is not desirable in this situation.

I modified one of my programs for another HP LCR meter to control the 4275A via GPIB. Wrote routines so it measures C/ESR from either Z/phase or R/X data (the latter is preferable, it bypasses all of the 4275A's range limitations), and plots the results.

This compare is of two different 100nF ceramic formulations. The X7R is the 100nF blue chiclets used throughout the 4275A, both for PS bypassing and DC blocking. The Y5S is a 100nF Murata disc that I got a few years ago. After getting some and seeing how good they measured (up to 100kHz back then), I bought a few hundred to have around.

If you want to make yourself sick, put the X7R in the bridge and watch the capacitance and DF change with time. At 100kHz with 1 VAC drive, it took nearly a minute for the value to settle. Change the frequency and watch it migrate again for half a minute. Then add some DC bias and watch the C go down and ESR go up. After this you won't put an X7R in an analog signal path ever again.

The Y5S was much better, settling to final value in <10sec, and showing very little change with freq change or DC.

This test was made after I had already replaced all of the blue X7R's on all analog boards with the Y5S's (except those used for PS decoupling). It was made with conditions similar to what they see in the instrument in the decoupling role: 1 VAC test level with 5V DC bias.
As you can see, the ESR of the X7R is much worse than the Y5S across the board, by a factor of 10 at 400kHz and worsening below that.
I had to go thru some of the cal procedures again after changing them.
It may be all in my head, but the instrument readings seem more stable, drift less, with the new caps in.

Next I want to take a close look at the calibration steps ( all two of them) that require the standard resistors and capacitors. They are used to calibrate the range resistors.

vtp:
Interesting findings.

Only explanation to your stability measurements under bias I can think of is that your old Y5S is a single disc capacitor and the X7Rs are MLCC. Today Y5S is only used in line rated X/Y-classified safety capacitors and they are single layer discs.

BTW, Q10 is a depletion mode MOSFET, it is the base/substrate bias that pushes its treshold voltage to around zero volts or below from clearly positive value. Figured this out some time ago.

precaud:

--- Quote from: vtp on May 06, 2018, 09:00:05 am ---Only explanation to your stability measurements under bias I can think of is that your old Y5S is a single disc capacitor and the X7Rs are MLCC.
--- End quote ---
Are you suggesting that MLCC of any class perform worse (stability-wise) than a disc under conditions of changing voltage/freq across them?


--- Quote ---Today Y5S is only used in line rated X/Y-classified safety capacitors and they are single layer discs.
--- End quote ---
Mine are collector's items, then   :P

These ceramic cap classifications all have to do with usable temperature range and stability vs temp change. I found this chart of capacitance change vs applied DCV:


Even with the cheaper electrode mat'ls, there is not much impact on capacitance at 5VDC. This agrees with what I found. So the impedance change is all in the increasing ESR.

All of which says X7R sucks for passing AC. Unless your design calls for an impedance modulator, get them out of the signal path for critical applications.

mrf245:
Hello, can anyone provide a download link for HP4275A service manual in high quality?
I found service manual in KEYSIGHT website, but its misty in ERROR check table and schematic. Many appreciate.

lotsmorebetter:
Hi to all who are reading this.
I am an audio hobbyist who has enjoyed the use of an HP 4275 LCR meter over the years. It has been great in every way that one would expect. Lately, not so. When I got Error Message 4, I sent it in for repairs and they wanted the price of a new one to repair it. Impossible.
I need someone from this community (preferably in the US, but I would consider Canada too) who has had some experience in repairing them and who will not charge the impossible to repair it.
Interested? Please send info to me either through this site or to my email address directly.
Thanks in advance.

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