Microelectronics | Hewlett-Packard 4276A LCZ Meter

Synopsis

I wanted to take a break from all that x86 system design work that I was doing and I decided to handle another electronics project that I had on my TODO list. This time I will concentrate my attention on the HP 4276A LCZ Meter. I will do some preventative maintenance and will replace the noisy fan and the dead Ni-Cd battery.

Since this unit contains some highly unobtanium active circuit parts (some weird hybrid SMD ICs), I will replace all failure prone capacitors. I don't want to risk breaking anything since in case of any electrical damage the whole unit might easily become bricked and unrepairable. Thus I reckon it's OK to replace everything that might fail, before it actually does.

This unit has an internal DC bias reference that is not working. I will investigate and fix this issue as well.

The front panel switches are a bit too stiff. I know these are serviceable so I will allocate time for this operation as well.

If you're reading this or you're interested in repairing this kind of units, I hope you're not faint of heart. I said it before, while some parts are serviceable, some are custom made for HP by HP or other companies. These are hard to obtain or even impossible to get anywhere except if you recover them from an identical (dead) unit.

Enough said, let's get to work.

Repair and Maintenance

As I wanted the best for this piece of test equipment, I used only genuine Japanese Nichicon capacitors, ordered from Mouser. Well actually most of the UBT series capacitors that I chose for this project are produced in Japan and the rest of the UPW series are produced in Malaysia. The metalized paper film capacitors are made in Germany by WIMA.

There are some tantalum capacitors of weird shape installed mostly on the digital section and the front panel PCB. While I highly appreciate tantalum capacitors for their stability and reliability, I also know they can fail. And especially older ones can go out in flames. When these fail, they usually go out shorting the supply rail. As I said, given the fact that this unit has unobtanium parts, I decided to replace all these capacitors by high reliability film parts instead. While this might sound a bit unheard of, actually these capacitors are only used as supply rail decoupling for the digital integrated circuits. They also act as small energy reservoirs for stabilizing the eventual power fluctuations that might appear on the + 5 V supply rail. So film is good. In addition, film capacitors have high ripple current capacity.

Replacing the Fan

The easiest operation is to replace the fan. And that is how I will start the adventure.

The old cooling fan has a weird three phase motor. I admit it's the first time I see a brushless motor with individual winding coil terminals and no motor controller on-board. There's a first time for everything! The motor controller is soldered on the backplane and it's constructed on a small ceramic PCB in SMD technology. It looks archaic but interesting. I will leave it there idling for the rest of its life.

I have found that J4 pin 1 is ground and pin 6 is + 8 V which is very convenient for me since I will be using the Noctua NF-A6x25 FLX 12 V DC fan as a replacement. I have added a small series resistor on the power supply wire of the fan in order to reduce its rotational speed a bit and create a friendlier atmosphere in the lab.

Here is the new fan installed in its place. I have secured all of the old unused fan coil wires with a small plastic clip. The fan motor controller on the PCB is now unused. I left it soldered on the backplane anyway since it's doing nothing, but consuming a bit of power, after all.

That's it for this part of the article. The old fan has reached the end of its usable life and so it hits the recycle bin.

Maintenance on the Digital Section

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Maintenance on the Analog Section

This is the analog process card that does the interface to the front panel unknown terminals and performs the analog measurements. This card is home to an assorted selection of parts such as passives and actives, coils, relays, precision resistors, precision capacitors, all sort of precision trimmers, hybrid ICs and experimental custom built hybrid SMD "integrated" circuits. Anyway I was only interested in replacing the electrolytic capacitors.

Check this weird single inline multi pin SONY branded hybrid IC that is coated in black epoxy. There are more of these experimental parts on this card.

Like the ones in the right side of this picture.

For the record, here are some ancient SMD ceramic PCB assemblies. Thankfully these are still serviceable.

Here is another black epoxy coated SONY branded IC. The blue electrolytic capacitors are UBT series. All the rest are UPW series, colored in brown.

This PCB had the most capacitors to change of them all. But somehow I liked the repetitiveness nature of this job. Very predictable and straightforward.

Maintenance on the Power Supply

This is a switching mode power supply with the dual power conversion sections well delimited and individually shielded with thick aluminum Faraday cages. I can't imagine any interference escaping these screens. In addition, an individual aluminum screen is embedded between each of the three vertical card that are installed in the backplane.

I used 125 °C rated electrolytic capacitors from Nichicon. Not because the power supply runs hot (because it really doesn't) but because those parts were the best I could source for this project. Let's start with the low voltage section.

There are some missing parts on the PCBA which leads me into thinking this design was maybe shared between similar devices. If I look at the test pins row, the + 12 V and - 12 V terminals are missing. These voltages are not used in this unit except on the DC bias card. But they are generated from the + 16 V and - 16 V rails with local linear regulators anyway.

From my experience I can say this power supply is a bit overengineered. But that's common with old HP gear. And this design proved to be rugged and very stable after all these years of service.

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Maintenance on the HP-IB Card

The HP-IB card is very simple and has only two capacitors to replace. One is aluminum electrolytic and the other one is a tantalum part.

This card is near the cooling fan so I had to clean it thoroughly since it was very dusty.

Maintenance on the DC Bias Card

The DC bias card had the topmost section soaked in some kind of conformal coating that was present on both components and solder sides. Once I removed the upper 33 uF / 16 V electrolytic capacitor, I had a hard time cleaning the area close to its terminals. In a split second I decided that most of the parts have to leave and I have to clean the entire section thoroughly. Which I did after all. This entire operation gave me some dizziness as I used high purity acetone to dilute and remove the conformal coating off the two sided PCB.

I installed precision sockets and I cleaned the ICs. Then I inserted the ICs in their respective sockets. The weird R1 and R2 hybrid resistor networks have been cleaned up and reinstalled in their respective places. New capacitors are in. I gave the PCB a good clean and finally I installed the rear aluminum bracket.

Even though Option 001 (DC bias) is recognized and is switchable from the front panel, it is not working at all. I did some quick measurements in key points on the schematic and it appears that the + 12 V supply rail reports 0 V instead. I traced this issue down to the U6 integrated circuit which is of type 78L12. While not visibly damaged, it doesn't regulate anything anymore. It is dead internally. + 16 V goes in and nothing goes out.

In the end I replaced both positive and negative linear regulators because if one failed, the other one might go out soon.

Replacing the Rechargeable Battery

The backplane is home to a 2.4 V Ni-Cd battery that is used to store front panel settings while power is off. There is a recharging circuit on the analog section PCB that will replenish the charge of the accumulator while the unit is powered on. The problem is the old battery has leaked electrolyte and damaged a bit the ground plane under the soldermask. Thankfully the corrosion didn't eat anything vital and I managed to easily clean it with isopropyl alcohol and white vinegar. Some parts of the soldermask pealed off in the process but the copper isn't exposed as everything is tinned under the green soldermask. So no issues on this side.

There is no manufacturer that makes an identical battery these days. Or at least I haven't found one. And even if it were one, I wouldn't want to risk another leakage in the future. So I came up with a new solution. I bought a 2 x AAA battery holder with connection wires and sticked it with double adhesive tape in place of the old battery. I soldered the wires as shown in picture.

Unfortunately, on the other side, one of the terminals leads to nowhere as it was just a fixture point for the four terminal battery pack that was previously installed. No worries, I soldered a bodge wire from that terminal to the ground on the other side where the negative terminal of the former battery was.

I installed a pair of Varta 1.2 V Ni-MH elements rated 800 mAh. It will take a lot of time to charge these with the charging circuit designed by HP. I am pretty sure this circuit can be tuned to source a greater charging current but I will not delve into this matter for now.

Problem solved. The battery holder appears to be made from one molded piece of plastic so I think it's quite leakage proof.

Maintenance on the Front Panel and Switches

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Adjustments

There are two easily done adjustments that have to be performed as per the instructions in the service manual.

First one is to adjust the + 5 V voltage to a steady + 5.1 V. And second is to adjust the oscillator frequency to exactly 21 kHz. I succeeded to adjust the frequency to 21.000923 kHz. I am amazed this old device can be tuned so accurately.

I checked the other voltages on the various test points on the power supply and all are within the specified range.

Old Capacitors Measurements

For scientific and knowledge purposes, let's measure all of the old capacitors that I replaced in this unit.

I am using the restored and aligned HP 4276A LCZ Meter with the following front panel settings.

  • DC bias: off
  • Display A: C (capacitance)
  • Display B: ESR/G (equivalent series resistance / conductance)
  • Circuit Mode: C2 (equivalent series circuit)
  • LC|Z| Range: auto
  • Frequency: 120 Hz
  • Measurement Speed: slow
  • Test Signal Level: high (1 V RMS)
  • Trigger: internal

You'd think it was a complicated and tedious job to manually collect all this data. But it was actually fun -- while also drinking a beer! I logged everything on paper and then I just updated this article based on my logs. The parts are ordered by rating from high to low based on capacitance and voltage.

CAPACITOR DATA
BRANDTYPESTYLERATINGCAPACITYESR
Nippon Chemi-ConElectrolyticRadial2200 uF / 16 V2570 uF0.06 Ω
Nippon Chemi-ConElectrolyticRadial2200 uF / 16 V2200 uF0.07 Ω
Nippon Chemi-ConElectrolyticRadial2200 uF / 16 V2390 uF0.06 Ω
Nippon Chemi-ConElectrolyticAxial2200 uF / 6.3 V2063 uF0.09 Ω
Nippon Chemi-ConElectrolyticRadial1000 uF / 35 V974 uF0.08 Ω
Nippon Chemi-ConElectrolyticRadial1000 uF / 35 V960 uF0.08 Ω
Nippon Chemi-ConElectrolyticRadial470 uF / 6.3 V412 uF0.30 Ω
Nippon Chemi-ConElectrolyticRadial470 uF / 6.3 V409 uF0.30 Ω
Nippon Chemi-ConElectrolyticRadial220 uF / 63 V209 uF0.22 Ω
Nippon Chemi-ConElectrolyticRadial220 uF / 63 V213 uF0.21 Ω
Nippon Chemi-ConElectrolyticRadial220 uF / 10 V181 uF0.71 Ω
MatsushitaElectrolyticRadial100 uF / 25 V100 uF1.29 Ω
MatsushitaElectrolyticRadial100 uF / 25 V100 uF1.26 Ω
MatsushitaElectrolyticRadial100 uF / 25 V100 uF1.38 Ω
Nippon Chemi-ConElectrolyticRadial33 uF / 16 V29.80 uF4.81 Ω
Nippon Chemi-ConElectrolyticRadial33 uF / 16 V30.30 uF4.40 Ω
Nippon Chemi-ConElectrolyticRadial33 uF / 16 V29.10 uF4.54 Ω
Nippon Chemi-ConElectrolyticRadial33 uF / 16 V28.10 uF4.93 Ω
Nippon Chemi-ConElectrolyticRadial33 uF / 16 V31.80 uF3.69 Ω
Nippon Chemi-ConElectrolyticRadial33 uF / 16 V28.20 uF4.46 Ω
Nippon Chemi-ConElectrolyticRadial33 uF / 16 V28.50 uF4.73 Ω
Nippon Chemi-ConElectrolyticRadial33 uF / 16 V28.15 uF4.74 Ω
Nippon Chemi-ConElectrolyticRadial33 uF / 16 V30.40 uF3.96 Ω
Nippon Chemi-ConElectrolyticRadial33 uF / 16 V29.41 uF4.74 Ω
Nippon Chemi-ConElectrolyticRadial33 uF / 16 V31.10 uF3.96 Ω
Nippon Chemi-ConElectrolyticRadial33 uF / 16 V28.62 uF4.76 Ω
Nippon Chemi-ConElectrolyticRadial33 uF / 16 V32.19 uF3.59 Ω
Nippon Chemi-ConElectrolyticRadial33 uF / 16 V28.39 uF4.66 Ω
Nippon Chemi-ConElectrolyticRadial33 uF / 16 V29.82 uF4.53 Ω
Nippon Chemi-ConElectrolyticRadial33 uF / 16 V29.15 uF4.45 Ω
Nippon Chemi-ConElectrolyticRadial33 uF / 16 V30.10 uF4.45 Ω
Nippon Chemi-ConElectrolyticRadial33 uF / 16 V30.10 uF4.05 Ω
Nippon Chemi-ConElectrolyticRadial33 uF / 16 V30.60 uF3.91 Ω
Nippon Chemi-ConElectrolyticRadial33 uF / 16 V31.07 uF4.10 Ω
Nippon Chemi-ConElectrolyticRadial33 uF / 16 V31.92 uF3.66 Ω
Nippon Chemi-ConElectrolyticRadial33 uF / 16 V29.40 uF4.59 Ω
Nippon Chemi-ConElectrolyticRadial33 uF / 16 V29.20 uF4.60 Ω
Nippon Chemi-ConElectrolyticRadial33 uF / 16 V31.80 uF3.71 Ω
Nippon Chemi-ConElectrolyticRadial33 uF / 16 V30.50 uF3.88 Ω
Nippon Chemi-ConElectrolyticRadial33 uF / 16 V30.00 uF4.34 Ω
Nippon Chemi-ConElectrolyticRadial33 uF / 16 V32.30 uF3.57 Ω
Nippon Chemi-ConElectrolyticRadial33 uF / 16 V30.00 uF4.44 Ω
Nippon Chemi-ConElectrolyticRadial33 uF / 16 V28.76 uF4.89 Ω
Nippon Chemi-ConElectrolyticRadial22 uF / 25 V22.30 uF3.85 Ω
Nippon Chemi-ConElectrolyticRadial22 uF / 25 V21.54 uF4.02 Ω
Nippon Chemi-ConElectrolyticRadial22 uF / 25 V19.10 uF6.56 Ω
Nippon Chemi-ConElectrolyticRadial22 uF / 25 V19.67 uF6.36 Ω
Nippon Chemi-ConElectrolyticRadial22 uF / 25 V19.10 uF5.94 Ω
Nippon Chemi-ConElectrolyticRadial22 uF / 25 V21.46 uF4.38 Ω
Nippon Chemi-ConElectrolyticRadial10 uF / 100 V10.75 uF1.94 Ω
Nippon Chemi-ConElectrolyticRadial10 uF / 100 V10.85 uF2.03 Ω
Nippon Chemi-ConElectrolyticRadial10 uF / 100 V10.80 uF2.24 Ω
Nippon Chemi-ConElectrolyticRadial10 uF / 100 V10.78 uF2.29 Ω
Nippon Chemi-ConElectrolyticRadial10 uF / 100 V10.83 uF2.34 Ω
Nippon Chemi-ConElectrolyticRadial10 uF / 100 V10.84 uF1.90 Ω
Nippon Chemi-ConElectrolyticRadial10 uF / 63 V10.20 uF3.00 Ω
Nippon Chemi-ConElectrolyticRadial10 uF / 63 V10.10 uF4.00 Ω
Nippon Chemi-ConElectrolyticRadial4.7 uF / 450 V5.54 uF9.97 Ω
Nippon Chemi-ConElectrolyticRadial4.7 uF / 450 V5.67 uF9.79 Ω
Nippon Chemi-ConElectrolyticRadial4.7 uF / 450 V5.76 uF9.60 Ω
Nippon Chemi-ConElectrolyticRadial4.7 uF / 450 V5.65 uF9.81 Ω
MatsushitaTantalumRadial4.7 uF / 25 V4.87 uF2.12 Ω
MatsushitaTantalumRadial4.7 uF / 25 V4.76 uF2.30 Ω
MatsushitaTantalumRadial4.7 uF / 25 V4.85 uF2.21 Ω
MatsushitaTantalumRadial4.7 uF / 25 V4.83 uF2.27 Ω
MatsushitaTantalumRadial4.7 uF / 25 V4.69 uF2.23 Ω
MatsushitaTantalumRadial4.7 uF / 25 V4.90 uF1.95 Ω
MatsushitaTantalumRadial4.7 uF / 25 V4.67 uF2.34 Ω
MatsushitaTantalumRadial4.7 uF / 25 V4.87 uF2.16 Ω
MatsushitaTantalumRadial4.7 uF / 25 V4.87 uF2.35 Ω
MatsushitaTantalumRadial4.7 uF / 25 V4.80 uF2.27 Ω
MatsushitaTantalumRadial4.7 uF / 25 V4.72 uF2.43 Ω
MatsushitaTantalumRadial4.7 uF / 25 V4.91 uF1.73 Ω
MatsushitaTantalumRadial4.7 uF / 25 V4.90 uF2.20 Ω
MatsushitaTantalumRadial4.7 uF / 25 V4.71 uF2.41 Ω
MatsushitaTantalumRadial4.7 uF / 25 V4.78 uF2.33 Ω
MatsushitaTantalumRadial4.7 uF / 25 V4.75 uF2.28 Ω
MatsushitaTantalumRadial4.7 uF / 25 V4.82 uF2.23 Ω
MatsushitaTantalumRadial4.7 uF / 25 V4.89 uF1.72 Ω
RIFAFilmRadial15 nF / 250 V30.05 nF11.6 kΩ
RIFAFilmRadial15 nF / 250 V25.65 nF10.1 kΩ

There you have it! Electrolytic capacitors marked 1982 are still going strong 40 years after they left the factory. Similar capacitors are pretty much consistent in reported parameters. Larger capacity parts, regardless of the rated voltage, are doing better than smaller lower capacity and low voltage capacitors. Some capacitors have an ESR that is to envy, like 0.06 Ω or 0.08 Ω. Seriously? I doubt many current production parts have an ESR this low.

As expected, the Matsushita tantalum capacitors still have very good characteristics and appear to be very stable. I don't have any experience with Matsushita branded tantalum capacitors so I can't offer more details on their reliability.

But there is absolutely no guarantee that tomorrow or the day after these capacitors will still be OK. They probably will. But why should I assume this risk?

Conclusions

Conclusions? Draw your own conclusions! The old Japanese capacitors are better than some of the new ones, especially if they are of el cheapo type.

But to be fair, here's a list of my conclusions.

  • The noisy fan replacement was a success in its own. I can totally work for hours with the machine powered on next to my ears without any issues whatsoever.
  • Besides the visibly damaged RIFA film capacitors, 95% or more of the replaced parts are still well within their rated tolerances. The new capacitors will give me peace of mind for the next decades to come.
  • I am impressed by the Matsushita capacitors that are spot on, reading 100 uF ±1 %. That's a bit insane.
  • The broken DC bias PCB was fixed and it is fully working. Again, this feature is very useful for measuring MLCCs and not only.
  • The old Ni-Cd battery has leaked and did quite a bit of a mess on the backplane PCB. While still using rechargeable Ni-MH cells, I am pretty sure that the new solution that I came up with will be better in time in all respects.
  • Front panel switches were revised and all are working correctly. Including the LOCAL switch that was previously completely dead.
  • The switching mode power supply and internal frequency generator stability is still amazing after all these years.

Now this unit goes back on my electronics workbench for some good years of quality service.


Copyright © 2004- Alexandru Groza
All rights reserved.
VER. 1.0 | REV. A