Microelectronics | Hewlett-Packard 4276A LCZ Meter
Synopsis
I wanted to take a break from all that x86 system design work that I was doing lately. Thus, 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, including 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 think it would be good 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. In addition, the LOCAL button doesn't operate at all. I know these are serviceable, so I will also allocate time for this operation.
If you're reading this, or you're interested in repairing this kind of unit, 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, SONY, or other companies. These are hard or even impossible to obtain except if you recover them from an identical donor unit.
Enough said, let's get to work.
Disclaimer
This article is not to be treated as a do-it-yourself tutorial on how to fix, restore, rebuild, or improve this unit. This was not my initial intention. But you can consider this whole content as a general guideline, should you decide to attempt any service procedure on an identical or similar unit.
The entire documentation is just a reflection of my work and I cannot be held responsible if you damage your unit, or even harm yourself in the process.
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 rectangular tantalum capacitors 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 with high-reliability film parts instead. While this might sound a bit unheard of, actually these capacitors are only used as supply rail decoupling for 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 an interesting three-phase motor. I admit it's the first time I have seen a brushless motor with individual winding coil terminals and no on-board motor control circuit. There's a first 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 I imagine this was cutting edge technology back then. I will leave it there idling for the rest of its life. I might investigate it at some point and reverse-engineer it, just for fun.
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 slightly reduce its rotational speed and create a friendlier atmosphere in the lab.
Here is the new fan installed in its place. I have secured all the old unused fan coil control wires with a small plastic clip. The fan motor controller on the PCB is now completely unused. I initially wanted to remove it altogether, but then I figured out it was best to leave it as it doesn't interfere with the normal operation of this unit.
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
The computer control section is built around the CMOS version of the Z80 microprocessor. I will dump the ROM so that I have a backup, should bit rot occur.
I replaced the axial electrolytic capacitor with a Vishay high temperature counterpart. This unit doesn't run hot at all, but if having access to the best electronic components, why not use them?
I replaced all IC decoupling capacitors with film parts. These will practically hold forever.
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 custom-made (maybe even 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 circuit board had a lot of capacitors. But somehow, I liked the repetitive 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 cards 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 to think this design was maybe shared between similar devices. If I look at the test pin 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 by local linear regulators anyway.
From my experience I would say this power supply is overengineered. But that's very common with old HP gear. And this design proved to be rugged and very stable after all these years of service.
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 located next to the cooling fan. I had to clean it thoroughly as 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 component 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 had to leave, and I had 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 from the two-layer PCB.
I installed precision sockets and I cleaned the ICs. Then I inserted the ICs into 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 I finally reinstalled 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 at 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. If one had already failed, the other one could go out anytime soon.
Replacing the Rechargeable Battery
The backplane is home to a 2.4 V Ni-Cd battery that is used to store the front panel settings while the unit is powered off. There is a recharging circuit section on the analog signal process PCB. This will slowly replenish the charge of the accumulator while the unit is powered on. The problem is the old battery has leaked electrolyte and damaged the ground plane by corroding it 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 there are no more issues.
There is no manufacturer that makes an identical battery these days. Or at least I haven't found any. 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 attached it with double adhesive tape in place of the old battery. I soldered the wires as shown in the picture.
Unfortunately, on the solder side, one of the terminals leads to nowhere as it was just a fixture point for the former four-terminal battery pack. No worries, I soldered a bodge wire from that terminal to the ground on the other side where the negative terminal of the old 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 amount of charging current, but I will not delve into this matter for now.
Problem solved. The battery holder appears to be made from a single molded piece of plastic, so I think it's quite leakage-proof.
Adjustments
There are two easily accessible adjustments that have to be done as per the instructions in the service manual.
The first one is to adjust the +5 V voltage to a steady +5.1 V. And the second is to adjust the oscillator frequency to exactly 21 kHz. I successfully adjusted 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 their specified range.
Old Capacitors Measurements
For scientific and knowledge purposes, let's measure all the old capacitors that I replaced in this unit.
For this purpose, I used exactly this freshly 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 | |||||
|---|---|---|---|---|---|
| Brand | Type | Style | Rating | Capacity | ESR |
| Matsushita | Electrolytic | Radial | 100 uF / 25 V | 100 uF | 1.29 Ω |
| Matsushita | Electrolytic | Radial | 100 uF / 25 V | 100 uF | 1.26 Ω |
| Matsushita | Electrolytic | Radial | 100 uF / 25 V | 100 uF | 1.38 Ω |
| Matsushita | Tantalum | Radial | 4.7 uF / 25 V | 4.87 uF | 2.12 Ω |
| Matsushita | Tantalum | Radial | 4.7 uF / 25 V | 4.76 uF | 2.30 Ω |
| Matsushita | Tantalum | Radial | 4.7 uF / 25 V | 4.85 uF | 2.21 Ω |
| Matsushita | Tantalum | Radial | 4.7 uF / 25 V | 4.83 uF | 2.27 Ω |
| Matsushita | Tantalum | Radial | 4.7 uF / 25 V | 4.69 uF | 2.23 Ω |
| Matsushita | Tantalum | Radial | 4.7 uF / 25 V | 4.90 uF | 1.95 Ω |
| Matsushita | Tantalum | Radial | 4.7 uF / 25 V | 4.67 uF | 2.34 Ω |
| Matsushita | Tantalum | Radial | 4.7 uF / 25 V | 4.87 uF | 2.16 Ω |
| Matsushita | Tantalum | Radial | 4.7 uF / 25 V | 4.87 uF | 2.35 Ω |
| Matsushita | Tantalum | Radial | 4.7 uF / 25 V | 4.80 uF | 2.27 Ω |
| Matsushita | Tantalum | Radial | 4.7 uF / 25 V | 4.72 uF | 2.43 Ω |
| Matsushita | Tantalum | Radial | 4.7 uF / 25 V | 4.91 uF | 1.73 Ω |
| Matsushita | Tantalum | Radial | 4.7 uF / 25 V | 4.90 uF | 2.20 Ω |
| Matsushita | Tantalum | Radial | 4.7 uF / 25 V | 4.71 uF | 2.41 Ω |
| Matsushita | Tantalum | Radial | 4.7 uF / 25 V | 4.78 uF | 2.33 Ω |
| Matsushita | Tantalum | Radial | 4.7 uF / 25 V | 4.75 uF | 2.28 Ω |
| Matsushita | Tantalum | Radial | 4.7 uF / 25 V | 4.82 uF | 2.23 Ω |
| Matsushita | Tantalum | Radial | 4.7 uF / 25 V | 4.89 uF | 1.72 Ω |
| Nippon Chemi-Con | Electrolytic | Radial | 2200 uF / 16 V | 2570 uF | 0.06 Ω |
| Nippon Chemi-Con | Electrolytic | Radial | 2200 uF / 16 V | 2200 uF | 0.07 Ω |
| Nippon Chemi-Con | Electrolytic | Radial | 2200 uF / 16 V | 2390 uF | 0.06 Ω |
| Nippon Chemi-Con | Electrolytic | Axial | 2200 uF / 6.3 V | 2063 uF | 0.09 Ω |
| Nippon Chemi-Con | Electrolytic | Radial | 1000 uF / 35 V | 974 uF | 0.08 Ω |
| Nippon Chemi-Con | Electrolytic | Radial | 1000 uF / 35 V | 960 uF | 0.08 Ω |
| Nippon Chemi-Con | Electrolytic | Radial | 470 uF / 6.3 V | 412 uF | 0.30 Ω |
| Nippon Chemi-Con | Electrolytic | Radial | 470 uF / 6.3 V | 409 uF | 0.30 Ω |
| Nippon Chemi-Con | Electrolytic | Radial | 220 uF / 63 V | 209 uF | 0.22 Ω |
| Nippon Chemi-Con | Electrolytic | Radial | 220 uF / 63 V | 213 uF | 0.21 Ω |
| Nippon Chemi-Con | Electrolytic | Radial | 220 uF / 10 V | 181 uF | 0.71 Ω |
| Nippon Chemi-Con | Electrolytic | Radial | 33 uF / 16 V | 29.80 uF | 4.81 Ω |
| Nippon Chemi-Con | Electrolytic | Radial | 33 uF / 16 V | 30.30 uF | 4.40 Ω |
| Nippon Chemi-Con | Electrolytic | Radial | 33 uF / 16 V | 29.10 uF | 4.54 Ω |
| Nippon Chemi-Con | Electrolytic | Radial | 33 uF / 16 V | 28.10 uF | 4.93 Ω |
| Nippon Chemi-Con | Electrolytic | Radial | 33 uF / 16 V | 31.80 uF | 3.69 Ω |
| Nippon Chemi-Con | Electrolytic | Radial | 33 uF / 16 V | 28.20 uF | 4.46 Ω |
| Nippon Chemi-Con | Electrolytic | Radial | 33 uF / 16 V | 28.50 uF | 4.73 Ω |
| Nippon Chemi-Con | Electrolytic | Radial | 33 uF / 16 V | 28.15 uF | 4.74 Ω |
| Nippon Chemi-Con | Electrolytic | Radial | 33 uF / 16 V | 30.40 uF | 3.96 Ω |
| Nippon Chemi-Con | Electrolytic | Radial | 33 uF / 16 V | 29.41 uF | 4.74 Ω |
| Nippon Chemi-Con | Electrolytic | Radial | 33 uF / 16 V | 31.10 uF | 3.96 Ω |
| Nippon Chemi-Con | Electrolytic | Radial | 33 uF / 16 V | 28.62 uF | 4.76 Ω |
| Nippon Chemi-Con | Electrolytic | Radial | 33 uF / 16 V | 32.19 uF | 3.59 Ω |
| Nippon Chemi-Con | Electrolytic | Radial | 33 uF / 16 V | 28.39 uF | 4.66 Ω |
| Nippon Chemi-Con | Electrolytic | Radial | 33 uF / 16 V | 29.82 uF | 4.53 Ω |
| Nippon Chemi-Con | Electrolytic | Radial | 33 uF / 16 V | 29.15 uF | 4.45 Ω |
| Nippon Chemi-Con | Electrolytic | Radial | 33 uF / 16 V | 30.10 uF | 4.45 Ω |
| Nippon Chemi-Con | Electrolytic | Radial | 33 uF / 16 V | 30.10 uF | 4.05 Ω |
| Nippon Chemi-Con | Electrolytic | Radial | 33 uF / 16 V | 30.60 uF | 3.91 Ω |
| Nippon Chemi-Con | Electrolytic | Radial | 33 uF / 16 V | 31.07 uF | 4.10 Ω |
| Nippon Chemi-Con | Electrolytic | Radial | 33 uF / 16 V | 31.92 uF | 3.66 Ω |
| Nippon Chemi-Con | Electrolytic | Radial | 33 uF / 16 V | 29.40 uF | 4.59 Ω |
| Nippon Chemi-Con | Electrolytic | Radial | 33 uF / 16 V | 29.20 uF | 4.60 Ω |
| Nippon Chemi-Con | Electrolytic | Radial | 33 uF / 16 V | 31.80 uF | 3.71 Ω |
| Nippon Chemi-Con | Electrolytic | Radial | 33 uF / 16 V | 30.50 uF | 3.88 Ω |
| Nippon Chemi-Con | Electrolytic | Radial | 33 uF / 16 V | 30.00 uF | 4.34 Ω |
| Nippon Chemi-Con | Electrolytic | Radial | 33 uF / 16 V | 32.30 uF | 3.57 Ω |
| Nippon Chemi-Con | Electrolytic | Radial | 33 uF / 16 V | 30.00 uF | 4.44 Ω |
| Nippon Chemi-Con | Electrolytic | Radial | 33 uF / 16 V | 28.76 uF | 4.89 Ω |
| Nippon Chemi-Con | Electrolytic | Radial | 22 uF / 25 V | 22.30 uF | 3.85 Ω |
| Nippon Chemi-Con | Electrolytic | Radial | 22 uF / 25 V | 21.54 uF | 4.02 Ω |
| Nippon Chemi-Con | Electrolytic | Radial | 22 uF / 25 V | 19.10 uF | 6.56 Ω |
| Nippon Chemi-Con | Electrolytic | Radial | 22 uF / 25 V | 19.67 uF | 6.36 Ω |
| Nippon Chemi-Con | Electrolytic | Radial | 22 uF / 25 V | 19.10 uF | 5.94 Ω |
| Nippon Chemi-Con | Electrolytic | Radial | 22 uF / 25 V | 21.46 uF | 4.38 Ω |
| Nippon Chemi-Con | Electrolytic | Radial | 10 uF / 100 V | 10.75 uF | 1.94 Ω |
| Nippon Chemi-Con | Electrolytic | Radial | 10 uF / 100 V | 10.85 uF | 2.03 Ω |
| Nippon Chemi-Con | Electrolytic | Radial | 10 uF / 100 V | 10.80 uF | 2.24 Ω |
| Nippon Chemi-Con | Electrolytic | Radial | 10 uF / 100 V | 10.78 uF | 2.29 Ω |
| Nippon Chemi-Con | Electrolytic | Radial | 10 uF / 100 V | 10.83 uF | 2.34 Ω |
| Nippon Chemi-Con | Electrolytic | Radial | 10 uF / 100 V | 10.84 uF | 1.90 Ω |
| Nippon Chemi-Con | Electrolytic | Radial | 10 uF / 63 V | 10.20 uF | 3.00 Ω |
| Nippon Chemi-Con | Electrolytic | Radial | 10 uF / 63 V | 10.10 uF | 4.00 Ω |
| Nippon Chemi-Con | Electrolytic | Radial | 4.7 uF / 450 V | 5.54 uF | 9.97 Ω |
| Nippon Chemi-Con | Electrolytic | Radial | 4.7 uF / 450 V | 5.67 uF | 9.79 Ω |
| Nippon Chemi-Con | Electrolytic | Radial | 4.7 uF / 450 V | 5.76 uF | 9.60 Ω |
| Nippon Chemi-Con | Electrolytic | Radial | 4.7 uF / 450 V | 5.65 uF | 9.81 Ω |
| RIFA | Film | Radial | 15 nF / 250 V | 30.05 nF | 11.6 kΩ |
| RIFA | Film | Radial | 15 nF / 250 V | 25.65 nF | 10.1 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? I guess I was frightened enough by the old tantalum capacitors blowing up unexpectedly on 1980s IBM system planars. There's no relation between those three-legged orangish tantalum capacitors and the rectangular Matsushita ones. But then again, why take the risk? All the new replacement capacitors were cheap compared to the whole value of a working HP LCZ meter.
Conclusions
Conclusions? Draw your own conclusions! The old Japanese capacitors are better than some of the new ones, especially if they are of the el cheapo type.
But to be fair, here's a list of my conclusions.
- The noisy fan replacement was a success on its own. I can totally work for hours with the machine powered on next to my ears without any fatigue.
- Besides the visibly damaged RIFA film capacitors, 95 % or more of the replaced parts are still well within their rated tolerances. But 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 made quite a bit of a mess on the backplane PCB. It's good that the corrosion didn't damage the card slots, or any of the signal traces. I am pretty sure that the Ni-MH solution that I came up with is better in all respects.
- All 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 are still amazing after all these years.
Finally, this unit can go back to my electronics workbench. I expect many years of quality service.
Your Help Matters
This content is provided as-is and is not for commercial purposes. It reflects my experiments and research and should be treated as such. I release my work to the public for educational purposes. I did all this at my own expense and in my free time. So if you like my work, or find it useful or inspiring for your projects, please consider making a donation.
Copyright © 2004- Alexandru Groza
All rights reserved.
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