Microelectronics | Hewlett-Packard 6033A Autoranging System DC Power Supply

Synopsis

I needed a power supply for one of my transceivers. The requirements were pretty simple as the power supply had to be able to source 13.8 V at 25 A, effortless. Finding a good power supply in a new condition that doesn't cost an arm and a leg is a bit difficult. Especially if you factor in the reliability clause.

Thus, I had to resort to the used market. As I was browsing various auction sites, I stumbled upon this HP 6033A unit. It won't supply the whole 25 A at 13.8 V, but according to the datasheet, it will do around 17.5 A, just fine. I decided to take the chance and bought it. Once it arrived, I tested it, and it proved to be in good working condition. Albeit noisy and containing the RIFA time ticking bombs inside. Did I mention the half a kilogram of bonus dust? And the dirty overall condition. Other than that, there is nothing wrong with it. Nobody messed with the internals.

I have some experience with old Hewlett-Packard test equipment and I found the service manual on the Keysight page for this power supply. Well done preserving the documentation for legacy products!

In the following lines, I'll share the steps that I followed in order to deem this unit as worthy of being part of my microelectronics lab.

Repair and Maintenance

First, I dismantled everything and washed the exterior of the aluminum case with soapy water. I removed the years of dirt, restoring the original gray color.

Next, I assessed the situation and made an inventory of all parts that I will replace.

I have this improvised screw sorting tray that I am often using for my projects. It came in handy this time. Just a mention regarding these screws. They are PZ style and not PH. You need to use a PZ2 (and sometimes a PZ1) screwdriver, otherwise you could damage the screw heads.

Finally, I spent a lot of time cleaning the PCBs of dust. Mostly, I have used compressed air and isopropyl alcohol. However, I ended up washing two printed circuit board assemblies as they were so dirty.

HP-IB Board

There is one RIFA cracked capacitor on this printed circuit board. It is located on the left edge of the card, next to the relay control socket. I replaced it with a new WIMA MKP-Y2 series safety film capacitor.

While here, I measured the two supply ripple filter electrolytic capacitors and they tested OK.

Here is a detailed view on the new WIMA film capacitor. It fits perfectly as there are other factory installed WIMA film capacitors in various positions within this power supply.

I like these diagnostics LEDs.

The ROM is coded 5080-2122 REV. A VER. 1.0 and is contained within a UV-erasable EPROM IC of type D27128A made by Intel. In other words, a 16 Kb EPROM.

I wanted to save the microprogram for future needs, should the existing ROM matrix degrade in any way. I carefully desoldered the IC, cleaned the pins, and used my TL866II programmer to read the whole memory. Then, I installed a Mill-Max precision machine socket and inserted the EPROM.

This is what the solder side looks like on the EPROM socket. As a personal note, I suspect the circuit layout is computer generated.

This board is now ready to return to the power supply.

But first, there is a stacked PCB that connects to the control board with two data cables. This PCB is fixed directly on top of the HP-IB board with plastic spacers. Besides cleaning the dust, there is nothing to do on this printed circuit board.

There are other factory installed WIMA film capacitors. They could have used cheap 1 uF / 63 V electrolytic capacitors. But they chose reliability over production cost. Good job. In time, it pays off. In case you wonder, the axial electrolytic capacitors are tantalum parts.

Let's advance to the next board.

Control Board

There was nothing for me to do on this board, besides a good cleaning. I took a picture of it, anyway. By the way, the PCB is coded 06023-60023.

I guess those two big electrolytic capacitors will have to be replaced at some point. For now, they still test well within their specs.

Power Mesh Board

Coded 06023-60022, the power mesh printed circuit board is a robust assembly containing a couple of parts that might need a bit of attention.

In my case, the two power MOSFETs and their heat sinks were so dirty that I had to disassemble them in order to clean everything. And I did well; the thermal compound hardened and was very brittle. The PCB was so dirty that I had to wash it in the bathtub. In the end, I dried it with compressed air and then let it in the sun for a couple of hours.

Here is the circuit board without the big heatsinks.

While here, I preventively replaced the 470 uF / 16 V with a newer capacitor of the same type, albeit rated at 25 V. In terms of construction and physical dimensions, it is identical to the old one. However, once I measured both of them on the LCZ Meter, the old one measured better than the new one. I left the new one, for now.

There is a resistor that looks a bit tired. It is a 10 Ω / 2 W part. In fact, there are three identical resistors on this circuit board assembly. Two of them are used as part of a loading circuit for the power MOSFETs while the remaining one is bypassing the CR7 output rectifier power diode.

I cleaned the heat sinks in the ultrasonic bath.

Next, I added new heat transfer thermal paste and installed the heatsinks and the MOSFETs back on the PCB assembly.

All good for now, let's move further.

Fan Replacement

The original fan is powered directly from the 115 Vac line. While it is very powerful, it is also very noisy. It does a very good job, especially when the unit is on 24/7 in a tight system rack, or in harsh environments. I plan on using this power supply with one of my HF transceivers. When the transceiver is receiving, the current draw does not exceed 1.5 A. When transmitting though, it can go as high as 10 to 15 A, depending on the power setting. Sure, it can go up to 23 A, but the HP unit will enter protection mode.

Given these prerequisites, I reckon a Noctua NF-A8 FLX fan will do just fine. However, there is no steady 12 V rail anywhere in the power supply. There is an 8.9 V, a bit low, and it's regulated through U4 on the power mesh board. This IC doesn't even have a heatsink on it (note to myself: maybe I should install one during the next revision). But, there's the 20 V rail that also serves as input to U4.

Getting 12 V out of 20 V can be done through a voltage dropping resistor of around 120 Ω. Considering the Noctua fan draws 0.07 A, total power dissipation on the resistor should be around 0.6 W. Conveniently, I dismantled two Noctua NA-RC10 low noise adapters and recovered two 51 Ω resistors. Next, I soldered them in series. They seem to be garden-variety carbon film parts, but they'll do fine for this purpose.

Thermo tube applied. For a clean appearance, the wires and the voltage dropping resistors will be additionally covered by the original Noctua varnish.

And conveniently enough, the 100 nF C12 on the power mesh PCB, serves as a good soldering point for stealing power for the Noctua fan.

I installed the new fan; the direction of airflow is from outside to inside.

I cleaned the outer fan grill with an ultrasonic cleaner. It was very corroded, and I thought I had to source a new one. But after 30 minutes in the ultrasonic bath, the thing came up like new.

The end result is that the power supply is not really silent, but it's not really loud either. I can work the HAM radio bands without getting myself a headache from the sound of the former howling wind turbine.

Old Parts Measurements

Next, I measured the old parts with my HP 4276A LCZ Meter.

All of them are cracked, and it's a miracle they haven't released any smoke, yet.

CAPACITOR DATA
Brand	Type	Style	Rating	Capacity	ESR
RIFA	Film	Radial	10 nF / 250 V	14.62 nF	10.2 kΩ
RIFA	Film	Radial	10 nF / 250 V	13.79 nF	5.0 kΩ
RIFA	Film	Radial	4.7 nF / 250 V	6.77 nF	19.3 kΩ
RIFA	Film	Radial	4.7 nF / 250 V	7.18 nF	16.9 kΩ
RIFA	Film	Radial	2.2 nF / 250 V	3.35 nF	73.4 kΩ
RIFA	Film	Radial	2.2 nF / 250 V	3.23 nF	59.0 kΩ
RIFA	Film	Radial	2.2 nF / 250 V	3.33 nF	45.8 kΩ

The RIFA capacitors behave more like unstable series RC networks (snubbers or dampers) as they have developed high resistance. Why unstable? Because the resistance rises slowly in time, if left under measurement for a couple of minutes. Of course, they went directly to the bin.

Conclusions

As with the majority of HP gear, it was a joy to work on this unit. I dismantled everything with little hassle, and the replacement parts were readily available. As always, I enjoyed reading the schematic diagram and the technical documentation for this unit. I learned some new things and improved my understanding on this series of power supplies.

I initially looked for WIMA metalized paper film capacitors, but I was unable to find them anymore. Apparently, they discontinued the entire MP series. Interestingly enough, I was able to buy them back in 2021, when I fixed my Hewlett-Packard 6624A System DC Power Supply. Oh well, good thing that I found these MKP replacements. I've seen that Kemet also offers safety film capacitors. And even identical RIFA parts are still available as new.

Finally, I found a place for the power supply, under the desk, near the transceiver.

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