This manual may contain references to HP or Hewlett-Packard. Please note that Hewlett-. Packard’s former test and measurement, semiconductor products and. Chapter 2 Getting Acquainted with the HP C How to Get the Most Out of the HP C “General Information” section of the Operating Manual has. This page is devoted to the repair of my HPC Network Analyzer. section or page of the HPC Service Manual PDF file) and got these results.
The Hewlett-Packard C Vector Network Analyzer is a massive literally instrument for measuring complex reflection and transmission parameters of the hp87553c under test DUT circuits.
The VNA class mannual instruments was not wide known to or used by the Ham Radio enthusiasts before recently because of very high cost of that type of equipment. The Test Set basically has a frequency doubler to multiply the source signal frequency by 2, RF couplers on the two test ports and RF switches to swap the ports. The cost of this type of equipment is still high these days and is in the range of several thousand dollars.
So when one dies on you it is not a good feeling. This was what I experienced one day when my VNA became erratic. The problem was with the device not powering on or restarting unexpectedly. There was no a specific pattern that would allow to predict as to when this may happen next time.
I had no choice other than to download the Service Manual and deal with the problem. When the power switch was activated, the VNA appeared to be starting up but as soon as the CRT display was about to come up it produced a hissing sound and the device shut down and immediately restarted in a continuous loop.
Clicking sounds produced by the attenuator and switch relays could also be heard.
The symptoms resembled the behavior of manuaal overloaded switching power supply that I have seen before in malfunctioned TVs or computer CRT displays. If the overload jp8753c was still present it caused the device to continuously cycle the power. Every once in a while the C was able to start and work for a period of time and then all of a sudden restarted as was described above.
The problem was intermittent, so the troubleshooting promised to be challenging. The Service Manual instructs to remove the boards one by one to identify which one may be mqnual the overload condition, which I did with no success.
I then tried disconnecting the power from the display and that at first let the power supply to start normally, so my first suspect was the display. At least mmanual the power supply was running and I was able to check the voltage and ripple levels on the pre- and post-regulators per the Service Manual and it all looked fine.
Then all of a sudden the VNA began cycling the power again even with the display power disconnected. So it appeared the problem was not in the display circuitry.
This time removing one or two boards in no particular order allowed the power supply to start. That puzzled me off completely. I got a gut feeling that the problem may be with the power supply itself.
The Service Manual suggested replacing the switching pre-regulator, which was not an option for me because I did not have a replacement.
EB5AGV’s Workbench: HPC repair
So I removed the switching pre-regulator box and opened it in a hope to find the answer there. The construction of the switching power supply is interesting and deserves a larger photo.
The pre-regulator box is made of two halves bolted together via a EMI strip gasket that seals the gap in the assembly to reduce emissions. It can be seen in the above picture along the edge of the right half of the box. The two halves of the box have connections with each other via disconnectable wire cables. The left half manuaal the box had the power stage and rectifiers.
8753C Network Analyzer, 30 kHz to 3 GHz
The right half was the line input and the controller circuit, which became the subject of my further investigation. After spending time with a pen, paper and a continuity meter to understand the schematics, and search on the Internet for the datasheets of the parts used in the pre-regulator, I got what I wanted which was understanding the power supply PWM controller circuit.
The following part of the schematic shows the critical elements of the PWM controller. The difference between the two was amplified and steered the PWM to regulate the output voltage of the pre-regulator. It then became apparent that the problem may be with the PWM controller which for some reason restarted intermittently. It can be seen from the schematic that this voltage is supposed to be stable and present on the pin 2 all of the time as long as the VNA power switch is turned on.
I connected a scope with a DC input to the pin 2 and That was the breakthrough and I realized that the shutdown condition was caused not by a overcurrent but by a undervoltage. Checked the pin 2 again, the voltage was changing erratically. I could not believe my eyes.
I thought that the resistor that connects the Zenner to the pin 2 must be bad. I soldered a new resistor in parallel to it but that did not help. There was no other circuits connected to the pin 2 other than manuao bypass capacitor which I marked on the schematic as C6. It was a small yellow axial 0. I cut one of the legs off and connected the ohmmeter across the capacitor – it measured ph8753c resistance. I was stunned – a one penny capacitor took down the multi-K dollar device.
To check my theory, I soldered a new 0.
The problem was solved. This was a very challenging but interesting project and a good exercise for the brain. Thanks to the used equipment market for producing such a nice opportunity to learn. The VNA Service Manual did not go to the level of details about the power supply that were needed to repair it. But with some determination and efforts even a very complex equipment can be repaired.
Also, it was unusual that this type of component, a small axial bypass capacitor, would develop resistance and caused that much of a problem, rendering the whole system unusable. Good example of something being at the wrong place at the wrong time! Hoping that the other few hundred bypass capacitors of this type in the VNA will have been doing fine for the next 10 years or so.
The device has a switching pre-regulator encased in a separate box and the post-regulator removable board with all the secondary linear regulators. In the picture on the right the post-regulator board can be seen in the middle of the frame by the back panel. The board has LEDs along the top of the board that indicate power status for each post-regulator channel. During normal operations all the LEDs are on and start flashing when the shutdown circuitry detects a overcurrent, overvoltage or undervoltage condition.
The controller board before the fix. The bad C6 capacitor is the yellow one in the middle of the picture to the left of the controller IC pin 2. The controller board after the fix. The bad C6 bypass capacitor was replaced with a new mica red one which can be seen in the center of the picture.
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