Putting back to life a malfunctioning HP 8753C VNA

A true detective story about a interesting repair project

The Hewlett-Packard 8753C Vector Network Analyzer is a massive (literally) instrument for measuring complex reflection and transmission parameters of the device under test (DUT) circuits. The VNA class of 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. It is now changing due to self-education and increased interest in RF measurements, affordable cost of the used equipment on the aftermarket and DYI VNA projects such as the N2PK VNA. So when I got a chance I put my hands on a HP8753C that was for sale at a local RF shop.

The HP8753C is a 3GHz device with a 6GHz option. The unit that I got was 6GHz one with a matching HP85047A S-parameters Test Set. 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 Test Set converts the 3GHz source frequency range coming out of the VNA source port into 6GHz range on the Set's DUT ports, but the 6GHz receiver is installed in the VNA itself as the 6GHz option.

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.

  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. The LEDs on the S-Parameters Test Set turned on and off as the VNA was cycling the power. Clicking sounds produced by the attenuator and switch relays could also be heard.

  The analysis

The symptoms resembled the behavior of a overloaded switching power supply that I have seen before in malfunctioned TVs or computer CRT displays. When such a malfunctioning device is powered on, the power supply attempts to come up and a startup hissing sound can be heard coming out of the switching power supply transformer and/or a burst of static produced by the CRT as the high voltage is applied to the CRT, but if there is a short circuit or some other problem on the secondary power lines, the power supply shuts itself down and depending on the design it may attempt to restart. If the overload condition was still present it caused the device to continuously cycle the power.

Every once in a while the 8753C 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 HP8753C Service Manual lists a set of procedures to check the power supply. 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 Service Manual instructs to remove the boards one by one to identify which one may be causing 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 now 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. 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 of the box had the power stage and rectifiers. 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 controller is built on the Motorola SG3527A pulse width modulator IC. There is a small transformer (not shown in this schematic) and a LM317 voltage regulator that produce a +10V voltage supplied to the VCC pin of the controller IC. The internal reference voltage regulator in the IC converts the VCC voltage into the +5.1V voltage that via the pin 16 is supplied to the external shutdown logic and to a circuit with a Zenner that converts the +5.1V into the +1.2V reference voltage that is applied via pin 2 to the non-inverting input of the PWM error amplifier. The inverting input of the error amplifier is connected via pin 1 to a voltage divider that produces +1.2V from the +5.1V sense feedback loop from the VNA. The feedback loop is simply a trace on the VNA motherboard that is connected to the +5.1V supply rail coming out of the post-regulator board with the LEDs which was described earlier. Therefore the PWM controller was comparing two +1.2V voltages, one produced by the Zenner in the pre-regulator, and the one coming from the sense feedback loop from the VNA motherboard. 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. The first thing that came to mind was to check the +5.1V feedback loop voltage coming from the VNA motherboard, but monitoring the +5.1V supply rail on the motherboard did not produce a useful result. The next thing to check was the +1.2V reference voltage on the PWM IC pin 2 generated from the VCC. 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... bingo - I noticed when the post-regulator detected a shutdown condition (the board LEDs were all flashing), the +1.2V reference voltage on the pin 2 dropped by 0.15V. That was the breakthrough and I realized that the shutdown condition was caused not by a overcurrent but by a undervoltage. It was the corrupted +1.2V reference voltage on the pin 2 that caused the switching pre-regulator output voltage to drop, which caused the post-regulator +5.1V supply rail to drop, which caused the post-regulator to detect a undervoltage condition and shutdown the PWM controller, who then tried to restart itself.

So far so good, but what was the reason for the +1.2V reference voltage on the pin 2 to change? I checked the +10V VCC, it was stable. Checked the +5.1V output on pin 16, it was stable. Checked the voltage across the Zenner, it was +1.2V and perfectly stable. 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. The problem was now isolated and caused by the +1.2V reference voltage being corrupted right on the pin 2 of the controller IC.

There was no other circuits connected to the pin 2 other than the bypass capacitor which I marked on the schematic as C6. It was a small yellow axial 0.22uF bypass capacitor. I cut one of the legs off and connected the ohmmeter across the capacitor - it measured a resistance. The capacitor somehow developed the resistance internally probably because of aging, and that resistance formed a voltage divider with the resistor connected to the Zenner, corrupting the +1.2V reference voltage on the pin 2 coming into the PWM error amplifier. I was stunned - a one penny capacitor took down the multi-K dollar device.

  The solution

To check my theory, I soldered a new 0.22uF mica capacitor and the VNA was back up and running. I monitored the pin 2 for a while, the +1.2V reference voltage was perfectly stable. The problem was solved. The following pictures show the affected part of the pre-regulator board with the PWM controller IC and C6 capacitor.

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.

Conclusion

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.

References

1. HP8753C Service Manual

2. Motorola SG3527A Pulse Width Modulator IC Datasheet.

3. HP 85047A S-Parameters Test Set Operating and Service Manual

In case you have questions, please feel free to email me at the address below.

Contact: miv@makarov.ca