I bought an agilent branded 6632B system power supply.
It's built end of 1999, owned by NOKIA, and last calibrated in 2006. Maybe it was used afterwards for a few years w/o further calibration.
On unpacking, it smelled strongly, like being stored for a longer period of time in humid condition.
It powered on, and operated properly, but all parameters were off a bit.
After successful calibration, everything was spot on during verification.
On the 2nd day, it worked for a few minutes only, then it went into protected mode, and errors 1, 2, 3, 4, 5, 80, 11 came on simultaneously.
That bunch of error messages indicated, that the communication between the main processor and the analogue part, like FPGA, EEPROM, A/Ds, D/As, and so on, failed.
It turned out, that the analogue circuits were not powered any more, as the +/-15V, +5V supplies were completely dead; F300 and F301, two 4A mini-fuses, were blown.
So the fault would probably be located somewhere in the analogue power supply components.
The most suspicious Ta caps C496, C497 showed no short, when measured relative to GND.
It was also possible to power + 15V (220mA) and - 15V (-48mA) by an external PSU.. even the fan came on.
So in first instance, I replaced the mini-fuse by two multi-fuses.
When powering the unit on again, it did a big bang, followed by a sizzling noise, and the Magic Smoke escaped from one of the suspicious Ta caps.
Now, both of them (1µF, 35V) were short.
I replaced them with 100nF SiBaTit caps, blue components, which should be sufficient for this purpose.
The unit now powered on w/o any problem, even the calibration was still fine.
The error root causes up to now:
1) Thou shalt not place Ta-caps directly on power entry
2) Thou shalt have resistors in series with Ta-caps, about 5 Ohm/Volt, to limit current and dI/dt
But that's not the end of the story, yet.
When I tried to measure the voltage directly across one of the replaced caps, expecting about 25V, I got no reasonable reading.
That was very strange, but a test of their common point showed no connection to ground.
So I assumed, that the over-current burnt the PCB trace.
But neither on top, nor on bottom of this two-layer-board, there was a trace to GND.
So both caps had been put in series, 55V in total across them.
The voltage distribution between both caps was undefined, maybe only given randomly by their individual leakage currents, so that one of them might have been loaded with the full 55V:
3) Thou shalt have a voltage balancing resistor network, if you place capacitors in series
4) Thou shalt not place too high voltage over Ta-caps, good reliability requires max. 2/3 of nominal voltage
Ta caps may withstand 30% overvoltage (surge voltage), for short time periods. Constant overvoltage will reduce their reliability and lifetime greatly.
After all, 25V as intended, would have been ok.
One of the capacitors may have had failed before, and the humidity inside their mold compound, or the drying of the components may have sped up the fault mechanism.
Now, the final root cause was revealed by looking closely into the schematics:
The design engineer simply forgot to place a dot on the line crossings, between the caps and the GND line.
Well, this unit was obviously produced for 20 years, in thousands of pieces, with these 3 fold design flaws, and I wonder, how many units failed due to this.
As this PSU family will be terminated mid of next year, I doubt that Keysight will make a further service note on this issue.
Frank
PS: Forgot to mention, that I of course bodged in a wire from GND of the 7815 to the common point of both capacitors, on the bottom side of the PCB.
I recommend to replace the Ta capacitors, also.
These ugly stickers, i.e. the calibration and the NOKIA inventory stickers, can be removed by carefully heating them with a hair dryer (not too hot!), and then peeling them off with a plastic scraper
Home
Help
Search
Login
Register
