HP 8753C VNA repair case

[Bud]

Here is a fascinating story about a tiny capacitor that took down a monster $$$K device.  I did this repair some time ago on my 8753C VNA.

VNA all of a sudden stopped working. Could not power it up anymore. It looked as if the protection circuit kept triggering  on one of the secondary voltage supply rails.  After troubleshooting the problem was traced to the power supply which stopped producing the proper +5.1V voltage.


Inside the power supply. The left half is the power stage and rectifiers. The right half is line input and the controller circuit, which was of my interest.


I had to reverse engineer the PS unit a bit around the controller IC - see a fragment of the schematic below . To my surprise, found a 0.22uF bypass capacitor developed a leak, corrupting the reference voltage on pin 2 (the controller IC error amplifier). That caused the power supply +5V channel voltage change erratically between 4.5 to 5.1V which apparently triggered the under voltage protection circuit on the 5.1V supply rail which in turn was shutting down the whole power supply.


And here it was the bad guy... darn penny cap that was causing multi $$$K pain in the arse


The bad capacitor was replaced with a mica 0.22uF new one. After the replacement the +5.1V voltage became stable within 0.1mV and the problem was gone. The VNA powers up normally now.

[TVman]

Well, what do you expect these days?
Do you expect it to last 100 + years with no problem?
(Conspiracy theory time)
Those designers purposely do that so that way they can charge you A grand just to fix that tiny, stupid capacitor.
It's just to get money.


And, Hey good job finding that fault.

Hunor

[AlfBaz]

Nice work tracking down that fault Bud.

SMPS's always give me ulcers, if you get it wrong sparks can fly.
The last one I fixed was for a wavetek signal gen. Frustratingly it looked a hell of a lot like a PC power supply but the voltages were all wrong. I found a bad cap, replaced it and still nothing until a member here pointed out that some of these psu's wont produce output without a load. I promptly loaded it up and bingo, worked like a charm