DC power supply, rattling before breaker trips

[nilium]

Hi all,

I salvaged an old HP 6268B regulated DC power supply and was just testing it out. As I crank up the voltage, a loud rattle occurs at about 25V 15V, which then disappears above 28V 17V or so. If I keep it between 25-28V 15-17V (rattling) for two or three seconds, the breaker trips. A quick glance inside revealed nothing suspicious.

Not really sure where to start investigating. Any pointers will be much appreciated.

NV

[David_AVD]

Just a pure guess without a schematic:

Could be the main filter capacitors are pretty much open circuit causing the transformer tap switching relay(s) to bobble about unhappily?

[aargee]

I don't know of the insides of this power supply but on my Topward supply something similar happens normally going above around 15V. It is caused by a relay selecting a higher winding on the transformer (I think), with a certain amount of hysteresis. In your case it does sound like a relay attempting something similar but with other issues, a problem with the transformer? Or the rapid switching is causing load issues for the breaker?

[Andy Watson]

"Rattle" ?

Is it a mains hum type of sound? These units employ a pair of thyristors to "pre-regulate" the supply to the transformer. My guess would be that there is something wrong with the thyristors and/or their control mechanism. One of the pair is not firing properly such that the transformer is being fed with partially rectified mains - leading to saturation of the core - high current levels and then the breaker trips.

First thing to do is to download and study the manual:
http://cp.literature.agilent.com/litweb/pdf/5950-1766.pdf

[RJFreeman]

First thought would be. is the Voltage set Potentiometer noisy at around that section, and this is ultimately tripping the crowbar circuit?
Although I would have thought the crowbar would trip hard...

The 'loud rattle' could be due to the pre-regulator causing the Transformer some grief (the 6268B like its cousin the 6269 has Traic based pre regulator so does not do tap switching as such)

BTW in case you don't have one already, the service manual (mostly readable)for the 6269B is available here:
http://bama.edebris.com/manuals/hp/6269b/

[nilium]

Thanks for the responses. Just to clarify, the rattle is similar to that of an arc welder as the weld is being made (though not normal in case of this PS). I will try to record a video of it tomorrow and post it in case it is useful. If anyone has any good thoughts on what to probe to gather better insight, please let me know.

NV

[RJFreeman]

Sounds like the noise is being caused by the pre-regulator (although this could be due to a noisy voltage setting pot).
try running the power supply with a remote voltage control as per figure 3-4 - if this is OK then you have a noisy pot.

[edpalmer42]

Let's back up a step.

You've got a 6268B.  I see in the options that it can be factory wired for either 110 or 220V.  What options are your unit equipped with and what do you have it plugged into?  When you say the breaker is blowing, is that the one on the front of the unit or the one feeding your outlet?

Blowing breakers means that you could be close to blowing the thing to kingdom come.  I'd be very nervous about turning it on at all until you do some careful thinking and testing.

Ed

[oldway]

The preregulator is oscillating at low frequency.
This instability occurs because the big electrolytic capacitor has lost his value.

[nilium]

Here's the video I promised http://goo.gl/Nk2NMc. In my original post, I misquoted 25-28V as the troublesome range. It's more like 15-17V, which is where the unit has been primarily used for most of its life. I also noticed it can't get to the maximum 24V but only to 18V despite turning the knob all the way.

Both the breaker in the breaker box and the in-unit one have tripped in separate occasions, never at the same time.

[lowimpedance]

As suggested I would look first at the pre-regulator for damage. You need to remove the RFI unit and look inside for burned stuff as a first step.
Now be careful here with the way its mounted, put it back the same way it was. Check the service manual relevant sections.
And also look at replacing electrolytics in the pre reg cct (and then the rest of the supplies control cct's).

[bktemp]

There is definitely something wrong with the preregulator. These power supplies make noises at normal operation because of the preregulator, but only when loaded.
You can almost hear the transformer saturating before it trips.
Be careful, the whole heatsink for the preregulator is at mains potential.