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HP 400F Power Supply Troubleshooting Help Needed!

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David D:
I have an HP 400F that has an issue with the power supply that's got me stumped  |O .

The original problem was that the power supply is supposed to deliver +26V and -26V and the negative rail was supplying -46V instead. The negative rail works the same as the positive rail, except that it gets its regulation from the +26V supply fed into Q7. Q7 was dead (open) so I replaced it with a new 2N3906 (as indicated in the factory manual). I also replaced all electrolytics with new.

The problem now is that with Q7 and the electrolytics replaced, now I'm only getting -1.4V to ground on the negative supply. Even though they checked ok, I tried replacing Q8 and Q9 and I still only have -1.4V on the negative rail.

Any measured voltages I'm getting that are different from the indicated factory voltages are indicated by the red arrows. Can someone help? What am I missing?  :-//


 

TheDefpom:
I would say to double check you put the caps in the right way around, with the negative rail ones + going to 0V... easy to mix that up.

Seems Q7 is turned on hard, so maybe check R26, R30, CR12

David D:
Thanks Scott.

I double checked all of the cap polarities and they're correct. I also checked R26, R30, and CR12 and they all check out ok. I also pulled and checked a couple of the tantalums (C24, C25) to make sure they're not shorting and they're fine.

I disconnected the negative rail jumper to separate the power supply from the rest of the meter and still get the same results, so the problem is isolated to the power supply itself. I did notice that the negative rail voltage starts off with around -8V when you first swith on the meter, but the voltage starts to drop after that.

Technical Ted:
This is a coincidence because I just bought one and repair it a few days ago! Mine also had a -26V PS issue. My C23 cap was shorted causing the voltage to drop to ~-6V with load. I don't have an answer for your issue other than check all the components/connections in the neg supply circuitry. Here are some general things I learned about HP's documentation for this meter.

1- The latest and greatest copy of the manual I could find was on Keysight's site, although it was a pretty rough scan copy.
2- Make sure you go through the last section of the manual and note any changes for your serial number range. Earlier serial numbers may have different circuitry/components. Mine was a later model 950- series with no changes to worry about.
3- I found a drawing error that I assume got missed when they changed the pre-amp from a negative -26V supply to a +26V supply. The drawing shows -26V going to the attenuator relays when in fact it is feed by +26V (confirmed on my unit with ohm meter and physically following traces.
4- What did you replace Q9 with? Did you find a direct replacement somewhere?
5- Note that if needed and the - jumper has been removed that you can load the circuit with a 560 ohm resistor to properly load the circuit directly. This was key for me. I found this by reading the noted manual. I had read earlier version of the manual and never read this in any of them. Only the one from Keysight's site states this: (paragraph 5-45)

Good luck. I still have the unit on my bench if you want me to check a voltage for you or whatever... don't know how much help I can be.

Ted

Technical Ted:
Oh, I forgot to mention that CR12 and CR13 are physically swapped in position from the layout diagram. Mine were marked on the board. This most likely depends on the version (serial number range) of your unit. I wanted to mention this because if you checked CR12 you may have actually been checking CR13, so I suggest to double check this.

Good luck,
Ted

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