Electronics > Repair

DSO-X 3024A Power Supply defect

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tapelu:

--- Quote from: Madmanguruman on December 04, 2020, 09:31:43 pm ---I'm liking the fact that the Murata unit is higher efficiency and most importantly does not have a perpendicular daughter board, like the meanwells. This should make the murata unit easier to repair, but then again, none of these units should need any attention for a long time.

The Murata PQC250-12 works just fine with three DSOX3014As that I've repaired - the oldest repair has had close to 1 year of constant use with no issues so far.

Like with other units, the wiring harness needs to be modified to make it work but it does offer remote sense as well as remote on/off (possibly with inverted logic, but the button will still work.)

I must also admit that I'm biased - I work at the Murata power supply design center where the PQC250-12 was designed and qualified.

Hi, how do I have to modify the wiring harness so that the power supply starts with the power switch? If I connect pin 1 to pin 8, the oscilloscope starts directly, but I want the power switch to be able to be used?

:-DD

--- End quote ---

Madmanguruman:
I'll share my reverse-engineering below.

The pinout of the Molex header (on the DSO-X side) is as follows:



1: +12V
2: +12V
3: +12V
4: +12V return
5: +12V return
6: +12V return
7: Return sense
8: +12V sense
9: Remote on/off

For the PQC250-12:

- Run 3 wires from the DSO-X Molex header pins 1-2-3 to +12V (any of J2 pins 1-6)
- Run 2 wires from the DSO-X Molex header pins 4-5 to +12V return (any of J2 pins 7-12)
- Run 1 wire from the DSO-X Molex header pin 6 to J3 pin 8 - this is the return for the remote on/off signal
- Run 1 wire from the DSO-X Molex header pin 7 to J3 pin 6 - this is the low-side remote sense
- Run 1 wire from the DSO-X Molex header pin 8 to J3 pin 5 - this is the high-side remote sense
- Run 1 wire from the DSO-X Molex header pin 9 to J3 pin 4 - this is the remote on/off signal
- The AC cable assembly will plug in directly without modifications.



Because the PQC250-12 is open-frame I usually put a thin insulating sheet behind the wiring harnesses / in front of the power supply.



Three DSOs with PQC250-12 are deployed in my lab, and all have passed their annual traceable calibration tests (done by an external accredited laboratory).

(Depending on the remote enable logic, you may find that the pushbutton is reversed: in=off, out=on - this can be changed via a few resistor pops/no-pops but why bother?)

tapelu:
 :-+
Perfect ! Many, many thanks for your effort !

ketch:
Hello. I need to know the value of the resistor from the photo.

martinr33:
I used madmanguruman's recommendation of a Murata power supply.

The only difference is, I used the existing sense lines from the oscilloscope board for the sense lines back to the small 8-pin connector. The reversed power switch is not bothersome.

I did try to troubleshoot the existing supply, but it was less cooperative than  I liked. The PSU has a lot more parts than  the example schematic for the 3842A (presumably for PFC) so there could be other failures.

My notes on that journey below.

Apply some 63/37 tin-lead solder to the pads before you try to remove parts. This technique is particularly effective on the metal frame. The existing lead free solder has a high melting point and is difficult to rework otherwise, the tin-lead trick makes it much easier to melt. Make sure that the solder flows for this trick to work. Alternatively, hunt down some indium based solder. 

1) It seems that units running on 220 - 250V might be more vulnerable to failure. The main cap is rated 400V, and can see 350V in some countries. Couple that with the local heat, and life could be short. Also, the 7A fuse is overrated for 250V, meaning more energy can let loose in a failure. Finally, some 250V countries (UK, for example) have very low impedance and can provide a lot of destructive current in a half cycle. 

2) Capacitor ripple current should be lower at 250V, but I think the higher voltage offsets any advantage of the lower current. A higher voltage electrolytic cap always improves life. 

3) The failure appears to cascade like this:
 - Main capacitor 100uF 400V cooks down to almost no capacitance
 - Switching FET shorts out under voltage/current stress caused by high resistance C1 

 - High current flows to ground through 0R11 2W current sense resistor on top side of board, which connects the drain of the switching FET to ground. Note that the fuse could blow here, limiting the damage to the fuse, the transistor, and the cap. There's no thermistor on the TI sample schematic, but this part is also at risk in the Agilent PSU. , - 0.11 ohm resistor blows open circuit. The drain of the main switching FET now rises to the voltage across the cap, 350V in 250V regions and 170V in 110V regions. The gate, now part of a shorted blob, follows. 

 - With the FET drain path to ground open,  the 350V now flows from the FET gate through the 2R2 gate drive resistor and into the output stage (pin 6)  of the switching PSU controller 3842A. Therefore, the 3842A does not survive if the 0.11 Ohm resistor blows. The 2R2 resistor also blows. 

- there is also a 2K2 resistor that connects to the sense input on the 3842A. This resistor can also blow, if the shorted internal 3842A drive path goes open circuit. The high voltage will flow to ground through the sense circuitry.

 - There's a final path to ground through the optocoupler phototransistor, but these transistors usually have a quite high current rating and seem to survive. 

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