R&S HMP4040 voltage spike at PSU turn-off

[rf-messkopf]

I'll dig out the cal certificate and post the date, but this will take a few days.

Okay, here it is: calibration date is 2019-02-27. This dates the implementation of the circuit change that prevents the voltage spike at mains turnoff to early 2019.

[electr_peter]

Short version:  there are some older HMP’s that show this behavior if channels are not disabled before switching off the device (the recommended shutdown procedure).  We implemented a hardware change several years ago that prevents this from happening, and this modification can be made by our service department if needed.

Regarding voltage spikes when switching channels off with mains button, does modification fall under warranty or "service circular"?

[pdenisowski]

Regarding voltage spikes when switching channels off with mains button, does modification fall under warranty or "service circular"?

I'd recommend contacting your local / regional service department:  I'm on the product management side and am not sure how service handles this, especially in different regions.

[mahi]

For those wondering how old their HMP power supply is and can't find the original calibration certificate, you can retrieve it via the Rohde & Schwarz website: Download calibration certificate. Just enter the device ID (case sensitive!) and it should recognize the device and offer a download link.


Anyway, to get back to this issue. I got in touch with Rohde & Schwarz customer support in Germany:

The (huge) voltage spikes when switching channels off with mains button is a known issue of the HMP series that can be fixed with Service Circular 16282 by Rohde & Schwarz Service.

Rohde & Schwarz does not provide direct service to private customers (at least not in Europe) so I had to inquire for pricing via a distributor or partner. I received a quote for my HMP4040 of just over 1100 EUR (without VAT and without shipping).

That's almost half of what a new HMP4040 costs. Ouch! That's bit more than I was hoping for (maybe I am naive)...

[EEVblog]

For those wondering how old their HMP power supply is and can't find the original calibration certificate, you can retrieve it via the Rohde & Schwarz website: Download calibration certificate. Just enter the device ID (case sensitive!) and it should recognize the device and offer a download link.

Wow, does any other manufacturer have this?

[mahi]

I took one for the team and had my HMP4040 serviced by Rohde & Schwarz (Service Circular 16282). Price... 1111 EUR without VAT or shipping. Ouch. It might be a bit cheaper if you can deal with R&S Service directly, but I had to go through a partner (Esenwein GmbH - very friendly people). The turnaround time was two weeks.

The power supply came back with a new calibration certificate and a elaborate 12 page test report (it was not re-adjusted, but still well within specs).


I've tested the power supply and can confirm the issue with the >10 V spike is fixed.

Measurements:

All outputs enabled and outputting 1 V before switching off with the mains power switch. The profile is identical to that from before the fix apart from the faster ramp-up to 1 V after switching off the power supply (compare against my screenshot in post #11). Thus the issue with the outputs staying at 1 V for 6 seconds after switch-off remains (but we know that is present in the latest revisions as well and it's not a big deal). The faster ramp-up seems to suggest some capacitance was changed:



Let's zoom in at the point of switch off. That's what we are most interested in. Gone are the massive >10 V spike on channel 1 and the 4-5 V spikes on the other channels. There's still a small spike of almost 2 V on channel 1 and 1.5 V on the other channels, but that is acceptable. Let's not forget that under normal conditions you should not run into this issue at all. It's only during a power failure or absent-mindedness that the power supply may be switched off without turning off the outputs first. This will not damage any devices under test:



Now with the voltage at 2 V before pressing the mains power switch:



The profile of channels 2-4 is exactly the same to that of the screenshot posted by rf-messkopf in post #33. This suggests my HMP4040 now behaves the same as the newer revisions. In fact, I'm quite sure the hardware modification applied to my power supply is exactly the same as what the newer revisions come with from the factory.

[mahi]

So, what was changed by Rohde & Schwarz Service?

I did make pictures of the power supply internals before sending it off. When I got it back I had a hard time finding what was changed. All the PCB numbers and stickers were the same, so no boards were replaced. In fact, it didn't even look as if the power supply was fully disassembled: None of the wires to the circuit boards seemed to have been (re)soldered.

After closer inspection I found 2 locations (per channel) with new solder flux: A 1000µF electrolytic capacitor was replaced by a smaller 470µF one and a previously unpopulated pad now has an 22µF 0603 chip capacitor. That's all I can find. If there's more, it is well hidden.

• Original: Panasonic 1000µF / 25V / 105°C (EEUFC1E102L)
• New: Panasonic 470µF / 63V / 105°C (EEUEB1J471)

• Original: unpopulated
• New: Unknown 22µF 0603 ceramic capacitor (measured in-circuit with an LCR meter at 120 Hz)

I can't tell what part of the circuit the components belong to. We do not have diagrams, it's a multi-layer PCB and the solder mask hides the traces very well.

Top:



Bottom:



The PCB version is 5800.5847.10. Earlier units (with higher output impedance) may have a slightly different PCB. I was unable to find PCB pictures of the earlier units.

The fact that the power supply did not need re-adjustment implies that the changes do not affect calibration. It is possible to perform these modifications with just taking off the cover. Channel 4 is a bit more challenging because of the cables passing over it. You should undo the connector to the interface board to move the ribbon cable out of the way, and perhaps take off the metal cable guide on the duct (2 screws) to get the cables to the rear terminals out of the way. That's probably how R&S Service did it. They definitely didn't take it apart completely. The biggest challenge is soldering the 0603 capacitor as it is a quite deep reach and close to a tall electrolytic capacitor and the heat sink.

Total cost of the fix? Less than 10 EUR in parts...

[electr_peter]

Thanks for an update and repair details, @mahi. Glad that R&S has responded and at least some issues were fixed in repair.
However, small voltage spike at PSU mains turn-off still remains. Users of HMP2000/4000 series (old and new models) should be aware this - in some case it can pose some risk to DUT, but likely not. Otherwise HMP are quite good PSUs.