Rohde & Schwarz NGPV powersupply repair - Page 1
Forum section: electronics / repair

author: TheUnnamedNewbie
author_rank: Super Contributor
timestamp: February 15, 2018, 02:36:56 pm
content: So, after successfully repairing (or at least partially) a multimeter a few weeks ago, I decided I wanted more of this repair-stuff. So I orderd some broken stuff on ebay, one of them was this big Rohde & Schwarz (big is a relative thing of course).I have wanted a high power power supply for a while now. This one fit the bill: I'm a big fan of Rohde & Schwarz, and this unit offers very good specs, and is rated 20 V 10 A. It was advertised as "No output regulation, always at maximum voltage". Sounded to me like a pretty ok problem to solve.I asked R&S for service info, and two hours later they sent me a full (almost, more on that later) service manual, operation manual and datasheet. Yay!Anyho, some pictures:This is a massive thing. It weighs a lot, mainly due to the massive, massive transformer.It came without top and bottom cover. I don't know if this is standard or not. When looking at the manuals, this was aimed at system applications, with the standard version just being programmable over GPIB and not having any front panel controls. The "Lab version" is an upgrade option that adds manual input options, so I can imagine that this might not be a nice "put-it-on-a-table" box and come without and top or bottom covers.It has a quite cool way of entering the voltage:Transformer:So, when powering on, nothing happens and the overvoltage protect kicks in. Turn on and then off the output, and it snaps to the maximum output voltage (which is, give or take, 26 V).However, it didn't always do this. I went in to check some voltages and found that the main 15 V line was being pulled down. After a few minutes I hunted it down to a shorted tantalum capacitor on the DAC board. Seems like the thing that was stopping this from just blowing to bits was the current limit in the LM7812 (I did manage to get it to blow to bits when I just put a bigger supply on that board, after removing all the ICs, to try and find where the short was).I have not yet replaced the capacitor, it was just an extra bit of supply line filtering, and I'm only gonna replace it if I can fix the supply, but with the capacitor removed, it does consistently snap to the high voltage.So now I was wondering what was going on with the supply. The behavior of the output seemed to indicate that the problem is somewhere in the output feedback loop. I first checked the output pass transistors (all 8 of them) but they were all ok. In order to check if the output regulator was just broken and the gates of the output transistors were just constantly at some high rail, I decided to see if current limiting worked. Turns out, current limit does work, and current regulation works fine, as can be seen on the following picture. 5 Ohm load, Keithley is showing the output current:Since that works, I know that it is likely that the voltage rails on all boards are fine. So my next steps will be to look at the DAC board and the regulator. Checking the outputs of the regulator opamps, and the reference voltages generated by the DAC. I hope it is not the DAC, since it is an old AD part that takes BCD input. In addition, for some reason the schematic for the DAC board is not included in the service manual. I have the BOM, and the PCB component position page, but no schematic....I found there are replacement parts. Will check back in when this is all done and when I have updates. And ofcourse, I always welcome questions and discussions.

author: TheUnnamedNewbie
author_rank: Super Contributor
timestamp: February 15, 2018, 04:51:42 pm
content: Well, it turns out there wasn't anything complex wrong with it. The tantalums are all failing, and one of them was shorting out a control line. The problem solved itself while I was testling. I orderd a bunch of replacement capacitors and will replace all tantalum capacitors next week, after which I'm going to look at load testing this thing.

author: Neomys Sapiens
author_rank: Super Contributor
timestamp: February 18, 2018, 01:11:40 am
content: Well, that would be a definitely successful recovery of a nice power supply. With limited effort. Hope it holds true. You are raising my appettite towards ordering defective PS.

author: TheUnnamedNewbie
author_rank: Super Contributor
timestamp: February 19, 2018, 10:11:29 am
content: Just waiting for the batch of replacement capacitors. I loaded it down over the weekend, and had it supply the maximum current at various voltages and it seemed to not even struggle a bit, as one would expect from what seems to be a high-end unit (at the time) from someone like Rohde & Schwarz. Very happy with this, worth every bit I payed for it.