Kinda screwed - Lecroy 9324 power supply repair - Page 1
Forum section: electronics / repair

author: ajrabassa
author_rank: Contributor
timestamp: December 10, 2015, 02:35:00 am
content: Hey everyone.I'm very new to electronics as a hobby - I do a lot of reading and light tinkering, but this is my first foray into trying to do anything for real. Unfortunately, it's not how I planned to start, but here we are.I just got a Lecroy 9324 that was was for sale as parts only, but was shown working at the time of shipping - no errors, waveform looked good. Just got it in, and it does not appear to turn on. There are no lights or indicators on the front panel to check, but the fans on the case aren't spinning - so I'm hoping it's just the power supply.I can't find the service manual to start probing around - anyone have any hints? Tried searching and google, no avail.Things I've checked so far:On power, no display and no fans spin. A high pitched whine comes out of the power supply. Took the power supply out of the scope and have it powered on my bench. On investigation, the sound is one of two inductors bunched very close together. I can't figure out which one is making the noise. Putting metal in/around the inductor does not change the pitch of the whine, but light pressure on any of the capacitors nearby changes the pitch drastically.The voltages do not appear okay. The PSU has +/- 5 and 15 volts listed as output (not on any particular pin mind you, just printed on the case) and I'm getting 2.5 and 12 volts with major variance between the pins.On the AC side, there's even stranger nonsense. I've got three pins: Pin 1, a brown wire and assumed hot; pin 2, blue wire and assumed neutral; pin 3, yellow with green stripe, ground.I have 124 volts between hot and neutral, but I also have 51 volts between neutral and ground, and 72 volts between ground and hot. Very interesting, after doing the addition of those numbers. Ground is not actually connected at the moment, because the ground jumps directly to the chassis before connecting to the power supply. I could hack together a bridge and connect it, but I feel it's better to leave it the hell alone until someone else chimes in who understands more than I do.Any suggestions or comments are welcome, this is not really how I was imagining starting out.Edit:Just found the correct pin-voltage chart for this power supply, it turns out all the DC voltages are correct (at least while not under power). Not sure where to go from here.

author: Tomorokoshi
author_rank: Super Contributor
timestamp: December 10, 2015, 03:38:15 am
content: Check to see if those electrolytic capacitors are shorted.

author: wblock
author_rank: Regular Contributor
timestamp: December 10, 2015, 04:34:53 am
content: Pictures would be good. Caps are always a prime suspect, but other things happen too. There is an article herehttp://www.azur-electronics.com/Pages/repair_lecroy_9310.aspxon a maybe-similar Lecroy 9310 that had a open power resistor. Does yours have a similar power supply?

author: ajrabassa
author_rank: Contributor
timestamp: December 10, 2015, 05:13:01 am
content: Thanks!Pictures:http://imgur.com/a/53XUlIf there are any more I can take, let me know. I am stuck using my half-broken cell phone camera, as that's the only thing I have that will focus close enough. Apologies for the slight blur as a result.Yes, it appears the 93xx series has the same power supply throughout. Unfortunately, I am having different symptoms than he's having and that resistor appears good.At this point I'm not sure it's the power supply, as the rail voltages are all good now that I'm using the correct ground pin. Probing the caps is proving difficult, as I don't have enough experience to know what I'm looking at/pick likely culprits. The only weird thing remaining on the power supply is the very high neutral-ground voltage when powered on.I don't see any dead caps while they're on the board; most of them read 280k+ ohms, while another series of them all read 380 ohms. The latter seems to be correct at it looks like there are some resistors in that range across the same trace. I took out one of the caps reading 380 ohms, and it shot up to 300k off the board. It seems like I'd have to take them all out if I wanted to do anything else with them. If this is worth it, let me know and I will do it.I may have to shelve this and pick up a different oscilloscope, but I'll keep trying any suggestions people throw at me in the meantime.

author: poot36
author_rank: Frequent Contributor
timestamp: December 10, 2015, 05:48:35 am
content: Try placing 3 to 5 watts of load on each power supply rail. That will usually let the power supply fire up properly and you should measure the correct voltage on each rail. Also now you know where the ground pin is on the power supply check that the scope is not shorted on any of the rails as well (could be a bad tantalum capacitor). Anything under 10 ohms should be investigated or any resistance that would cause the power supply to exceed its ratings on any of the rails.Just be sure that the power supply is working correctly because you don't want to fry the rest of the scope like Dave did on a similar Lecory. If you can use a dummy load on the power supply to simulate a full load on it. The cheapest way to do this is use nicrome wire from a broken toaster and do some math to figure out what lengths you need to get the proper resistance for each output on the power supply. Just keep in mind that some supplies may have a different maximum load on each rail when all rails are loaded.The different voltages you are seeing between the live nuteral and ground are caused by the X and Y suspression capacitors to reduce emi been fead back into the line. Don't worry about that it is normal for a switch mode power supply.

author: ArcticGeek
author_rank: Regular Contributor
timestamp: December 10, 2015, 02:25:34 pm
content: I've seen Lecroy 9354 and 9320 schematics on the web, in fact I think I still have them somewhere on my home computer. These schematics include the power supply schematic. Would those models be close enough to the 9324?

author: ajrabassa
author_rank: Contributor
timestamp: December 10, 2015, 05:28:02 pm
content: I have a few spools of resistance wire, so I can easily check the supply under some load - hadn't thought of using it that way, now kicking myself. I'll give this a shot when I get home.As for the power supply, it *appears* that the entire 93xx series uses the same power supply, but I can't be positive. At least on visual inspection, they seem identical.

author: PA0PBZ
author_rank: Super Contributor
timestamp: December 10, 2015, 06:58:33 pm
content: Quote from: ajrabassa on December 10, 2015, 05:28:02 pmNow you know that the rail voltages are good without a load you could plug the scope back on and check the rails again, that will tell you if one of the voltages drops.Then you can find out if it is the power supply giving up or there is a short somewhere in the scope and work from there.

author: ajrabassa
author_rank: Contributor
timestamp: December 11, 2015, 01:55:51 am
content: Before I can investigate the mainboard safely, I need to know what this connector is called - I've got to make some wiring harnesses so I can insert things into the board without killing myself or destroying the scope. I'm going to try to troubleshoot by only taking apart as much as is absolutely required, and I'm willing to wait and make cables if that's what it takes. I'd like to buy ~30 or so single-pin connectors so I can mix and match shunts and Y cables.Here it is:There's 3 of these power wiring bundles in the scope, each with a different number of pins.Results from load testing:Power supply voltages solid; no drop at 0.8A on the 5.2v rails, and the drop I see on the 15v rails at 2.8A is well below the tolerance of my multimeter. From here, it's definitely on to testing the mainboard.

author: Macbeth
author_rank: Super Contributor
timestamp: December 11, 2015, 02:56:43 am
content: Quote from: ajrabassa on December 10, 2015, 02:35:00 amLOL, just read up to this and apparently the Euro IEC mains standard is strange nonsenseHere in the UK 240 VAC 50 Hz, Brown = Live, Blue = Neutral, Green/Yellow = Earth

author: poot36
author_rank: Frequent Contributor
timestamp: December 11, 2015, 03:22:39 am
content: Where the power supply connects to the scope measure the resistance between ground and the 5.2V rail and ground and the 15V rail. Also you can use the diode test function of your multimeter if it has one to see if there is a leaky semiconductor or cap on the rails.

author: ajrabassa
author_rank: Contributor
timestamp: December 11, 2015, 06:18:42 am
content: Quote from: Macbeth on December 11, 2015, 02:56:43 amHaha, I was actually referring to the voltages between neutral and ground - I wasn't aware switchmode supplies put a big voltage differential between the two. I have no idea what colors relate to what in electrical standards!