Lecroy LC534L dead supply, don't start on external supply - Page 1
Forum section: electronics / repair

author: Halide
author_rank: Contributor
timestamp: October 06, 2018, 12:01:59 pm
content: HiI have the Lecroy LC534L scope with dead power supply. I tried powered it up with external power supply. On Vcc rail i use +5V, on Vee -5V. I started all power rails together and only standby led is on. I don't know what to do with "LINE" rail. Mayby the orginal psu have the special power rails startup sequence. Can you get some tips?The service manual is similar to LeCroy 9374M-L-TM Service Manual.

author: DaJMasta
author_rank: Super Contributor
timestamp: October 06, 2018, 06:32:39 pm
content: Haven't used or worked on the scope myself, but worked on a LC684 series that was similar to the LC584... which could be a distant relative to yours. Regardless, I expect it has more rails than the two. I think the LC680 had at least +-15V and +-12V in addition to those, and maybe had another. It may also have a voltage monitor chip that turns the power supply off automatically if the rails aren't present in the first couple seconds of booting - the LC684 did.I think the easiest option to get this to power up is to fix the power supply. While it's unlikely there's tight requirements for powerup order and timing, you can at least be sure it's giving you all the right rails when fixed all at the right time.

author: Halide
author_rank: Contributor
timestamp: October 06, 2018, 08:29:55 pm
content: HiI tried to repair the power supply but i don't have a experience with servicing supplys. I found the damaged Q1 transistor and rv1 varistor. When I desoldered the q1 i noticed that the pads responsible for emitter and collector are still shorted. I be afraid that the T2 transformer is blow too. Maybe you have some advice what I should do next.

author: nctnico
author_rank: Super Contributor
timestamp: October 06, 2018, 10:02:47 pm
content: A transformer winding always tests as a short. Maybe there is a diode in parallel with the transistor.

author: Halide
author_rank: Contributor
timestamp: October 07, 2018, 06:46:10 am
content: I attached the schematic of this power supply. I found the tin between this pads, my bad. What I should check next? The fuses still blow off. I noticed that the C1 and C2 are very hot. I found this topichttps://www.eevblog.com/forum/repair/unknown-part-in-a-lecroy-power-supply/. I think it is a good way to remove that. Should I connected some pads with wire when I desoldered that part?

author: JFJ
author_rank: Regular Contributor
timestamp: October 07, 2018, 12:45:37 pm
content: Quote from: Halide on October 07, 2018, 06:46:10 amI believe that your oscilloscope automatically adapts to the line voltage present. If the automatic voltage selector has failed, and is stuck in 115V mode, then C1 and C2 would be connected in parallel across your line supply, instead of in series (as required for 230V operation). That may explain the fuse failures and the capacitors becoming hot.If you have access to a step-down transformer or a suitable variac, then you might try powering the oscilloscope from 115V. Alternatively, the power supply 'E' connector could be manually jumpered for 230V (leaving the automatic line voltage selector disconnected)

author: JFJ
author_rank: Regular Contributor
timestamp: October 07, 2018, 03:32:45 pm
content: Quote from: JFJ on October 07, 2018, 12:45:37 pmThat may not be the case, if your schematic is correct, because the C1 -ve is shown hardwired to the C2 +ve.

author: Halide
author_rank: Contributor
timestamp: October 08, 2018, 04:41:36 pm
content: Hi,Thanks for replay. Tomorrow I'll check this black box responsible for automatic voltage selecting.

author: Halide
author_rank: Contributor
timestamp: October 10, 2018, 03:13:05 pm
content: HiI remove this black box and the fuses don't blow. Now I have another problem. I don't have any output voltage. I checked the R4 and it have a 100k resistance( on schematic it have 47k)- I replaced it. I remove the RV1 because it was shorted- I don't replaced it by new one. How can I check the hybrid module?

author: DaJMasta
author_rank: Super Contributor
timestamp: October 10, 2018, 04:32:15 pm
content: I would definitely put back the 100k resistor unless it looked burned. It is common for hardware revisions to have slightly different parts and designs, so unless the part seems obviously wrong (like it's fully open or just a few ohms), it's probably not a good idea to replace it.

author: Halide
author_rank: Contributor
timestamp: October 10, 2018, 05:07:37 pm
content: I know that the some parts can have other values between revisions however resistor barcode claims it should be 47k.

author: rhb
author_rank: Super Contributor
timestamp: October 10, 2018, 09:50:01 pm
content: On the DDA-125/LC684DLX, the PSU tests the load on the 12-15 V rails. If it's not correct it shuts down and leaves the fan running. The STANDBY light will blink very briefly before going out, I bought one on eBay. It arrived and worked fine. Next day it just blinked the LED and sat with only the PSU internal fan running. I opened it up and reseated all the connectors I could see. It worked again. Next day it didn't, so I took it apart.In my case the issue is a burnt connector pair on the PSU plug. I cleaned and straightened it and it works. But I need to find a replacement.

author: Halide
author_rank: Contributor
timestamp: October 11, 2018, 06:39:00 pm
content: I will try to load the rails and start the supply. You said that if the load is not enough only the fan stay on. In my scope the fan don't start. I supposed that the PSU is completely dead. I think it is the good way to check the hybrid. Maybe you have some advice about checking this part.

author: rhb
author_rank: Super Contributor
timestamp: October 12, 2018, 01:00:50 am
content: I'd get to work with a DMM starting at the IEC power connector. Look for things like soldered lead fuses.

author: Halide
author_rank: Contributor
timestamp: October 14, 2018, 09:23:46 am
content: Hi,Ok, I check the IEC power connector. The scope stay unused in the cellar about few years.

author: Halide
author_rank: Contributor
timestamp: October 15, 2018, 10:52:44 am
content: Hi,I checked the IEC connector, every think is ok. I still supposed that the hybrid module is broke. How I can check it?

author: shakalnokturn
author_rank: Super Contributor
timestamp: October 15, 2018, 01:39:05 pm
content: If you subscribe to yahoo groups "lecroy owners", you'll find quite a bit on these PSU's, including some of my own notes, I/V curves to check your module against out of circuit if you have a scope with a component tester function, notes on the main components to check when T1 goes short.Beware of large juicy electrolytics on secondary, the 2 small ones on the secondary side near the module are usually worth replacing.Also beware of opto...

author: Halide
author_rank: Contributor
timestamp: October 15, 2018, 05:54:31 pm
content: I subscribe the yahoo "lecroy owners", could you give me a link to this topic?I will check this caps and opto.

author: shakalnokturn
author_rank: Super Contributor
timestamp: October 15, 2018, 07:49:27 pm
content: https://groups.yahoo.com/neo/groups/LeCroy_Owners_Group/files/100004 (A1) PWM/This could give you a starting point, use some imagination and the "search conversations" there have been several topics on repairs in the past.

author: Halide
author_rank: Contributor
timestamp: October 21, 2018, 03:50:55 pm
content: I probably find the problem. The T2 8,9 and 10 taps are totally shorted. Maybe someone know which transformer Can I use as a spare part. I tried search this part in google using information on trafo sticker, but I can't find anything.

author: shakalnokturn
author_rank: Super Contributor
timestamp: October 22, 2018, 07:40:21 am
content: If you have used a DMM to test your transformer I have no doubt that it looked "shorted", the problem is that a DMM measures resistance (DC) and SMPS transformer windings have very low resistance.Either you have a way to measure inductance to check the windings or you'll need to test it at 30kHz (the SMPS frequency).Your T2 transformer is probably in perfect condition if it looks shorted, the way it is used in this supply it is more likely to break open than shorted.

author: Halide
author_rank: Contributor
timestamp: October 22, 2018, 10:40:42 am
content: So it is quite normal that this transformer looked shorted between this taps? If I measure it( using diode test) between other taps its shows approximately 18 ohms( 3 and 4). In this taps I have zero ohms read( 8,9,10).