[Solved] LeCroy LC 334 disturbed screen, PLL VCO fail. Question about inductor - Page 1
Forum section: electronics / testgear

author: Uunoctium
author_rank: Frequent Contributor
timestamp: May 08, 2025, 08:03:05 pm
content: Yes, the next LeCroy LCxxx passed away.Less than 30 operation hours.Fetched it from a company where nobody was left over, who could operate that scope. I spend it a new backup battery and floppy drive in '17 and never used it since. Too heavy, too noisy.At moment i need the math functions, esp FFT for Fet biasing aligment in Schoeps microphones. First surprise was a loud bang and magic smoke – tanalum on MB totally burnt with complete short. After replacement the well known issue "CL measure_gain_and offset: cntl 4" and funny coloured hallucinations on screen instead of an trace.Groups.io published an short document from LC? which points to an faulty inductor on 500Mcy PLL Subboard. Mine measured at approx 4.5 Ohms which means: badThere is also a tiny schematics snippet from the sm in the paper. L100 = (OP) 31A6N8I need only a hint for the magnitude. Is it 6.8pH, 68pH or 680pH?Thanks!

author: madires
author_rank: Super Contributor
timestamp: May 08, 2025, 08:21:09 pm
content: I'd say 6.8 nH.

author: Uunoctium
author_rank: Frequent Contributor
timestamp: May 14, 2025, 04:45:46 pm
content: Updateit seems the failure is gone after replacing the inductor. I followed the hint above and soldered in a new 6.8nH. Somewhat, Mouser had in stock and suitable for RF applications.The type is Kyocera AVX L06036R8CGWTR, 0,627 €/each.Many thanks to the guys @ LC groups.io - esp that one, who was sharing the document.It is already linked in eevblog in this thread:https://www.eevblog.com/forum/repair/lecroy-lc564/?

author: Uunoctium
author_rank: Frequent Contributor
timestamp: May 16, 2025, 10:10:16 am
content: Where is this inductor located? Inside the tin can.From the schematics the detail of VCO:LeCroy_LC5xx_LC3xx_VCO_PLL.pdfMost of LC334 is identical to LC574a, whom detailed service manual is easily to find. Didier (KO4BB) hosts it also, but page availability gets worse last time.

author: Kl0neMan
author_rank: Newbie
timestamp: June 25, 2025, 12:18:40 am
content: I think my LC534AL is doing this too:Quote from: Rémi on February 09, 2018, 07:39:10 amI am unsure which time-base ranges are affected, but I know it is NOT all of them. It does not exhibit the behavior on the very long time-base settings (where the output looks like a strip chart), nor on the shortest ones. I guess I need to dive into that can on the Acquistion board and see what I find. Did you use an SMD soldering setup to do the removal and replacement of the failed inductor? I don't have one of those - only a Hakko FR-301 and FX-888D. Can the replacement be accomplished without damage using those?I also replaced the power supply, as mine just quit powering up one day. I did not toss the old one, hoping to repair it some day. There were no blown fuses, nor bulging electrolytics. I will also take a look at that resistor mentioned in the thread.

author: Uunoctium
author_rank: Frequent Contributor
timestamp: July 25, 2025, 05:44:50 pm
content: Sorry for the late answer. I didn't turn on the notification switch so i was aware of your question by accident only.No, i didn't use special SMD soldering equipmet - a normal Ersa Icon did this job. The board is before lead-free aera and the metal cover cap comes easily out. I recommend to measure the inductors resistance before replacing it. To make sure he's the real culprit.After replacement the Scope works till today without any malfunction.Can't say anything on PSU repair, exept a hint from an professional testing gear repair guy. He said the LC PSU is the same OEM as in Tek's TDS and the main cause of blowing is a magic hybrid which automatically selects between the different main voltages. His solution is to remove this hybrid and hardwiring the desired voltage.

author: Kl0neMan
author_rank: Newbie
timestamp: July 30, 2025, 06:51:29 am
content: >>Sorry for the late answer. I didn't turn on the notification switch so i was aware of your question by accident only.No, i didn't use special SMD soldering equipmet - a normal Ersa Icon did this job. The board is before lead-free aera and the metal cover cap comes easily out. I recommend to measure the inductors resistance before replacing it. To make sure he's the real culprit.After replacement the Scope works till today without any malfunction.<<Cool, and thank you for the reply. - I just returned to the topic myself after looking for some other information regarding repairing it. I do have an LCR meter that can measure the inductor, once I pull it. My iron is a Hakko FX-888D, that should do the job in a satisfactory manner . This old beast could probably use a bunch of new electrolytic capacitors too, but I i am not much into voodoo. I prefer to get it stable first, and then take my time with such things. I also need to get a new rechargeable disk battery for it, and reenter any codes for options it is supposed to have if they are lost. It needs a new bezel, and I wish I could find a 3D printed one or a template for having one made for me. I also have a LC534A that I could scavenge the bezel from if I cannot do anything else.