LeCroy WaveSurfer 434, blown FET, PFC-circuit - Page 1
Forum section: electronics / repair

author: gunther_p
author_rank: Newbie
timestamp: June 11, 2025, 02:50:13 pm
content: Hello everyone,I’m working on a LeCroy WaveSurfer 434 (MFG date: 2004) that was donated from my company because it’s not working. Initially, it had blown fuses in the mains power circuit.According to our records, the scope was officially repaired at least once in the past.Upon examining the PFC board (Rev. 4.XX), I found that transistor 2Q1 (originally a STP20NM60FP) had a short between drain and source. Judging by the soldering marks, it had already been replaced before.Attachments:PFC schematic is on page 31/43. The supply for PFC controller is on page 38/43. In addition 2 pics of pfc board.I noticed the schematic lists a STP20NM50FP for 2Q1. A online post at "groups io" suggests STF26NM60N as a better replacement, since the NM50 version is obsolete.Interestingly, my board uses a MC33262 PFC controller instead of the FA5500AP shown in the schematic. I assume this is a compatible replacement.I initially replaced the fuses and installed a new STP20NM60FP. The scope worked fine for about three startups. But after a normal shutdown via windows shutdown command, the scope suddenly emitted a loud bang. The bang was simultaneous with the switch-off (I have a made a video). The same issue returned: blown fuses and a destroyed 2Q1 FET.I now suspect that using the STF26NM60N may offer better durability in this application. I wonder if I even should use a FET with more VGS like STF31N65M5? Could the lower RDS_on be a problem?Additionally, the resistors 2R12 (150 mΩ) and 2R25 (100 mΩ) are also defective, as their measured resistance is much higher than expected.Regarding the large electrolytic capacitor 2C8 (150µF, 450V): it looks fine visually (no bulging), but considering its age, I think it may be dried out. It’s a United Chemicon LXQ series:ELXQ451VSN151MQ30S (tanδ = 0.2, Irms = 0.88 A). Unfortunately, this part is hard to find, but I believe the following would be a suitable replacement:ELXS451VSN151MQ30S (same dimensions, tanδ = 0.20, Irms = 0.93 A).I also noticed a trim potentiometer labeled “PFC ADJ” (2R27) in the feedback loop of the PFC controller. However, I’ll leave it untouched for now to avoid misadjustments. I guess that the DC output voltage can be adjusted there.My plan:Replace 2Q1 with STF26NM60N or "better" one, Replace dead resistors.Replace 2C8 with a new electrolytic capacitorReplace all Caps at the left side from the FET (ECQUL, ECQE4, replacements: B32923C3474K189, B32922H3224M, ECQ-E4105KFW)After that i want to measure the behaviour of the PFC controller and the FET. For that I want to measure the Gate Voltage and then the V_DS voltage.I have a newer Lecroy Wavesurfer and a PP020 probe (10:1, Max. Input Voltage: Circuits not directly connected to Mains: 500 Vrms, 1500 V transient overvoltage, Measurement Category II*: 400 Vrms)Long story short: I need a voltage divider to be on the "very save" side. I would divide the V_GS with 900k/100k and measure this /10 (divided by 10) voltage with the PP020 probe. I could try to compensate the probe and the devider via the internal scope compensation before measurement. What do you think?The scope says: max power 200VA. I have a Isolation transformer with 100VA max that should be able to power up the scope and navigate into the BIOS of the internal PC. To ensure that, i could disconnect the hard drive. I guess that Windows booted with Software running could exceed the max power rating of the transformer. I would not connect the Earth-GND and connect only the DUT-scope resulting in a isolated from mains DUT. The scope for measurement will be normaly powered.Does anyone have additional suggestions or experience with this issue?Thanks in advance!

author: gunther_p
author_rank: Newbie
timestamp: June 18, 2025, 10:29:55 am
content: Hi,i testet the 910k/100k resistor divider prior to the scope probe at the internal CAL port of the scope. There are only a few distortions visible, but the most important thing is the additional divider for voltage reduction. I'll go into the race with that.I will use The 2Q1 FET source pin for measurement GND. So i can display Drain voltage (with divider) and V_GS voltage with another probe (withoud divider). Everything with the 100VA isolating transformer.Any futher suggestions?

author: wraper
author_rank: Unknown
timestamp: June 18, 2025, 10:55:43 am
content: I would replace the IC, just in case, unless it makes sourcing the components significantly harder/longer. Its output often gets hit through the gate when a MOSFET blows up, especially in cases when source resistor goes open. Also I always thoroughly check if all gate circuit components are fine. Did you verify if PFC output voltage is around 385V?