Broken LeCroy 9424 :D - Page 1
Forum section: electronics / beginners

author: mortos360
author_rank: Regular Contributor
timestamp: September 06, 2013, 11:00:12 am
content: I finally got one of the cute "little" LeCroy scopesIt wasnt easy to get 1 cheap here in EU, but I finally managed... it was 84 Euro inc. shipping with only 2 channels working, channel 2 and 4.I bought it so I have something new to play with and learn, I dont really need it working, but of course if I can fixI am still very new to electronics and everything I know about scopes is basically from daves blog.Any idea (other then checking voltages) where to start with this?I will try to get a service manual today.. as my first step.Every little tip I can get would help and later when I have some more informations I will post pictures and updates.Thanks

author: digsys
author_rank: Unknown
timestamp: September 06, 2013, 12:01:02 pm
content: I bought the 2nd one imported to OZ, many years ago, and still have it. Cost over $30K back then, so you got a bargain :-)It has a heat fault on ch2, which goes away after ~30 mins, but otherwise even the screen is new (no burn at all).I have the service manual, but it's a heck of a lot to scan. If you don't find one, maybe I can scan a few needed pages?Once I find a couple newer LeCroys to replace it, I'll likely let it go, maybe strip it and sell the parts (It's very heavy to ship).Keep in touch on how you go, maybe we can do a deal :-) .. don't worry, It was written off eons ago, I'm too sentimentalto get rid of it, besides it still works great. It was an awesome scope in it's time ... a legend. Great find mate.

author: AlfBaz
author_rank: Super Contributor
timestamp: September 06, 2013, 12:09:16 pm
content: Here's a link to the service manualhttp://www.ko4bb.com/Manuals/LeCroy/LeCroy_9424_Digital_Storage_Oscilloscope_Service_Manual_May_1993.pdf

author: mortos360
author_rank: Regular Contributor
timestamp: September 06, 2013, 12:24:20 pm
content: @digsysIll keep that in mind.. thanks!@AlfBazI found the same link just before.. thanks tho!!

author: mortos360
author_rank: Regular Contributor
timestamp: September 08, 2013, 10:12:40 am
content: It seems that somebody already have try to repair this scope and some parts are missing (see picture)Does anybody know where I can get a new front end board for the 9424, other then ebay?

author: seancsnm
author_rank: Regular Contributor
timestamp: January 10, 2015, 07:48:34 am
content: Any chance a service manual can be re-uploaded? The link is dead...And yes, I'm new here.

author: digsys
author_rank: Unknown
timestamp: January 10, 2015, 09:54:10 am
content: Quote from: seancsnmI have a version atwww.digsys.com.au/ftp/LeCroy9424.zip(or pdf) Not sure if it's the same one, but if it's not suitable, I have others.Unlocked now, let me know when you're done. No hurry.

author: seancsnm
author_rank: Regular Contributor
timestamp: January 10, 2015, 06:47:19 pm
content: Got it. Thank you! If I don't have any luck (which is quite likely), I may post some more information as to what the problem is.

author: seancsnm
author_rank: Regular Contributor
timestamp: January 10, 2015, 09:56:08 pm
content: Ok so it looks like it is a triggering issue. Anything to do with the trigger causes all kinds of strange things to happen. I am using the probe calibration 1 KHz (?) signal (which was verified to work) to do testing and the behavior of all four channels is virtually identical. Triggering to a channel not in use causes the waveform to be a flat slightly noisy line - the sort you would expect from a channel with no input signal. However, when triggering from the channel receiving the 1 KHz signal, things start to go crazy. Sometimes the signal quickly alternates between a flat 0 and a flat 1 with no baseline (adjusting the trigger level actually changes this baseline a bit), and sometimes the channel makes a strange curvy waveform that seems to repeat consistently. And sometimes, the behavior sporadically bounces between the two.So I really don't know. Maybe a bad cap on the trigger board?

author: AlfBaz
author_rank: Super Contributor
timestamp: January 10, 2015, 10:13:45 pm
content: Quote from: seancsnm on January 10, 2015, 07:48:34 amIt appears ko4bb has done some changesnew linkhttp://www.ko4bb.com/manuals/download.php?file=LeCroy/LeCroy_9424_Digital_Storage_Oscilloscope_Service_Manual_May_1993.pdf

author: seancsnm
author_rank: Regular Contributor
timestamp: January 11, 2015, 02:23:39 am
content: Thanks to you too, though as far as I can see, it is the same as what digsys posted. Anyways, I have been using my good o-scope to look at the signal that leaves the (analog board? The board pictured by mortos above). All four signals are pretty strange (I will upload pics in a minute), so I feel like the issue may lie in that board. Anyone know what the board (pictured by OP) is called?Edit:The images:The first image is what the waveform looks like before I change the volts/division setting for the channel I am looking at. Once I change the volts/div setting for any given channel, it looks like one of the pictures that follow (2-10), in order of 50 mV/div to 1 V/div (or possibly vise versa - it is not clear to me what the significance of the changes from waveform to waveform is).The other thing to note is that when the volts/division setting changes for a channel that I am not looking at, the waveform changes while the relays change their positions, but then goes back to what it originally was. Also, all of these waveforms come from the "normal" trigger mode. Nothing changes (audibly or visibly) when I change settings while the O-scope is in its default "single" trigger mode....and spoiler tags don't work for me.Another edit:I don't know if this has anything to do with the problem, but the voltage adjust pot on the front end cannot bring the voltage up to what it needs to be (6V on TP1, pins 3 and 4). Images below show the voltage range. I'm going to see if I can calibrate the PSU and bring it up to what it needs to be. That much ripple makes me wonder if too much current is being drawn.Edit:Line voltages on the main power supply look good for the most part, but -5.2 V is the lowest (magnitude) that the potentiometer for that rail will allow. No ripple on any of the rails either. Note that I did the adjustments with the scope fully running.Going back to the front end, the larger LeCroy branded ICs on the front end board get pretty warm after the scope has been on for a bit, but I don't think they are getting hot enough to say there is a partial short circuit in any of them. Still, I am not sure what could be causing the issue with that board power supply. I wonder if a regulator has gone bad?Edit:Looking at the schematic, it looks like there is a feedback loop between the LM78GCP regulator and the other three regulators (LM7805, and two LM7905s). My guess is that this forms some sort of op-amp based feedback power supply that is more efficient than a simple linear regulator, but I really don't understand the many different types of feedback based power supplies. What confuses me is how there is so much ripple at the output of the LM78GP regulator - about 1.6 V at a frequency of about 16 KHz. And similar ripple out of all of the voltage rails for this front end power supply. I wonder if it's enough to cause issues with the signal acquisition... I would think the answer would be yes, but I don't have a whole lot of experience...

author: seancsnm
author_rank: Regular Contributor
timestamp: January 11, 2015, 08:17:19 pm
content: Sorry for the double post, but the last post was getting pretty long...It looks like this guy pretty much has come to the same conclusion as me. I might try what he tried to see if it makes any difference. What still concerns me is the feedback loop on the board though. I will probably test individual components first.http://www.electronicspoint.com/threads/lecroy-9424-calibration-faillure.259847/Edit: Well, it seems I may have found one of the problems. I removed the LM78G regulator for testing, and it seems to work fine - it produces a stable voltage at a 120 mA load while being driven by about 10 V. But when the input voltage is increased above about 10 V, the output voltage drops to just about 200 mV almost instantaneously from 10 V to greater than 10V - my bench power supply does not have the precision to know exactly where the drop occurs, but it's clear that this behavior is not normal. I wonder if I can find an equivalent component to replace this thing with.Looks like this could work:http://www.nteinc.com/specs/900to999/pdf/nte953.pdfAlso, if anyone is following this, please feel free to share opinions, suggestions, ideas, etc. I really have no idea what I'm doing, so it would be appreciated!

author: seancsnm
author_rank: Regular Contributor
timestamp: January 12, 2015, 01:40:39 am
content: I'm gonna get slapped pretty hard for triple posting, but...It looks like we have a winner. I had an LM317 laying around that just happened to work for the fix. Below are the images and hand calculations done to try to convince myself that the regulator would work. It turns out that with the potentiometer all the way down (0 ohms), the voltage coming out is just a tad bit over 6 V, so I obviously messed something up. The 800 ohm resistor shown in my hand-drawn schematic is actually an 820 ohm resistor, but that would put the minimum voltage at just about 4.7 V, so it seems that the resistance or voltage drop introduced by the diodes in the circuit was indeed significant, so I should not have neglected that component.Measuring the resistance between the pins manually is a bit baffling. It looks like there is the expected 300 ohms of resistance between output and control, but only 334 ohms of resistance between control and ground. How this thing is able to works is beyond my level of understanding.Anyways, thanks for the manual again. I don't think I would have been able to get the scope where it is without them.

author: digsys
author_rank: Unknown
timestamp: January 12, 2015, 02:13:33 am
content: Quote from: seancsnmWouldn't concern yourself with that !! Technical content has no limits :-) You should see how long some nerd fight posts get !!!Quote from: seancsnmEasy, measuring resistance IN circuit is always an UNKNOWN. When you measured the adjust resistor, you ALSO had in parallel the 300 SET resistor in series withwhatever load resistance happened to be there. The only REAL way is to lift a leg, but since you've fixed the problem, who cares :-)Excellent job finding the fault, If only I could get unlazy and fix mine :-)

author: seancsnm
author_rank: Regular Contributor
timestamp: January 12, 2015, 03:27:18 am
content: Quote from: digsys on January 12, 2015, 02:13:33 amYou do have a good point.The O-scope gets a somewhat reasonable signal on all channels over a range of frequencies but it looks like the job might not be quite done yet. The input signal still looks like it has some capacitive issues, especially with lower frequency signals. But, it could be possible that the issue is related to the probe I used. It was home-made, and although its output signal looks pretty decent on my Tektronix TDS 640, it could be possible that the Tektronix is automatically compensating for any sort of capacitive issues that are occurring. I don't have time to continue messing around with it, but I will probably play around a bit more once I get another block of time and some good probes.Anyways, I hope this info might help someone else.

author: seancsnm
author_rank: Regular Contributor
timestamp: January 24, 2015, 09:51:02 pm
content: Does anyone know how often the scope should autocalibrate, or if there is a way to change the autocalibration period? It seems to be calibrating about every 30 seconds. Looking at the manual, it looks like the only information it gives is that "it autocalibrates periodically." Looking through the scope menus isn't much help either. Is this normal? I'd like to find a way to turn it off, as having a series of relays click every 30 seconds is a bit annoying.Anyone know if this is normal? If it is, I can see why these scopes didn't get very popular.

author: digsys
author_rank: Unknown
timestamp: January 24, 2015, 10:40:03 pm
content: Quote from: seancsnmYes very normal. The main reason it runs cal so often at the start is tempco. As parts warm up it should become less and less.Mine usually settles after 1-2 mins, depending on room temperature. It IS annoying, but the way I look at it .. I get a FREE cal each time :-)If it's NOT settling down within 2 mins, then you may have a drift problem, or sensor issue?Quote from: seancsnmNaaaa. You have to remember when they first came out (and I bought mine straight off the assembly line), technology was still in it's infancy,with regards high end DSOs. I paid $30K for mine and having a scope that would self-calibrate was awesome.The only reason they may? not have been popular would have been the price !

author: seancsnm
author_rank: Regular Contributor
timestamp: January 24, 2015, 11:03:00 pm
content: Ok, the scope has been running for at least an hour now, and it is calibrating far less often, so its calibration rate has definitely tapered off. It's great to know it's all running good.And $30k new? I guess that is the price of being at the edge of the game. And it might explain why everyone and their neighbor didn't have one.

author: digsys
author_rank: Unknown
timestamp: January 24, 2015, 11:28:35 pm
content: Quote from: seancsnmAn hour is a bit long though. Mine usually sorts itself out within a couple minutes, dropping to 1 per 10mins, then 1 per hr after ~ 1hr.If you can put up with it, probably not worth chasing, finding a drifting component will be tricky.Quote from: seancsnmCorrect. I had spent ~$40K on HPs, Teks etc etc in the years before that, and was so pi$$ed of with poor design / specs. The LeCroy blew my mind :-)I even had govt agencies "borrow" mine ... until they could afford their own :-) They had a "front end" with an antennae that they could park in front ofbuildings to read CRT screens (long before strict radiated emissions came in). The LeCroy would rebuild the sync and you would get a near perfect(or quite readable) image of the CRT :-) It was soooo cool.