LeCroy 9400A - FIXED! (Was: Transport damage, no/missing deflection) - Page 1
Forum section: electronics / repair

author: Ysjoelfir
author_rank: Frequent Contributor
timestamp: September 09, 2015, 02:21:58 pm
content: Hello everyone,I "recently" (2 months ago or so..) got a LeCroy 9400A oscilloscope. It worked flawless at the seller, but during shipping it got damaged because of the bad package (he used the carrying grip as some sort of shock absorber... a single piece of styrofoam was the only packaging material whatsoever!!). Needles to say, it didn't work at the arrival. I actually don't care about the broken handle as it will be used at a stationary place. Even the broken off power supplys that rattled around before openung aren't that much of a problem as I surely will find a way to mount them without renewing those tiiiiiny thin wobbly plastik thingys that broke off.The main Problem is, that I don't have any deflection at all right now. Sometimes it shows a vertical deflection (a single line going from top to bottom), but then it flickers and the line disapears all of the sudden.I checked the four PSUs and they work fine, I also checked if something gets to the yoke. It doesnt, no signal whatsoever. How would you go on? I actually have a qualification of repairing TVs, but that service manual and the circuit diagrams somehow confuses me, besides the fact that I did not work on a TV for 5 years now...Any advice?A service manual can be found here:http://www.ko4bb.com/manuals/index.php?dir=LeCroyUpdate 10.09.2015: Now the disappearing stopped, even though I didn't change anything. Could have been a cold joint or a bad connection, I have to investigate this later.Update again: Found a ripped off trimmpot. now i have half of the screen shown....14.9.2015: I fixed itWorking fine now!

author: helius
author_rank: Super Contributor
timestamp: September 09, 2015, 02:42:14 pm
content: Sorry to hear your scope was damaged, it always pains me when people don't understand how to ship things.I have a very similar scope, a 9410. Is there something specific you'd like me to check?Edit: There are several cards in these scopes, I assume you've already tried to reseat them.

author: Ysjoelfir
author_rank: Frequent Contributor
timestamp: September 10, 2015, 06:49:50 am
content: Hi helius and thank you for your reply!It would help me to see what you can measure at the pin headers/plugs of the deflection yoke. especially the one with the red/blue wiring, but the other one would be interesting, too.Could you measure this right after the self test ended, without pressing any button? As i can't see what is written on the screen I don't press any buttons. And I guess the signals would be quite simmilar when it's showing the self test results.If this would be a television I would guess there should be some kind of triangle or sawtooth wave, but I have no Idea what to expect here. I measured with a Hameg HM412 something.... strange. looked more like some sort of interference than a real signal. sadly the servicemanual doesn't say anything about signals and waveforms and measuring points at all, as far as I didn't overread it by scrolling through those 450 pages.Strangely the working part of the deflection (with the yellow/green wiring) shows a signal that looks quite simmilar to a (C)VBS signal, what I didn't expect at all.Strange too: Now I have always a vertical line, no disappearing whatsoever anymore. Don't know why, I didn't change anything at all. Have to look for that later.

author: grumpydoc
author_rank: Super Contributor
timestamp: September 10, 2015, 07:35:43 am
content: The CRT circuit on these LeCroy 'scopes is basically just a monochrome monitor so very TV-likeThe vertical & horizontal scans are probably flipped though - it is on the 93xx series scopes the scan being top to bottom, then left to right if memory serves.

author: Ysjoelfir
author_rank: Frequent Contributor
timestamp: September 10, 2015, 08:00:07 am
content: Yes, it is quite simmilar to a CRT TV. I also found a sample waveform in the service manual around page 114. There is also a description of the amplifier circuit. I get back to the scope and probe around a bit, let's see what we can get.

author: Quarlo Klobrigney
author_rank: Super Contributor
timestamp: September 10, 2015, 08:30:19 am
content: Looks like J5 is the X output from the display board to the horizontal part of the yoke. Check that the B- & B+ are both at the top and bottom of the FETs and that there is a signal at testpoint T21. You could disconnect the op-amp drive and try to insert a signal from a 6V transformer via an electrolytic blocking capacitor making sure the op-amp driver is totally out of the circuit. This would at least give you some X axis deflection. All this of course if there was no signal from the op-amp in the beginning. I am looking at the schematics only on PDF page 30, not the full service manual.Page 31 same same for the Y axis.Edit: As helius says, there is no continuous scan signal. It is "made" by the input signal applied. So my suggestion of adding drive above should give the results to at least check the output circuit(s).Quote from: Ysjoelfir on September 10, 2015, 08:00:07 am

author: helius
author_rank: Super Contributor
timestamp: September 10, 2015, 08:57:17 am
content: Quote from: Ysjoelfir on September 10, 2015, 06:49:50 amThe 9400 series use a vector X-Y display, not a raster, so there is no real difference between vertical and horizontal. This also prevents the output from being scoped using TV triggers. The deflection circuits are two identical sets for X and Y.

author: Ysjoelfir
author_rank: Frequent Contributor
timestamp: September 10, 2015, 11:43:56 am
content: Thanks again for your reply! I got a quite nice progress I think.At first a look at the workbench. Quite uncomfortable to work at that lecroy thing. I bet at the lab they had riser cards to work at a single card without having the whole scope around it.Second: I had to change the scope I use to probe around in there . I know, it's not quite the best choice, BUT, I know that that thing is quite good in calibration and its absolutely reliable. My father used it for years until I got it. I have some other scopes around (like Hameg HM412 + 512, Tektronix D10/5103, Tektronix 422) but that little Hameg thing works perfectly. I changed it because I didn't realy believe in what the 412 used before showed me. Guess I have to go through that at nextHowever, I probed around a bit, looked at the "S1" zero Ohms resistor connecting the output stage with the integrator and linearity correction part, compared that with the working channel (at this point I noticed already that both channels are identical, nevertheless thanks helius for pointing that out!) and noticed that the signal at those bridge resistors are quite near - with the little difference that the working channel had like 1,5VPP and the not working one had 50mVPP. If you look at the graphs you could assume that that isn't a real signal but rather a interference. Which it indeed was, as I saw after another hour of probing around, thinking etc... but then I noticed, that the trimmer for XO was missingI never found it in the package, nor flying around in the scope, so I wasn't aware that there was something missing. Bloody transport damageSo I fixed that in a very professional manner (in need of a 500 ohms trimpot..), shown in the fifth picture, which fixed it.... nearly. just look at the last picture. I feel quite reassured now, knowing that the eeproms and custom DACs and whatever working in there is not destroyed at all. Overall this half sided error should be somewhere around the IRF9523 (Q58), at least a simple measurement shows that at testpoint T19 a signal simmilar to the signals at the working channels testpoint is measureable, but at testpoint T18 there is nothing. So i measured the resistance over the IRF and noticed that there is quite a difference between the working channel and the not working one. Measured over Gate and Source of the FET i get around 330 ohms. the working channel is at 430 ohms as you would expect, looking at the diagram. But again, measuring the 430 ohms resistor (one side desoldered) going to the gate of the FET, that thing is perfectly fine. now I'm confused again.By the way - don't you all love disconnecting those high voltage plugs at CRTs?

author: Ysjoelfir
author_rank: Frequent Contributor
timestamp: September 10, 2015, 02:08:01 pm
content: I kept working on it and got to a point where i can say: guess I fixed it!Its working / showing a complete display again. What did I do? I had the feeling that something was wrong with the FET, even though it measured quite good. So I desoldered it, replaced it, and also replaced the resistor. I saw, that the resistor was sensible to heat and changed his value rapidly after desoldering completely, and the FET was sensible to the touch - when I bent it a bit, there was a medium low resistance across drain and source of around 70k. So I got two stupid errors at one time. Damn. whatever, I replaced the resistorwith a used *coughs* one, because I didn't had a 430R so I had to take a salvaged one from my "strange values" boxwith a new one and looked at my databook for a a decent replacement of the IRF9523. I found a new IRF 9630 which should work fine, It has a little lower RDS(on) of 0,6 to 0,8R, a little lower drain current capability of 4,8 to 5A, but a higher Drain-Source Voltage of -200 to -100V. All relevant other ratings are identical if I didn't overlook something.Put that Board back in, attached all wires and fired it up! And it worked! Now all I have to do is get a new 500R trimpot...

author: Quarlo Klobrigney
author_rank: Super Contributor
timestamp: September 10, 2015, 02:24:34 pm
content: "the FET was sensible to the touch"Careful now. 30VDC between each rail with an AC voltage of? Don't get bit!Good deal on the missing pot. It never amazes me how many repairs start with fixing what the other guy did and then go on to the real problem!

author: Ysjoelfir
author_rank: Frequent Contributor
timestamp: September 10, 2015, 02:48:48 pm
content: Quote from: Quarlo Klobrigney on September 10, 2015, 02:24:34 pmIt was sensible to the touch after I desoldered itIt may have been before, too, but I could not measure that in circuit

author: helius
author_rank: Super Contributor
timestamp: September 11, 2015, 12:04:55 pm
content: I had a suspicion it could be a transistor, since it looked like the deflection was only working to the right side and not the left. Looked like what you would get when one side of a push-pull was dead.Enjoy the scope, they aren't worth much now, but were well made and should last.

author: Ysjoelfir
author_rank: Frequent Contributor
timestamp: September 12, 2015, 07:23:18 pm
content: I also thought it had to be something in the push pull amp, and measuring the testpoints gave me proof that one side didn't work, but actualy finding the error was more of a coincidence I guess. Now I'm waiting for new caps and the 500R trimpot to make it last some years

author: Ysjoelfir
author_rank: Frequent Contributor
timestamp: September 14, 2015, 02:14:22 pm
content: I guess it's ready now. At tme moment I'm waiting for more projects to order parts for (because I don't like paying 6€ postage for a 15 cents trimpot...) while playing around with my working Lecroy 9400A!You can actualy see the missing / misconfigured trimmer-replacement on the screen, as it is shifted verticaly to the right as cou can see on the pictureI also found out, why the transport damage was that hard. While disassembling i didn't notice, that the retaining bar that holds down the plugin cards (and fixes the back of the PSUs) didn't have any screws in it...now while reassembling i noticed that they were missing and as you can think, those four heavy duty PSUs broke down their retaining clips and flew around the chassis. luckily the CRT was not harmed at all.I would bet that nothing would have happened if those screws were in the retaining bar as they were designed to be...Now I have to learn how to use that puppy...Does anybody know if you can hack that thing, as there were some versions with a FFT option? Someone here at the forums also mentioned that but at that time nobody had any information. ->https://www.eevblog.com/forum/testgear/lecroy-9400a-options/Edit: should have attached the pic...

author: helius
author_rank: Super Contributor
timestamp: September 14, 2015, 04:43:36 pm
content: Quotehttps://www.eevblog.com/forum/testgear/a-question-about-a-lecroy-9450-'options-ic-chip'/I'm not 100% sure if the 9400/9400A use the same GAL as the 9410/9424/9450. There are some differences in the design between the two series.But LeCroy has released the code for the Option chip, so it's as if they endorse the modification.

author: Ysjoelfir
author_rank: Frequent Contributor
timestamp: September 14, 2015, 07:36:01 pm
content: Hey, thanks for the info heliusI read through the thread and found out that they don't use the same chip. I also found what you already wrote, that they released the code for their legacy products chips. I registered at the lecroy users yahoo group, maybe there are more informations about enabling those options, we will find out after they accepted my registration!