Missing lines on display, Lecroy LT264M repair - Page 1
Forum section: electronics / repair

author: Jontm
author_rank: Contributor
timestamp: February 19, 2023, 01:36:46 pm
content: Hi!Made a post a while back on my sick Lecroy LT264M that I was trying to repair. End result there was for now was that the trigger is somehow broken.This was a couple of years ago. Now i found the will to have a look at it again. Had the same issue as before, but on the third boot it suddenly started having problems with the screen. There seems to be some line missing. Almost looks like every other line. See the attached photos.I removed the scenen and backlight and inspected. Everything looks fine. I booted it up with the scenen so that I could try to apply some pressure to the melted on flat flex connectors to check if there could be some bad contact, but everyone looks fine.Anyone got any idea what could cause issues like this ?For information i also probably got some kind of firmware corruption as clicking the panels button crashes the scope with some error for a memory address it seems to have problems accessing.

author: alm
author_rank: Super Contributor
timestamp: February 19, 2023, 04:29:05 pm
content: I haven't worked on these scopes, but the display fault really looks like a missing signal between the TFT driver and panel to me. if not in the cable, then maybe the connector, PCB trace or even defect in the driver IC.Regarding the crashes I would look at ripple on the digital power supply rails (dried up caps?) and since the memory is a standard PC133 SDRAM memory module, I would also try to swap out the memory.

author: Jontm
author_rank: Contributor
timestamp: February 19, 2023, 04:45:02 pm
content: Thanks for the feedback! The flat flex cables going to the display does look a bit iffy. I will order some new and test.Have already changed memory once. It did not boot at all with the original memory, but maybe I should try a third stick.If that do not work i will probe around the TFT driver and see what I can find.Will also check the PSU rails, i believe they where OK the last time I tried to fix it, but then the screen was working ok.

author: alm
author_rank: Super Contributor
timestamp: February 19, 2023, 07:08:11 pm
content: My logic is that while firmware corruption is certainly plausible, I prefer to look at things that are easier to fix, like changing out a DIMM, first.For firmware, you could try flashing it with the documented flashing methods (ScopeExplorer, floppy drive, optionally emulated). But I would make very sure the rest of the digital board works fine so you don't brick your scope with a failed flash. I have an LT374M if that helps. If the firmware is easy to extract, like in a socketed DIP EEPROM, then I could help with that. I haven't checked if the firmware upgrade file is a plain bin file or something else.

author: Jontm
author_rank: Contributor
timestamp: February 19, 2023, 09:00:56 pm
content: Wow. That is very generous of you, but thankfully that will not be necessary. The firmware is available in the LeCroy user group. I have ordered new FFC's, FFC sockets and even a VGA chip from Ebay now. Hopefully i should then be covered.I think i tried a floppy disk flash here a couple of years ago, but will try using GPIB and ScopeExplorer after I have changed flat flex cables. Just need to borrow a USB to GPIB cable from work, surprised by how expensive those cables are.The first think i should try is probably to connect a display to the VGA out to see if i get something from that. Then I guess I could rule out firmware for the display at least.Who knows, maybe changing flat flex cables solves the trigger issues as well. The contacts on the FFCs are quite corroded. I ordered new sockets as well, as i saw they where cheap and they are only specified for 10 mating cycles...

author: Jontm
author_rank: Contributor
timestamp: February 24, 2023, 09:52:45 pm
content: Unfortunately changing flat flex cables did not solve it. Have not been able to test the VGA output yet, however i got the RS232 up and to my suprise it seems like the display corruption is even there when capturing from rs232 in scope explorer...Edit: It looked like it was two good vertical lines and then two bad. Repeatedly. However i tried to turn on analog presist and it seems like it is not that clear cut, see the additional attached photo.I have a "new" CHIPS vga IC coming, but does anyone have any idea if this corruption could be from CPU or VGA chip ? I guess the CHIPS 65545 does not have any onboard code i need to try to dump before changing it out when i get the replacement IC ?

author: shakalnokturn
author_rank: Super Contributor
timestamp: February 25, 2023, 02:40:06 am
content: I'm not the most experienced around nor have I looked at the schematic before posting.I'd expect that the screen copy is done by reading out of the video RAM so your display problem would come from either the display RAM, Chips 65545 (they are known to fail) or anything sharing the same buses.All this is on the top PCB so why not scope the RAM for something odd.I'd expect the same on VGA out.With all these different issues I assume the power rails have been checked for DC and ripple ?

author: Jontm
author_rank: Contributor
timestamp: February 25, 2023, 03:07:45 pm
content: Thanks. That makes sense. All the rails that I have measured so far seems to measure good. Before the VGA went bad i was able to test that most of the rest of the scope works fine. It does however still fail trigger cal and the trigger seems to be permanently on, except in auto mode. Going to single or normal on any of the channels or other trigger options cause the scope to stop no matter the trigger level.However, none of that really matters right now that I don't even have working VGA 😜Probably spenning way too much time on this thing, probably better off just buying a new scope. However I like the challange of troubleshooting electronics sometimes, that and the fact that I probably actually don't need a scope at all.I have soldered out the old Chips 65545, now i just have to wait for the replacement to arrive.You helped me out quite a lot the last time I attempted to fix this scope a few years ago.https://www.eevblog.com/forum/repair/lecroy-lt264m-repair-does-not-power-on/I was able to solve the error when hitting the panels button by doing a general reset (F2, F5 and Ch1), but the scope is quite unusable without the screen. Last time I gave up troubleshooting on it, i only had the trigger left to fix.Edit:On second thought, maybe I should have replaced the VGA memory first, these seem quite a bit easier to solder off...If it is a bus issue, I don't really know where to start. Seems like a lot of difficult work to verify all things on the same bus, but maybe it is just me that is unexperienced with busses and digital things...

author: shakalnokturn
author_rank: Super Contributor
timestamp: February 26, 2023, 01:11:29 am
content: I skimmed back over your old topic, this scope seems quite challenging, you also seem about as stubborn as I do.The way I'd usually go about your display problem is scope the RAM busses first starting with Data lines as a signal integrity check making sure voltage levels make sense and that no line looks very different from the others in terms of activity or amplitude.Then move on to Address lines then enables whatever...Yes it would have been easier starting with RAM if you must start swapping parts blindly but you could get lucky with the Chips graphics.