Tek TDS754D repair - Page 1
Forum section: electronics / repair

author: Ice-Tea
author_rank: Super Contributor
timestamp: May 20, 2016, 10:13:18 am
content: Bought a stack of scopes at an auction before. All scopes were in great shape, except for the only Tek I bought. Being a 4ch, 500MHz beast it's just too good to abandon.The scope was offered with a known issue: very low brightness. I first hoped it could be cranked up in a menu (and the auction Gremlin missed it) but allas: already cranked up to 100%. Barely readably at 100%, pretty much gone at 90%. Unfortunatey, missery didn't stop there: tried the diagnosis menu and none of the tests completed: scope hanged, rebooted... So: off with it's head!PS: If anyone has the service manual for this thing, I'd be grateful...Four screws release a part of the backcover. Figured I would need to undo all these screws at the back first, but the entire shell slides off, no trouble!This reveals the main logic board. Some custom Tek chips and a whole lot of discretes! Very clean layout and not a patch anywere. Actually, I haven't found a single scratch or patch wire anywhere in this entire unit! This board is not screwed anywhere. There are two plastic spacers keeping it in place, as soon as you remove those, the board slides right out (after you unplug all connections off course). There is a pushbutton, rocker switch and DIP switch. I'd love to know what these are for...Note how the flatcables have an additional connector that is not used. This is the 1M unit, I guess another logic board is added for the 2M unit. A bit odd: aside from the flatcables, there's a PCB connection as well. Not sure why not a simple flatcable, too high speed? Dunno...Back of the logic board: again ultra clean. Only decoupling caps here...Flipping the unit reveals the analog part Again: pretty damn clean. Note the cast shielding piece for the AFE. Beneath the logic board there's only metal casing/shielding with the PSU and CRT circuitory beneath it. To get to it, the backpanel and a lot of screws have to go:Not much there: some coax for the trigger/aux, a small circuit board for RS and parallel ports.The PSU and CRT. Looks pretty damn good for a 15+ yo unit except... Hang on.. What do we have here?Bulge much? I'll be fixing this before going any further. To be continued!

author: tautech
author_rank: Super Contributor
timestamp: May 20, 2016, 10:25:51 am
content: http://www.testequipmenthq.com/datasheets/TEKTRONIX-TDS754D-Datasheet.pdfhttp://www.spectroscopic.com/Tektronix/TDS600Manual2.pdfand 700D unitshttp://elektrotanya.com/tektronix_tds520d_tds540d_tds580d_tds654c_tds680c_tds684c_tds694c_tds714l_tds724d_tds754d_tds784d_tds794d_service_manual.pdf/download.html4.3 mb 330 pageshttps://www.eevblog.com/forum/testgear/hackingupgrading-old-scope-(tds754d)/

author: AndyC_772
author_rank: Super Contributor
timestamp: May 20, 2016, 10:33:11 am
content: There's quite a bit of information around on these scopes. You're quite right that they're perfectly decent equipment when working properly... which, sadly, isn't all that often unless they've been refurbished.There is a service manual but it's not much use for component level repair. The only more detailed information is in the TDS520B Component Service Manual, which (of course) is for a different model, but it may still give some useful pointers. You should be able to find these with a few minutes on Google.The biggest issue by far with these scopes is the leaking capacitors - but the 754D doesn't have them except on the serial card. Do replace these to avoid irreparable PCB damage (which may well have happened already).Also, the batteries in the Dallas memory modules on the logic board are likely to be long dead. Not sure what's stored in them on this particular model.There's a CRT brightness pot on the driver board, which IIRC you can access from the side through a hole without having to open the scope up too far. If the CRT (or its driver) does pack up completely, there's always the VGA port on the back. Or you can fit an LCD conversion kit, but these are quite expensive and IMHO not really worth the cost given the value of the scope.

author: Jwalling
author_rank: Unknown
timestamp: May 20, 2016, 06:16:40 pm
content: After you fix the re-boot problem:Looking at the memory controllers and burst SRAM on the ACQ board, your scope is serial prefix B040 or later.Option 2M can be enabled via GPIB. You already have the hardware.Do a search for "PASSWORD PITBULL" and find out more.EDIT: If you really suspect that mains bulk cap of bulging, you can carefully use an X-acto blade and remove the plastic cover on the top. You may have to remove a bit of the plastic wrapper to do this. Anyway, the cap Is not part of the metal can itself. I doubt that's where your problem lies, though. I suspect the cap is really not bulging, just the plastic bit is domed.Jay

author: Ice-Tea
author_rank: Super Contributor
timestamp: May 20, 2016, 07:37:44 pm
content: Oh? What's the free board holder space for, then?

author: nctnico
author_rank: Super Contributor
timestamp: May 20, 2016, 07:40:00 pm
content: The TDS520B Component Service Manual is a treasure trove on diagnosing the TDS500/600/700 series so be sure to get it!The extra board position if for option 5: TV triggering.

author: Jwalling
author_rank: Unknown
timestamp: May 20, 2016, 07:59:09 pm
content: Quote from: Ice-Tea on May 20, 2016, 07:37:44 pmFor option HD (Hard drive) This option goes below the CPU. If you had option 05 (Video trigger) it would go above the CPU board.Jay

author: Ice-Tea
author_rank: Super Contributor
timestamp: May 23, 2016, 02:48:32 pm
content: Quote from: Jwalling on May 20, 2016, 06:16:40 pmRemoved the plastic cap as per your advise. Still bulges, albeit neither dramatic nor dirty. Yanked it out anyway. No ESR meter handy though, measures 450uF.EDIT: 0.35 ohms. So should be OK. Bummer.

author: Jwalling
author_rank: Unknown
timestamp: May 23, 2016, 03:55:01 pm
content: A long shot perhaps, but try running the scope with the ribbon cable to the HV assembly disconnected, and use an external display instead.Make sure you have a fan blowing across the heatsinks on the ACQ board.Jay

author: Ice-Tea
author_rank: Super Contributor
timestamp: May 23, 2016, 05:16:21 pm
content: Finally found the brightness/contrast screws. Seriously, how small are these pots?To get to the contrast screw, you have to tear out the floppy drive. Stupid.Anyway, tossed that slightly-bulging-but-apparently-OK cap back in. With the screws I can get brightness to acceptable levels (turning them up too high loses focus, makes it all too blurry. But: good enough. Launching any kind of diagnostic still reboots the unit though. So went ahead and ordered a replacement cap just in case. If that doesn't help I'll be digging in deeper in the voltage rails.

author: Ice-Tea
author_rank: Super Contributor
timestamp: July 15, 2016, 06:56:21 am
content: Hmmm... replaced the cap, no improvement. Unit still rebooting when launching diagnostics.

author: george.b
author_rank: Frequent Contributor
timestamp: July 15, 2016, 07:58:42 am
content: And how's the low voltage side doing? Have you checked for ripple?

author: Ice-Tea
author_rank: Super Contributor
timestamp: July 15, 2016, 08:55:15 am
content: The ripple on the.. eh... I think 20V is ridiculous. 2-3V if memory serves. According to the manual within spec but that's next on my list.

author: TAMHAN
author_rank: Frequent Contributor
timestamp: July 15, 2016, 03:29:02 pm
content: Hello,congratulations on your score.If you happen to give up on the unit and want to make a quick buck, I would be very interested in the Aquisition board. My TDS754D - incidentially also with a dead screen - has one error there.Re the dim screen: this is a common problem which can be fixed by either attaching an external monitor or buying a 249USD kit with an LCD.

author: Ice-Tea
author_rank: Super Contributor
timestamp: July 15, 2016, 03:34:51 pm
content: I'll keep the offer in mind, thanks!

author: TAMHAN
author_rank: Frequent Contributor
timestamp: July 15, 2016, 05:55:16 pm
content: Hello,in fact, you just motivated me.I took my TDS754D apart and checked for "dirty" or bad connections on the attenuator, and all looks perfectly stable. So next, I will try to replace the U416 memory IS which it complains about in self diag. Feeding time now, will report in on the error in my own thread tomorrow to stop myself from derailing you.

author: Ice-Tea
author_rank: Super Contributor
timestamp: July 15, 2016, 06:00:35 pm
content: Feel free to derail. The more info the more I like it. I'll be repairing more Teks in the future, so...

author: TAMHAN
author_rank: Frequent Contributor
timestamp: July 15, 2016, 06:59:57 pm
content: Well. I just reassembled mine after performing a free eye and basic magnification analysis of the Aquisiteur board. Mine is a different model than yours tho, as I have the 8MB of memory and easier to solder aka no BGA IS on there (can upload pics if you so desire). Of course, I found no dead solder points at first glance.So, now the plan is this. Run a bunch of detailed self tests, as they generate memory errors (digSpeedMemDiag) not found in the normal bootup which dies only with digHFStepDiag. Then, I need to find a way to scroll the error log to the side as the really interesting information is cut off.Then, I will probably just sit down with the relevant part of the TDS520B schematic and the addresses collected, and try to find out which line is fooked. To be honest, my wife was like fuck it lets exchange the memory IS on pure luck - but a quick Google of the type (71024) showed not a single reference to those going INRI. Oh, and one more thing - while they seem out of stock forever (no longer made) at Farnells, I found a source here:http://eu.mouser.com/ProductDetail/IDT/71024S20YGI/?qs=JcGQCygHkIa4aJVsYrW44A%3d%3dImportant, brain fart for Icetea: running the Diag ALWAYS reboots the Tek. So, just reboot him again and see what the error log got enriched by. Oh, and there is a little numeric display on the top of the Proc board which tells you where the planar went tits up. Finally, you could also use a RS232 adapter to read the console of the SmallTalk OS.So, if you want to help me: did you ever see a dead SRAM before?

author: TAMHAN
author_rank: Frequent Contributor
timestamp: July 15, 2016, 07:02:59 pm
content: Forgot to add, on the risk of being beaten by Icetea.Icetea, do this as your scope might work well:a) connect VGA monitorb) connect probe to cal outfor foo = 1 to 4c) connect to port food) enter dpoe) press auto setf) magnify waveform, checking for odditiesOh, and check the debug DIP switches.Need to run now, feeding time for real.Tam

author: Ice-Tea
author_rank: Super Contributor
timestamp: July 15, 2016, 07:09:05 pm
content: Quote from: TAMHAN on July 15, 2016, 06:59:57 pmMooh? What do you mean?Quote

author: AndyC_772
author_rank: Super Contributor
timestamp: July 15, 2016, 07:33:20 pm
content: Quote from: TAMHAN on July 15, 2016, 06:59:57 pmI've had a dead SRAM IC on a TDS540. I tracked down the offending IC by deliberately disabling ICs to see what areas of the memory map threw up errors. Assuming they're in some kind of sensible order on the PCB, you can probably do a binary search to narrow down the offending part without having to test them all.IIRC one of the strobes on each IC was connected via a 0R link which was easily removed to disable the part.

author: TAMHAN
author_rank: Frequent Contributor
timestamp: July 16, 2016, 06:01:52 am
content: Hello,sorry for causing a bit of chaos.@Ice-Tea: I meant that the behaviour you see could be quite normal. A TEK Dpo ALWAYS reboots when running the tests. Plus, the scope has a serial header on the planar where it emits status information as it boots. Harvesting this might be helpful.@AndyC: thanks. In fact, the error log even alerts me to the U416 IC...might exchange that one on a for kicks basis.Tam

author: Ice-Tea
author_rank: Super Contributor
timestamp: July 16, 2016, 08:34:19 am
content: So, I was expecting a status page with pass/fail info but that's not the wat it works? That's both stupid and good news. Unit could actually be fine!

author: TAMHAN
author_rank: Frequent Contributor
timestamp: July 16, 2016, 01:45:54 pm
content: Working on a video for you

author: TAMHAN
author_rank: Frequent Contributor
timestamp: July 16, 2016, 04:21:40 pm
content: https://www.youtube.com/watch?v=uJ6pUMIM9Ks&feature=youtu.beHere you all go! Please give criticism, its my first video with the new camcorder and the light boxii from my client!

author: Ice-Tea
author_rank: Super Contributor
timestamp: September 19, 2017, 06:21:22 pm
content: Crap. Tam, I totally missed this. I appologise for not commenting or thanking you before. Just picked the project back up and this is great as a reference of how things should be.Thanks again...

author: TAMHAN
author_rank: Frequent Contributor
timestamp: September 21, 2017, 11:00:57 am
content: No worries, man. You just remind me again that I finally need to ship the scope to Jay - these TDS754D have a knack at "slinking out" and being forgotten

author: Ice-Tea
author_rank: Super Contributor
timestamp: September 21, 2017, 11:21:04 am
content: Just closed up mine. In the end, there was nothing wrong with it

author: TAMHAN
author_rank: Frequent Contributor
timestamp: September 21, 2017, 09:56:08 pm
content: Mine is badly shot up, the acquisitor is toast...

author: denimdragon
author_rank: Regular Contributor
timestamp: September 30, 2017, 06:06:05 pm
content: Quote from: TAMHAN on September 21, 2017, 09:56:08 pmI have 2 of these coming in next week and 2 784D. I'm going to have to go through them to make sure they are all ok. I want to keep one of the 784D for personal and get rid of my 684C. I might have to get a few pointers from you guys!