Tektronix TDS420 Oscilloscope smells like a fish market  - Page 1
Forum section: electronics / repair

author: OptoIsolated3
author_rank: Newbie
timestamp: May 30, 2020, 04:05:43 pm
content: If anyone can help or lend some insight. I have a TDS420a. When I originally got it and powered it up there was a strong fish odor. The odor has went away and the scope still functions properly. I have opened it up and haven't seen any leakage but it wasn't a through inspection . My professor thinks it could be an old transformer off gassing Should I look into recapping the scope? and if so Does anybody have any tips or advice about recapping? Any help is appreciated and let me know if I'm doing anything wrong this is my first post, Thank you!

author: TheMG
author_rank: Frequent Contributor
timestamp: May 30, 2020, 05:16:33 pm
content: Fishy smell in my experience comes from leaking electrolytic capacitors.Look very carefully around the bottom of every electrolytic cap, sometimes it's not very obvious. Wouldn't hurt to poke around with an ESR meter if you have one too.I would check this even if it appears to be working properly, electrolyte leaking onto a PCB can over time corrode traces.

author: Smoky
author_rank: Frequent Contributor
timestamp: May 30, 2020, 06:04:58 pm
content: Here's my guess.On the main board there are voltage regulators near small groups of electrolytic capacitors.The caps have leaked and fluid has crept under the regulators.When you turn on the oscilloscope, the regulators heat up and bring up that smell.If this is the case, eventually, a hole will burn into the PC board.I'd clean each board and test a few capacitors even though the leaking capacitor issue was more of a problem in the earlier TDS line of scopes.

author: shakalnokturn
author_rank: Super Contributor
timestamp: May 31, 2020, 12:14:27 am
content: Wouldn't the TDS420A be one of the "earlier line of TDS scopes" considering that some of the TDS500, TDS600 also suffered from poor capacitors?

author: Smoky
author_rank: Frequent Contributor
timestamp: May 31, 2020, 01:19:57 am
content: I own a TDS420 and it's dated 1992. I believe the "A" versions were dated around 1996.As for OptoIsolted3's problem, since he wasn't soldering anything at the time when he smelled that foul odor, that to me means, a leaky capacitor is, or located near a component that is, getting hot enough to make it smell like that.

author: CaptDon
author_rank: Super Contributor
timestamp: May 31, 2020, 02:10:05 am
content: Don't know what world that professor lived in to think a fishy smellwould be an off-gassing old transformer??? 'Green' uncured transformerssmell like hot pitch or asphalt and do indeed stink. Sylvania had thatproblem with some of their televisions that used a constant voltagetransformer which runs very hot and some slipped out without goingthrough the curing process. That sickly fish smell is INDEED bad capacitorsleaking electrolyte. You may even have had one that partially shortedand popped and cleared the fault and no longer smells. Most of the 400,500and 600 as well as 400A, 500A and 600A series had faulty caps andabsolutely need replaced if still original. I just got done doing a 644Aand a 644B. By the time they got to the 'C' series they got it figured out.They even went to the 'Sure to Short' tantalum series in later models.Oh, and you'll just love the polyfilm (RISA?) exploding capacitors acrossthe A.C. mains. They should have included a built-in fire suppressionsystem in the power supply section. Oh, and pray the 420 power supplynever eats the 'green unobtainium hybrid' during a power surge orlightning tickle. I have a 420 (non-A model) about to hit the dumpster.No floppy, No USB, it is a worthless hulk now that the power supplywon't run.

author: Smoky
author_rank: Frequent Contributor
timestamp: May 31, 2020, 02:21:54 am
content: Excuse me, but CaptDon, before you decide on dropping your TDS420 in the dumpster, I'll gladly pay shipping to have it brought to my doorstep

author: james_s
author_rank: Super Contributor
timestamp: May 31, 2020, 04:30:49 am
content: It's the surface mount electrolytic capacitors on the boards, particularly the acquisition board but there are at least a few of them on almost every PCB in there. First thing to do is unplug it NOW because the leaking electrolyte is damaging the boards and electrolysis rots out traces and vias even faster. You have to wash the boards thoroughly and replace every one of those capacitors, I've done two TDS400 series scopes so far, the job is not too bad if you catch it early but ignored it creates a HUGE mess.The fishy odor is an immediate tipoff, that's the classic stench of leaky caps, I thought that was common knowledge but maybe those stupid SMT electrolytics aren't used so much anymore. I always replace them with tantalum chip types.

author: james_s
author_rank: Super Contributor
timestamp: May 31, 2020, 04:32:33 am
content: Quote from: CaptDon on May 31, 2020, 02:10:05 amThe hybrid has been reverse engineered and somebody designed a replacement board for it, I forget who but I came across it a while back on this forum. There's not really much in it, some op amps, a handful of transistors, diodes and a few passives. Nothing exotic.That power supply was a real mystery for a long time but then I finally got my head around how most of it works. There's a schematic for a variation of it in a Lecroy scope manual. The Lecroy version has different polarities of some of the outputs but the PCB is identical and there are only minor changes to the output section.

author: madao
author_rank: Frequent Contributor
timestamp: May 31, 2020, 06:43:43 am
content: TDS420A is too new about problem with SMD eletrolytics. Problwm with SMD eletrolytics is solved since 1994/5. But probably: Eletrolytics on PSU make a problem with leaking caps.I have a TDS430A corpse, SMD elektrolytics is fine, but not PSU.TDS420 , pleas don't trow it away, despite low 100Ms/s samplerate. In germany, i sell always with sucess him for over 100€. I wan't keep him, but repair: YESGreetingsMatt

author: CaptDon
author_rank: Super Contributor
timestamp: May 31, 2020, 11:53:45 am
content: Smokey, The unit was working perfectly with one slight weirdness.When you powered it on it would run about 10 minutes exactly, thenpower down. When you hit the power button again it would power onand run the rest of the day. We took a huge power surge at my workplacewhen a car hit the utility pole outside. The scope was not powered onat the time, but the power supply portion is always on and it smoked.I made an attempt at repairing the supply but the hybrid is bad. I amsure the rest of the scope is perfect and I had even replaced both coincells about 2 years back. I could put the scope back together and wrapup the power supply separate. Nothing is butchered. I would suspectbecause of the weight it would be around $50 to ship. I used the correctedLeCroy power supply schematics to troubleshoot. It is a TDS420 andnot an A model. They only sell for about $150 on Ebay and the powersupplies when listed sell for about $125 so we decided to just shoveit under the bench. I can put it back together and send you the serialnumber. What city and state do you live in? I'll try to get a shippingestimate. B.T.W. it is super clean with no physical damage and the C.R.T.is bright and well focused. It was a pleasure to use this scope untilthe power supply smoked with a shorted switch transistor.

author: Smoky
author_rank: Frequent Contributor
timestamp: May 31, 2020, 05:57:57 pm
content: Thank you CaptDonPM on the way!

author: james_s
author_rank: Super Contributor
timestamp: May 31, 2020, 09:54:04 pm
content: Quote from: madao on May 31, 2020, 06:43:43 amI just went and looked, the one I have is a TDS410A, it was not leaking nearly as bad as the TDS460 but it certainly had some bad SMD caps, and now that you mention it the output filter capacitors in the PSU were leaking too. It's best to replace them all, even good quality SMD electrolytics don't last forever and 1995 was 25 years ago.

author: OptoIsolated3
author_rank: Newbie
timestamp: June 02, 2020, 03:50:41 pm
content: Thanks for all the reply's, I'm going to replace all the smd electrolytic capacitors in the scope. Should I replace the lithium batteries as well? Also if any one can point me in the right direction finding a procedure to replace the batteries, I hear its a tricky thing to do and there is a potential to lose the chips memory but I haven't found much googling things like "Oscilloscope lithium battery change" or any other combination of search terms I can think of. Is there a name for this?

author: Smoky
author_rank: Frequent Contributor
timestamp: June 02, 2020, 05:09:15 pm
content: On the back of the PC board you will see the two solder joints where the battery connector ties in.Solder two short jumper wires to those solder joints and then attach a new temporary battery in parallel.You can then un-clip the old battery harness and replace the cell without losing power to the board.When you're finished, de-solder the temporary jumper wires/battery.

author: james_s
author_rank: Super Contributor
timestamp: June 02, 2020, 08:34:29 pm
content: I unplugged the batteries in mine and it sat that way for months. As far as I could tell, the only thing those batteries keep alive are saved waveforms, settings, time/date and SPC constants. I ran SPC on my TDS420 and it passed, everything was then good to go, only takes a few minutes and you should run SPC occasionally anyway.There is no critical calibration data stored in NVRAM that I can see.

author: Smoky
author_rank: Frequent Contributor
timestamp: June 04, 2020, 02:41:53 am
content: James, I was careful when changing-out the batteries because the TDS420 that I have has the FFT option installed. I'm not sure how that option is stored but I didn't want to take a chance on losing it. Here's a shot I took today when I fed a 12kHz signal into it:I plan to work on CaptDon's TDS420 and I was thinking about trying to activate options 1M and 2F within it. Has there been a clear path to success in doing that?Anyway, since I have plenty of extra parts to do so, would anyone know which board the "options" are stored on?For the audio work that I do, the TDS420 fits the bill

author: OptoIsolated3
author_rank: Newbie
timestamp: June 04, 2020, 04:09:02 pm
content: Update: Upon further inspection nearly all smd electrolytic capacitors have leaked. The THT electrolytic capacitors seem fine. The time in between the initial inspection was about 5 months ago and the scope was powered on for about ~40 min. This caused corrosion in multiple places. My digi-key order came and i'm replacing all of those smd caps. I'm going to desolder them, then wash the boards in with a light detergent and water, then rinse with distilled water. After that ill let it dry and then do a pass with IPA and compressed air. Here is some Pics of the board!https://imgur.com/a/oudEQJf

author: james_s
author_rank: Super Contributor
timestamp: June 04, 2020, 07:08:32 pm
content: Quote from: Smoky on June 04, 2020, 02:41:53 amThe options are stored the same way the other TDS scopes and can be enabled via GPIB or serial commands. Note that the TDS400 series does not support all of these options and some require physical hardware to be present but those it does support can be enabled with the same commands as the bigger models. Also there are two dip switches on the CPU board you have to flip instead of the single switch as in the TDS500/600/700.At least in my case, option 1M required installing 6 additional SRAM chips on the DSP board, I did it on mine just for fun, cost about $13 from Digikey. Look on the board and see if there are vacant spots, I added the chips and it just worked, I didn't have to enable anything in software for that.Power onSet calibration protect switche to unprotectedSend "password pitbull" via the GPIB.Send:WORDCONSTANT:ATPUT 327686,1Option 1M (130,000 record length)WORDCONSTANT:ATPUT 327687,1Option 05 (Video trigger) *WORDCONSTANT:ATPUT 327688,1Option 13 (RS-232/Centronics Hardcopy Interface Ports) *WORDCONSTANT:ATPUT 327689,1Option 2F (Advanced DSP math)WORDCONSTANT:ATPUT 327690,1Option 1F (Floppy disk) *WORDCONSTANT:ATPUT 327692,1Option 2C (Communication Signal Analyzer)WORDCONSTANT:ATPUT 327693,1Option 3C (P6701B with system calibration) *WORDCONSTANT:ATPUT 327694,1Option 4C (P6703B with system calibration) *WORDCONSTANT:ATPUT 327695,1Option 2M (8 M acquisition length) *Return the calibration switch the normal working position.Cycle the power and see the initial banner.Edited to make the strings you actually send to the scope green so it's more clear.

author: james_s
author_rank: Super Contributor
timestamp: June 04, 2020, 07:16:18 pm
content: Quote from: OptoIsolated3 on June 04, 2020, 04:09:02 pmYep, looks about right, glad you caught it before things got too bad. I have a TDS460 that was more severely damaged, I still don't have that fully working, SPC fails on it. The newer scopes are not quite as prone to leakage as the older ones but those SMD lytics *always* go bad. They were notorious in 90s camcorders, you'd pull it out of the closet, open the tape door and the smell of rotting fish was a dead giveaway. Those things contained dozens and dozens of them and were a real nightmare to work on. 68k Macs are full of them too, I re-capped a lot of IIci's, LC's, SE's, etc. They leak and it makes an awful mess, you really have to be sure you get all the electrolyte out from underneath ICs and out of vias.There's a crazy sounding trick someone showed me for removing the old caps that works amazingly well. Don't try to desolder them, you'll rip up pads for sure. Instead grab the cap with needle nose pliers, push down slightly and twist, it will pop right off and then you can separately wipe each pin off the pad with a soldering iron. I've changed hundreds of them with that technique and have yet to damage a board.Oh, also replace them a few at a time so you don't lose track. I remember there being some confusing values on there, like some are 10V at one value, and some are 10uF at whatever voltage and they look similar, you've got a bunch of detailed pictures at least which should help keep track.

author: Smoky
author_rank: Frequent Contributor
timestamp: June 04, 2020, 11:27:44 pm
content: Honestly, I don't like what I'm seeing.Btw, under this ribbon cable are more electrolytic capacitors:Take a very close look at this voltage regulator. These will burn a hole through the board in no time:The voltage regulator on the front panel board looks like it has caused some damage due to the leaking electrolyte. There are traces under it that can short/rot:In almost 100% of the cases, the traces that flow under those small tantalum caps and SMD resistors lose continuity. The bubbly look around them appears to be extreme heat damage.Now that I've seen this crazy leakage in the "A" version scopes, check the capacitors mounted on the backplane board too. They are through-hole type capacitors but I've seen some ugly damage done by them.Thanks for the tips James! I have two extra display boards to scrounge extra memory from

author: Smoky
author_rank: Frequent Contributor
timestamp: June 05, 2020, 03:48:19 am
content: I looked through my repair photographs and found one that may help troubleshoot your attenuator board.Here is your picture of U134, the 14051B CMOS Multiplexer:Here is a picture I took when I repaired a trace on my attenuator board. Follow the traces and check to see if you lack continuity under that chip, if so, it may need to come off or be jumped:

author: TheDefpom
author_rank: Frequent Contributor
timestamp: June 05, 2020, 11:37:47 pm
content: That smell sounds like the main IEC input filter, they smell like that when they fail, it could still be a leaking capacitor but I have had a few of those IEC filters go on me and they stink...(the fun of repairing OLD gear and upping the input voltage from 115VAC to 240VAC)

author: james_s
author_rank: Super Contributor
timestamp: June 06, 2020, 05:28:33 am
content: Quote from: TheDefpom on June 05, 2020, 11:37:47 pmNo, it's the SMT electrolytic capacitors all over the boards, this has already been confirmed.I've had those line filter capacitors burn up a couple of times and it's a whole different matter. They sizzle and smoke and stink and then fizzle out, it's nothing like the fish smell of leaky electrolytics.

author: madao
author_rank: Frequent Contributor
timestamp: June 06, 2020, 11:45:55 am
content: Pic with bad SMD caps, it came from TDS420 and not TDS420A, correct ?

author: james_s
author_rank: Super Contributor
timestamp: June 06, 2020, 11:18:47 pm
content: Quote from: madao on June 06, 2020, 11:45:55 amNo, the OP said this about the TDS420A: "Update: Upon further inspection nearly all smd electrolytic capacitors have leaked. The THT electrolytic capacitors seem fine. The time in between the initial inspection was about 5 months ago and the scope was powered on for about ~40 min."

author: madao
author_rank: Frequent Contributor
timestamp: June 07, 2020, 04:54:12 am
content: Ok,smokys pics show me a TDS420 and not TDS420A.many reason tell it.1.) Data code of IC: 1992 TDS420A is since 1996 on market.2.) Frontplate haven't space for floppy disc drive.

author: Smoky
author_rank: Frequent Contributor
timestamp: June 07, 2020, 06:04:29 am
content: The front panel board the OP shows is definitely used in a TDS420, not a TDS420A.I just checked the part number too. Q9C-0861-01 is the part number to the front panel board in my TDS420.There's definitely something "fishy" going on

author: james_s
author_rank: Super Contributor
timestamp: June 07, 2020, 06:42:42 pm
content: Maybe somebody replaced the bezel of a TDS420 with one from a TDS420A?Either way it's kind of irrelevant, we know the caps are leaking and even if they weren't leaking I'd replace them. The newer ones may be less likely to leak but they're not immune. The 'A' model is newer than the non-letter model but it's still decades old.

author: OptoIsolated3
author_rank: Newbie
timestamp: June 10, 2020, 03:49:57 pm
content: Correct, my mistake its the TDS420 not the TDS420a

author: OptoIsolated3
author_rank: Newbie
timestamp: June 10, 2020, 04:31:46 pm
content: Update: I have replaced all the caps and washed the board thoroughly in IPA and reassembled, everything is working. It passed the self check on start up. I have temped the areas with previous corrosion that showed signs of heat damage and they are within normal operating temperatures. Thank you for all your comments and help! Here is a video of the first start up.https://youtu.be/aTi6GmeMTqE

author: Smoky
author_rank: Frequent Contributor
timestamp: June 10, 2020, 11:53:45 pm
content: Way to goyou jumped a major hurdle.The next critical step is for your TDS420 to pass the Signal Path Compensation tests.Let your scope run for 10-20 minutes to warm it up and then run the SPC test.I get into the habit of turning the intensity of the CRT down to zero whenever the scope idles.Good luck!

author: james_s
author_rank: Super Contributor
timestamp: June 11, 2020, 02:39:58 am
content: It's always good to run the intensity as low as is comfortable to use but no real need to dial it down to zero. As long as the cathode is hot it's slowly getting used up. Fortunately these tubes seem to last a very long time, haven't encountered a weak one yet.

author: CaptDon
author_rank: Super Contributor
timestamp: June 11, 2020, 01:25:33 pm
content: Maybe a Tektronix CRT designer could jump in here and explainthe details but there is some kind of cathode poisoning that happensto a hot cathode with zero emission. We know this from real worldexperience in broadcast transmitters. If you leave a standby transmitterin hot standby mode with filaments lit but never call for emission (timeframe 6 to 12 months) the peak emission of the cathode will be degradedmuch more than expected. If you actually 'operate' the transmitter foran hour or two once a week you will get almost the entire 'normal'operating hours from the tube. It would take me hours to find the articleand it may be as old as the M.I.T. RadLab series on CRT's, but operationin standby at low intensity may actually be more beneficial than zerointensity for reasons I never fully understood.

author: Smoky
author_rank: Frequent Contributor
timestamp: June 11, 2020, 04:15:58 pm
content: Thank you for sending me the Tektronix TDS420 manuals CaptDon!I looked up "Intensity" in the Index and it directs you to page 3-22:So turning the Intensity knob fully CCW does not bring it to zero.I practice this habit to reduce the chance of Phosphor burn-in.