My $35 TDS460 Oscilloscope Repair Project - Page 1
Forum section: electronics / repair

author: Smoky
author_rank: Frequent Contributor
timestamp: June 19, 2020, 05:30:25 am
content: Well here she is, fresh off of the Ebay chopping block!When I read through the description, it appeared to fit the bill. It didn't have a back panel cover or a handle, but that isn't a problem thanks to CaptDon, I have those extra parts in a box on the shelf.The serial number is a high one and the chassis is stamped June of 1994.The PC boards look really nice but the seller claims the scope failed to pass the initial self-tests and SPC.The power supply looks good too even though it's dirty as can be.Within an hour of un-boxing, the scope was stripped and the boards dried from a hot soapy scrub and ready for their re-capping session.The part number of the Acquisition Board is 671-1685-11 and she looks pretty good:Those are the old caps ready to be twisted off. I will do continuity tests on every nearby trace and via. I don't see anything that alarms me yet**The Server crashed on EEVBlog in the past so it's March 25th 2025 and I'm back to upload all of the pictures again**

author: andy3055
author_rank: Super Contributor
timestamp: June 19, 2020, 06:58:44 am
content: Waiting to follow your adventure. Good luck!

author: Smoky
author_rank: Frequent Contributor
timestamp: June 19, 2020, 07:14:40 am
content: Thanks Andy!I just cleared and prepped the Attenuator Board, part number 671-1686-06. No issues whatsoever. All pads cleaned up really nice and no loss in continuity to be foundThe Display board, 671-1784-03, has one 33uf 10v cap on it. The pads look great too.We're on a roll

author: andy3055
author_rank: Super Contributor
timestamp: June 19, 2020, 07:26:59 am
content: BTW, whatever happened to that amp repair where you were dealing with an STK module?

author: Smoky
author_rank: Frequent Contributor
timestamp: June 19, 2020, 07:49:35 am
content: That was the Pioneer SX-580 receiver. It's fixed.Rick, the fella who designed the RCS16 circuit had me comment on AudioKarma.com about my experience. I mentioned the 2SC945A transistor voltage imbalance and a member chimed in and said that there were three 1S2473 diodes that need to be replaced with 1N4148's in the rectifier circuit. One of the old diodes was intermittent and caused the transistor to sit there and cook. Mr Merlynski is his "screen" name and he's labelled as the "Pioneer" God over there

author: Smoky
author_rank: Frequent Contributor
timestamp: June 19, 2020, 09:11:26 am
content: Here's the DSP Board, part number 671-1684-01, cleaned and ready. The original battery still measures 3.10 Volts:...and this is the save of the day. Both of these Front Panel Boards have the same part number, 671-1687-01. The one on the right came with the TDS460. The board on the left came from CaptDon's donated TDS420 and it's in perfect shape:Look close at the damage I found on the TDS460 board. A corroded ceramic capacitor was lifted upwards on its pads, so I removed it. The trace leads to pin 4 of the connector. Notice how the electrolyte crept up the pin and turned it green:This had to be a major failure point causing the scope not to pass its self-tests.Thank goodness it's not the chassis soft-key connector**I just checked, it's the connector for the BNC ribbon cable**We're still batting a thousand. Thanks to CaptDon!

author: CaptDon
author_rank: Super Contributor
timestamp: June 19, 2020, 01:41:07 pm
content: Wish that out of all the 420/460 units one of themwas an 'A' model with the floppy drive option. I doextensive R&D and QA/QC for my employer (Youknow who they are) and it is very important to meto be able to submit .jpg or .bmp screen shots aswell as .csv waveform captures that can either berecreated on an AFG or imported to MatLab fordiagnostic chart creation and so forth. That wasone of the reasons the 420 ended up not beingrepaired for our lab use. If it had the floppy theymay have actually bought a power supply and letme repair the unit. I saved it from the dumpsterfor about 1.5 years but lab space was gettingcramped. I am so glad it found a good home.I have been a ham since 1973 and almost everypiece of gear I own was a dumpster dive. I am hopingthe Berea Hamfest doesn't get cancelled for 2020.Dayton/Xenia and Butler Pa. were cancelled andthey are great fests normally. Don't know if theHoward County Md fallfest is on or not.Cheers mates!!

author: madao
author_rank: Frequent Contributor
timestamp: June 19, 2020, 06:27:17 pm
content: This TDS460 is build in years 1994. Since this time is SMD eletrolytics issuses solved.SPC fails, have you checking all attentuator step and all four channel. I have a TDS420A with SPC fail. one range of input is dead. (overvoltage )..You can look, what is difference between TDS420 and TDS460Input PCB, few capacitor on signal trace, remove it.Acqusisitionsboard, ID-resistor on edge of PCB, near 40pol connector. change this few resistor.Result: TDS420 start as TDS460 with verified bandwidth of 350Mhz.greetingsmatt

author: Smoky
author_rank: Frequent Contributor
timestamp: June 19, 2020, 06:54:11 pm
content: Quote from: madao on June 19, 2020, 06:27:17 pmMatt, I may try that trick on one of the two TDS420's that I haveAs for the leaking capacitors, the damage on the Front Panel Board of this TDS460 was indeed caused by electrolyte. The other boards also showed signs of leakage too but with little to no damage.I can tell that this TDS460 was ordered as a rack-mount unit. There is no markings whatsoever around the handle mount area.Wait until you see the next few pictures of the CRT circuit...

author: Smoky
author_rank: Frequent Contributor
timestamp: June 19, 2020, 07:15:55 pm
content: Quote from: CaptDon on June 19, 2020, 01:41:07 pmCaptDon, the Raleigh Hamfest was cancelled this year too, but I'm hoping the big Hamfest in Shelby, NC is still going to happen this Fall. It's an outdoor event so it just might happen.Anyway, the chassis of the TDS460 was stripped and dunked in the tub. She's squeaky-clean.What's interesting too is that Tektronix put a second serial number sticker under the Attenuator Board.Now on to the CRT. Take a look at the "burn-in" that's on the CRT from the TDS460! This thing was probably turned on 25 years ago and never shut offLook at the chip in the glass too:But don't fear, CaptDon to the rescue! I cleaned his CRT last week and put it in a box thinking I would never use it again.We're turning the corner and now it's time to re-build the SMPS

author: james_s
author_rank: Super Contributor
timestamp: June 20, 2020, 02:47:07 am
content: That burn doesn't look bad at all, I have one with significantly darker burn and you don't really notice it when it's turned on.I forget if the non-A models even have a spot for a floppy. I have a TDS410A with no floppy, it has a FPC connector for it on the CPU board so I tried connecting a random slim floppy drive that came from something else. It fit but was not recognized, even after trying all of the option enable commands. I don't know whether it requires a special drive to be compatible or if there's something else that has to be done.

author: Smoky
author_rank: Frequent Contributor
timestamp: June 20, 2020, 06:16:38 am
content: The power supply is done. A thorough cleaning, all new premium capacitors, a new Vishay Optocoupler, new LM339N and LM324N IC's, and a re-flowing of the solder joints. The next step in the project is the re-capping of the Attenuator board

author: madao
author_rank: Frequent Contributor
timestamp: June 20, 2020, 06:37:29 am
content: Quote from: james_s on June 20, 2020, 02:47:07 amDrive is Teac FD-05C, not special.You can buy gotek SRF1M44-DU26 (important DU26 Version) and remove two jumper (JC and JE ), it runs.Hack to TDS460.Here two attached pictures: red circle show difference between 150 Mhz and 350 Mhz modell.( I am glad burned TDS460 acqusition board is not mine)(Input PCB came from TDS430A, but you'll understanding, where is other capacitor of input 3 & 4. ;-) )EDIT: it is odd, pictures is replaced, but i have problem with uploading of one pictures...try...try...try...success ..

author: james_s
author_rank: Super Contributor
timestamp: June 20, 2020, 07:02:51 am
content: Oddly while the thumbnails look correct, when I actually click to view a larger photo I see pictures of a hard drive swap in a TDS5054?

author: Smoky
author_rank: Frequent Contributor
timestamp: June 20, 2020, 08:31:01 pm
content: I moved on to re-capping the Attenuator board since I seen signs of electrolyte damage. I found two SMD's that had corroded solder joints. I lifted the parts from the board, cleaned the pads, and soldered them back in place. I also found several traces with a layer of "blackened" conformal coating. That's an indicator that the electrolyte has crept under it and is doing its damage. I scrape those areas clean and follow that with a tinning process.The capacitors on the left are heat-shrunk and laid on their side since a ribbon cable passes over them.The Front Panel board was close to flawless. It came from CaptDon's TDS420. I removed the rotary switches and flushed them with a shot of DeoxIt D5. The board got a re-cap job and everything was put back together.The re-capping of the Acquisition board is also finished. I found no issues during the process.

author: Smoky
author_rank: Frequent Contributor
timestamp: June 21, 2020, 03:12:00 am
content: With only the Back-plane board left to be re-capped, I wanted to shift gears and bench-test the power supply ahead of time.I was told not to bring it up slowly using the Variac, but that doesn't seem to make sense to me, especially since it's just been re-capped. I'm guessing that it should be brought-up at a moderate pace?If all goes well, does it make sense to measure the low-voltage outputs for AC Ripple?I'm not sure if a maximum allowable specification was ever published for these things.One thing to mention, the two new 680uf 250v bulk capacitors were tested for leakage and value. They both measured very close to 595uf. I was just concerned that being lower in value that it may allow an increased amount of ripple to pass.What better time to check than while it's sitting here on the bench

author: james_s
author_rank: Super Contributor
timestamp: June 21, 2020, 04:32:28 am
content: Quote from: Smoky on June 21, 2020, 03:12:00 amI tried doing that with one of mine and it blew up, a power resistor in the primary side opened up and I think something else failed to. Listen to the advice with these, it's not wrong, don't try to bring the voltage up slowly, just make sure the input is fused appropriately and plug it in.

author: Smoky
author_rank: Frequent Contributor
timestamp: June 21, 2020, 04:44:38 am
content: Thank you James, the power supply livesI scrolled up the voltage pretty quick.All pins are at proper voltage too. Here's pin 8 (+15):Here's the power supply clipped into its cradle ready to be installed:

author: madao
author_rank: Frequent Contributor
timestamp: June 21, 2020, 05:12:05 am
content: Quote from: james_s on June 20, 2020, 07:02:51 amIt is really odd, i have replaced both pictures again, one pictures is correct.Othter pictures, Server refuse him and i need 4 try to uploading -> wrong pictures. Last try -> sucess.

author: Smoky
author_rank: Frequent Contributor
timestamp: June 21, 2020, 05:43:56 am
content: Madao, check this out.I have two spare TDS420 parts boards.The two bottom boards in the picture are from TDS420's.The top board is the TDS460 that I'm fixing*Madao, Here is a fourth board!It's my TDS420 with FFT and it also has the Centronics/RS-232 option card installed:

author: james_s
author_rank: Super Contributor
timestamp: June 21, 2020, 05:45:53 am
content: Quote from: madao on June 21, 2020, 05:12:05 amStrange, well that worked, I'm able to see the correct pictures now.So just remove those capacitors? Does the scope need to be calibrated or will it increase the bandwidth just removing them?

author: Smoky
author_rank: Frequent Contributor
timestamp: June 21, 2020, 06:18:39 am
content: Check this out too. Here are shots of the Attenuator boards from a TDS420 and TDS460.The SMD's are symmetric around the IC's.Notice that a green SMD marked "000" appears on the TDS460 Attenuator board and the white SMD's are missing.TDS420:TDS460:

author: Smoky
author_rank: Frequent Contributor
timestamp: June 21, 2020, 07:20:30 am
content: While I had these Acquisition boards stacked, I started looking for more of these green "000" SMD's.I found them in the area near where the ribbon connector from the Option Cards comes in.All I can say is, this TDS460 that I'm working on does not have a Centronics/RS-232 Card installed. Blank plates/filers are installed.It would be cool to find out what is being diverted/deleted/routed by these green things

author: madao
author_rank: Frequent Contributor
timestamp: June 21, 2020, 07:25:18 am
content: Quote from: james_s on June 21, 2020, 05:45:53 amYes, capacitor is bandwidth-brake. Calibration is not neccesary, a luck! Inaccuary of amplitude by high frequency is possible.0-Ohm Resistor on PCB edge is just a jumper, or "ID-Resistor" (ID = indentify )Other 0 ohm Resistor on 40pin connector, probably a ID-Resistor for memory size. I haven't verified their function.I have picked your pic and drawing few circle. Red circle is Identify of model (TDS420 or TDS460 ), yellow is possible ID for memory / Option .. it is already not verified. I have compare it with my pictures.

author: Smoky
author_rank: Frequent Contributor
timestamp: June 21, 2020, 07:44:57 am
content: This is good stuff!I own two restored TDS420's. One with FFT (2F is printed on the serial number sticker). The other has no options other than the Centronics/RS-232 card.I may slip the covers off to see what is present in the corner of their Acquisition boardsThe re-capping of the Back-plane board is now behind me! Thanks to Hakko too. I don't use the de-soldering tool very often, but when the circuit board is clean and it's through-hole parts are in need of removal, it can't be beat:What's left on the list to be done is the replacement of the CPU board battery. After that, it's assembly time.At this point, I can only hope I caught everything that needed fixing. The seller of the scope said that the TDS460 fails its initial self-tests and the Signal Path Compensation routine. I didn't power it up when I got it, I headed straight for the screwdriver!*I went ahead and set the SMPS up on the bench to measure the amount of ripple on the +15 rail (pin 8 & 9). I used a 1x Tektronix P6101 probe and a TDS420 oscilloscope. Here's how the waveform looked DC coupled:I then switched the scope to AC coupled. I needed to activate the "zoom" function to get a better look at the ripple:The waveform would show a spike about every second which is at 60 cycles/Hz causing the whole waveform to bounce a little. Is that spike normal and just due to the switching action of the power supply? Anyway, the +15 VDC rail is rock solid with no fluctuation when measured on the DMM.Anywho, back together she goes!

author: andy3055
author_rank: Super Contributor
timestamp: June 22, 2020, 04:46:02 pm
content: Looks promising!

author: Smoky
author_rank: Frequent Contributor
timestamp: June 23, 2020, 12:23:02 am
content: Thanks Andy!I would classify this particular TDS460 as their "base" model. It's not loaded with any of the extra options like FFT, RS-232/Centronics, etc. I detail these scopes to the max! From the inspection of every trace down to the polishing of every screw. I take great pride working on this American-made equipment!Ultimately, my goal was to have one extra TDS oscilloscope for "high-voltage" measurements, but now I have three! Last year, my Sencore SC-61 oscilloscope kicked the bucket. With the Sencore 10x Lo Cap probes, I could measure up to 3KVDC. For some reason, the digital read-out on the SC-61 started flashing, and without warning, it stopped working all together.Since my passion is working on mono tube amplifiers, I was put out of commission. Here's one of two Six Pac mono audio amplifiers that I recently restored:Two weeks ago, I made an offer on a brand new Tektronix probe. It's a P5100. What was a plus too was that the cover of the package had the same color scheme printed on these older TDS scope's users and service manual binders. Anywho, the seller accepted my offer:The P5100 is rated up to 2500VDC and it's not big and clunky either. I'm going to use this probe with one of the TDS420's on a separate bench in my garage. I also own a P6009 100x probe that is good to 1500VDC. I'm also going to buy a nice differential probe package too as I learn more about this electronics stuffHere's a look at my next mono tube amplifier repair project. Cary Audio CAD 211 FE's. The B+ averages around 1380VDC in these things. I traded labor for them a few years back

author: andy3055
author_rank: Super Contributor
timestamp: June 23, 2020, 01:12:04 am
content: You seem to spend a lot of time on these. I used to do the same when I was younger. But since moving to the US, I have not done much. Most people I know have the more modern stuff. The only person I know who has a hybrid amp is one of work friends who bought an Ampeg SVT 4 last year from a swapmeet. That is the one I fixeed for him and busted the scope by leaving it on overnight accidentally. I just finished fixing the scope a few days back.

author: andy3055
author_rank: Super Contributor
timestamp: June 23, 2020, 01:29:38 am
content: BTW, what did you use to clean the boards in that scope?

author: Smoky
author_rank: Frequent Contributor
timestamp: June 23, 2020, 01:57:59 am
content: It all started when a professor from NC State gave me his non-working TDS420 three years ago!I have a ton of classic audio equipment and I needed to learn how to use an oscilloscope. So what better way to learn electronics than to fix the oscilloscope you intend to learn fromI scrub the PC boards with Dawn dish-washing detergent and used an air compressor and a heat gun to blow them dry.The #1 problem that I have found while restoring these three scopes is the way the traces will rot under the SMD's when the leaking electrolyte streams down the board.If you find a crusty solder joint, scrape it down to the pad. Flux it and let it boil-off. Clean with alcohol and inspect it. Chances are, the pad has lost continuity. Tin/repair.Btw, here's a P6101 probe attached to the probe compensation post on the rebuilt TDS460:I also put a 30MHz square wave into it from the Siglent waveform generator:...and here are the three Tektronix scopes that have been restored. Two TDS420's and one TDS460Here's a test that I performed on the three scopes at the same time. All were warmed up and SPC was executed. I limited bandwidth on all three scopes to 20mHz and 20.0 MS/s. The Siglent waveform generator was set to a Sine wave of 15Hz at 2Vrms.All three scopes read 15Hz on all 4 of their channels. The frequency would bounce around slightly but the center measurement was 15Hz.The voltages:TDS420, Serial #B021008Ch 1, 1.980Ch 2, 1.982Ch 3, 1.982Ch 4, 1.982TDS420, Serial #B020953Ch 1, 2.004Ch 2, 2.004Ch 3, 2.004Ch 4, 2.000TDS460, Serial #B021375Ch 1, 2.002Ch 2, 2.000Ch 3, 2.008Ch 4, 2.008

author: andy3055
author_rank: Super Contributor
timestamp: June 23, 2020, 03:31:16 am
content: Thanks for sharing the story. Very nice work!

author: james_s
author_rank: Super Contributor
timestamp: June 23, 2020, 04:20:52 am
content: Quote from: Smoky on June 23, 2020, 12:23:02 amNice to see I'm not the only one who does that. I've restored about half a dozen Tek DSOs of various series, they always clean up real nice. I'm a big fan of the older Tek scopes, the TDS700 and TDS3000 are my favorites. One of these days I still need to get out my TDS460 again and see if I can solve the SPC failure, that's the only remaining issue it has.