TDS460 SPC failure errors

[james_s]

I've got this TDS460 mostly working finally, power supply is solid, replaced all the leaky capacitors on the acquisition and attenuator boards, the scope powers up and passes all the self tests, all four channels work, overall it's starting to look pretty good.

Unfortunately there is one glaring flaw remaining, SPC fails every time with the same two error messages:

EXECUTION ERROR Characterized values out of bounds Vertical

SEVERE EXECUTION ERROR 2260 Calibration failed Triggers

Is anyone familiar with these specific errors or have ideas on narrowing things down? When I got my TDS700 it was failing SPC most of the time due to faulty relays in the attenuators but I don't remember what the specific fault codes were. That one would occasionally pass SPC but this I tried running it about 10 times and it failed every time with the same two errors. Even if I could just narrow it to one specific board or better yet a specific channel on a specific board that would give me something to go on. The corrosion was worse on the the acquisition board, I've cleaned it up pretty well but there is still some corroded solder on a handful of passives. I don't want to just blindly shotgun stuff and risk introducing new problems.

[shakalnokturn]

I don't know the 400 series well other than their PSU.
What we does the acquisition board actually contain?

Other than SPC fails, do you have any on-screen symptom such as an offset when inputs are grounded and offsets adjusted to 0V?
How about amplitude does the display conform with a given DC input?

[james_s]

There does appear to be a slight offset however I have  seen that symptom on other TDS scopes when SPC had not been run. Also I noticed when measuring a sinewave the RMS voltage on the readout was about 10% high. I have not had a chance to do extensive testing yet but I didn't notice any difference between the channels.

The acquisition board is similar to other DSOs, it contains ADCs, ASICs, SRAM and triggering, all the stuff to sample the analog signal from the attenuator and load it into RAM. Unfortunately there is no schematic for any of the boards, something I find inexcusable for a piece of professional test equipment. HP was still providing full schematics in this era.

[madao]

Pleas check  Trigger,  does run trigger at all mode (include Logic & Glitch-Trigger)

If  one trigger-function is dead -> SPC fails. Troubleshooting is not easy, yes ! :-(

I have seeing it at  TDS520. (Glitch-Trigger bad)

Greetings
Matt

[james_s]

I'll have a look, I set it aside temporarily to work on other projects but I'll likely get motivated to pull it out again sometime soon and have another poke at it.

Is it possible to get any more detailed information on what failed? I noticed there's an internal serial console that is aimed at debugging situations where the display is not working, can it give me any additional detail on the SPC failures? I find it annoying that all of the self tests pass with flying colors, and the troubleshooting charts in the service manual all end with "The scope is ok" when clearly it is not.

[madao]

Check, Trigger works at all channel and both edge, then check  line-trigger, ext-trigger(?)  , glitch & pulse (complex stuff) trigger must be checked.  "
SEVERE EXECUTION ERROR 2260 Calibration failed Triggers" tell   channel SPC ist success but not with trigger-stuff.

My  TDS520 has  failed SPC, because  glitch-trigger is bad.  (setting value is  not matching with  result, if it is over 1µs )
Repair success; yes, but  it is now a TDS540 with TDS540A acquisition board.

greeting
matt

[Smoky]

James, I think I have one of many places for you to look And to all of you TDS420 and TDS460 lovers, here's another juicy tidbit of the typical "cap" damage I'm sure you have encountered once or twice or, likely, more! Anyway, it's a rainy day in Raleigh but a perfect day to do electronics repair!

I watched a Tektronix Company video and it stated that if "SPC" fails it's a "hardware" problem. I'm not sure if that is always the case or not, but that's what the video stated.

I happen to have a spare attenuator board from a TDS420 which, I imagine, is similar to a TDS460. Its part number is 671-1680-05. I focused in on the area of the group of four 33uf 10v capacitors that are prone to leak:




So I removed the old capacitors and cleaned their pads. I did continuity tests on all of their traces and I got a thumbs-up. I next turned to the resistors and tantalum capacitors that are nearby.

To the right of the 14051B Analog Multiplexer/Demultiplexer I found resistor 5621 not "connecting" to its pass-through, so I removed it:

          

I then tested the resistor, and it is at 5.63 kOhms:



But the funny thing was, I wasn't getting any continuity going to the left trace that sneaks under the 14051B either, so something is "open" in that direction too.

I also noticed that pin "7" on the 14051B looked burnt:



So, off goes the 14051B




What a funky-mess under that chip! And sure enough, that resistor takes a bee-line straight to pin 7! So I cleaned up the area and started to do continuity tests. Btw, look at all of the pass-throughs under that small part 





The two red dots in the lower picture are where there are breaks in the traces right at the pads. All of the remaining traces and pass-throughs pass their continuity tests.

I looked up what a 14051B is and the identity of pin "7":




So James, there's one place to look

One other thing, I read that the 33uf 10v capacitors used with the LM2940CT voltage regulator on the attenuator board needs to have an ESR no greater than 1.2 Ohms based on its current.  The capacitors used with the LM2990T should be under 10 Ohms. Question is, which of the four capacitors filters the LM2940CT???

I looked for a low ESR cap on Mouser and found this polymer one:

https://www.mouser.com/ProductDetail/Nichicon/PCS1A330MCL1GS?qs=sGAEpiMZZMsIwzbKW1rlgcLka6aSC%252BMP6mMfrKeBnCE%3D

Scratch that! I just read down the spec sheet of the LM2940CT!




So I see why Tektronix used tantalum capacitors in tandem with the standard electrolytic SMD capacitors

Anyway, I plan to practice repairing the rest of the board and then trying it out in my working TDS420 in the future. I imagine that the 14051B chips are still available?


*Also*, the TDS420/460 Quick Reference Guide I sent to Ebaman.com is now available for download.

[james_s]

Cool, yes I do indeed have some very similar damage on mine, this definitely warrants a closer look. It's not so much that I love the TDS400 instruments as I really hate to let something get the better of me. I've come this far and can't give up now or it will annoy me forever.

For a guy claiming to not know much about electronics you've been doing some pretty advanced repair and restoration work.

[james_s]

The lack of schematics for these is super annoying though. I find it absolutely inexcusible that a company like Tek stopped supplying schematics. I have some HP gear of a similar era that contains full schematics in the manual.

[Smoky]

Here's another topic that I think may relate.

Here is a picture of a "Backplane" board within an actual TDS460. The owner started to have problems with the scope not passing self-tests because of leaking "Marcon" brand capacitors:





Here is my spare spare Backplane board. It doesn't have the "Marcon" brand capacitors installed on it. They are Nichicon and Nippon Chemi-Con caps:




And that white ribbon cable plugs right into the attenuator board.

I plan to rebuild both of the boards that I own to be safe too.

These scope repairs are fun when I have some down time.

...I don't work when it rains

[james_s]

Shoot I forgot there were even capacitors on the backplane. I think those are ok so far, however I had capacitors leak in the power supply so I think it's worth replacing the backplane caps as well.

[shakalnokturn]

True it isn't an easy task without schematics, it forces one to look all over the place rather than probe to follow a specific suspect.
From my limited experience with the TDS500/600 series I would expect your trouble to be somewhere around an analog MUX or quad OpAmp, one of those that scatters the various control voltages from the common DAC.