My $35 TDS460 Oscilloscope Repair Project

[Smoky]

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 nice 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.

This will be my last TDS400 series project, so I'll make it a good one 

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

[andy3055]

Waiting to follow your adventure. Good luck!

[Smoky]

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 found

The Display board, 671-1784-03, has one 33uf 10v cap on it. The pads look great.

We're on a roll 

[andy3055]

BTW, whatever happened to that amp repair where you were dealing with an STK module?

[Smoky]

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

[Smoky]

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 with the orange label 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 :

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!

[CaptDon]

Wish that out of all the 420/460 units one of them
was an 'A' model with the floppy drive option. I do
extensive R&D and QA/QC for my employer (You
know who they are) and it is very important to me
to be able to submit .jpg or .bmp screen shots as
well as .csv waveform captures that can either be
recreated on an AFG or imported to MatLab for
diagnostic chart creation and so forth. That was
one of the reasons the 420 ended up not being
repaired for our lab use. If it had the floppy they
may have actually bought a power supply and let
me repair the unit. I saved it from the dumpster
for about 1.5 years but lab space was getting
cramped. I am so glad it found a good home.
I have been a ham since 1973 and almost every
piece of gear I own was a dumpster dive. I am hoping
the Berea Hamfest doesn't get cancelled for 2020.
Dayton/Xenia and Butler Pa. were cancelled and
they are great fests normally. Don't know if the
Howard County Md fallfest is on or not.
Cheers mates!!

[madao]

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 TDS460
Input 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.


greetings
matt

[Smoky]

You can look, what is difference between  TDS420 and TDS460
Input 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.

Matt, I may try that trick on one of the two TDS420's that I have 

As 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...

[Smoky]

I am hoping the Berea Hamfest doesn't get cancelled for 2020.
Dayton/Xenia and Butler Pa. were cancelled and
they are great fests normally. Don't know if the
Howard County Md fallfest is on or not.
Cheers mates!!

CaptDon, 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 off   Look 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 

[james_s]

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.

[Smoky]

The power supply is done. All new capacitors, a thorough cleaning, and the re-flowing of the solder joints. The next step is the re-capping of the Attenuator board 

[madao]

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.

Drive 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 ..

[james_s]

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?

[Smoky]

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.

[Smoky]

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 

[james_s]

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?

I 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.

[Smoky]

Thank you James, the power supply lives 

I scrolled up the voltage pretty quick.

All pins are at proper voltage too. Here's pin 8 (+15).

[madao]

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?

It 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. 

[Smoky]

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.

[james_s]

It 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. 

Strange, 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?

[Smoky]

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.

Also, they use this "000" SMD on the three Acquisition boards above that I stacked. Are they just jumpers?

[Smoky]

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

[madao]

It 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. 

Strange, 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?

Yes,  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.

[Smoky]

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 boards 

The 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 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 it.

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? And is it the diodes that are causing it? Anyway, the +15 VDC rail is rock solid with no fluctuation when measured on the DMM.

Is this a situation where a soft-recovery diode would be beneficial/used?

Anyway, together she goes!