EEVblog #1185 - Dumpster Tek TDS540D Scope

[jnissen]

I've carefully Dremeled away the plastic backing on similar Dallas parts (Real time clocks) and managed to find the battery and connections inside. I never tried to keep it powered on when I did this but it should be possible to parallel a new battery and then trim out the old one. I ended up using Lithium A123 type batteries and should be good for another 20 years! Saved an old Tek Logic analyzer that was dead otherwise.

BTW - Yes it takes time and patience but you can chip away the epoxy in there with a soldering iron. Heat will break it down fairly easily.

[induzer]

Thankfully, the 500D/600D/700D models store the CAL constants in two I2C-EEPROMs, placed on the aquisition board right near the fan.

Of course one of mine was brick and the calibration process is quite a journey ... 

I did not fully explore the dallas yet, but the most reasonable seems to be the installed options. Which are alterable via GPIB  (forum knows)

Dave, the Dallas nVSRAMs routinely last about 20 years so those are close to the end of their life.  Before getting too involved in repairing the display, know that the calibration data is stored in the nVSRAMs and the calibration procedures are very involved requiring special equipment and software.

Replacing the nVSRAMs is not too difficult however I have not successfully been able to read a nVSRAM which is that old.  If you try this, I suggest testing your read and write procedures on an expendable nVSRAM of the same type before removing the ones inside the instrument.

-------------------------------------------

About the color update - it takes patience and some evenings finding some hi-res PCB pics of the CPU board.

https://www.eevblog.com/forum/repair/conversion-tektronix-tds500-to-tds700-color-oscilloscope/msg2231910/#msg2231910

greetings, induzer

[free_electron]

On the monitor. Check out the little electrolytic cap next to the heatsink , ( sort of underneath the neck of the tube ) that is most likely the culprit.
The monitor works, just the deflection is compressed.

On the 'color'. These scopes uses a monochrome tube with a color LCD 'Shutter' in front of it.
They write an image with the R shutter enabled, to show the red color , one with the B , one with the G. so the colors are time multiplexed. that is why your VGA output looks weird in that test mode. you see the individual planes of the color.

Even if you were able to engage color on this scope your VGA output will be monochrome.

For their age these scopes had very high video resolution. They couldn't make small picture tubes that could produce such high detail ( the shadowmask openings would have to be very small ) so they went with a monochrome tube and switchable color filter ( look up  lcd shutter )

on many o fthe color versions of these scope the shutters went bad , or the filling material between tube and shutter ( a kind of gel) started 'rotting' ...
many many many of these scopes were scrapped due to that. if the shutters went bad all colors went wonky.



someone made an fpga based 'converter from tektronix  to lcd
https://forum.sparkfun.com/viewtopic.php?f=14&t=23147

http://jvgavila.com/tds524a.htm

The TEK forum has lots of repair information for these beasts
https://forum.tek.com/viewtopic.php?t=138172

and of course the eevblog itself
https://www.eevblog.com/forum/repair/conversion-tektronix-tds500-to-tds700-color-oscilloscope/

i fixed a number of those beasts. 'in the old days' ( like 5 years ago . don;t have time for repairs ainymore)

[digik]

Dave, i know it's a little bit off-topic but can you tell me what's the brand of that blue screwdriver @ 13:00 on the left side of the oscilloscope? I know that you use only high quality tools, so those must be very good ones. Also can you make a review (and possibly a comparison) between those wera, pb tools and wiha screwdrivers? it would be a nice video

[EEVblog]

[EEVblog]

Dave, i know it's a little bit off-topic but can you tell me what's the brand of that blue screwdriver @ 13:00 on the left side of the oscilloscope?
I know that you use only high quality tools, so those must be very good ones. Also can you make a review (and possibly a comparison) between those wera, pb tools and wiha screwdrivers? it would be a nice video

LOL< it's just a no-name cheapie. Wera, PB, and Wiha are all excellent, chose whatever one you like.

[EEVblog]

Even if you were able to engage color on this scope your VGA output will be monochrome.

Apparently the mod actually changes your scope to the 700 series colour version and the VGA works accordingly.

[David Hess]

Thankfully, the 500D/600D/700D models store the CAL constants in two I2C-EEPROMs, placed on the aquisition board right near the fan.

Oh, do they?  That is different from the earlier TDS models which I am more familiar with and a godsend.  Storing almost unreplaceable calibration constants in lithium backed up SRAM should be a sin.

For their age these scopes had very high video resolution. They couldn't make small picture tubes that could produce such high detail ( the shadowmask openings would have to be very small ) so they went with a monochrome tube and switchable color filter ( look up  lcd shutter )

These oscilloscopes still have very high resolution and I do not think color CRTs ever achieved this kind of resolution so there was some justification for the liquid crystal shutter technology.  Vector CRTs can have even higher resolution and Tektronix first used their liquid crystal shutter technology there.  HP used a custom raster scan black and white CRT on some of their early DSOs which had doubled horizontal resolution.

[EEVblog]

On the monitor. Check out the little electrolytic cap next to the heatsink , ( sort of underneath the neck of the tube ) that is most likely the culprit.

Nope. 1uF 350V measured 5ohms at 100kHz

[free_electron]

these are the suspected caps.

The horizontal sweep is compressed. These caps heat up due to the proximity of the heatsink.
Many older crt monitors in scopes suffer form that problem.  they may measure good but replace them . under dc bias their capacitance becomes snot

[snoopy]

I have attached the schematic of the monitor section for a TDS544A although that would be the color version I believe ?

[Shmidtey]

these are the suspected caps.

The horizontal sweep is compressed. These caps heat up due to the proximity of the heatsink.
Many older crt monitors in scopes suffer form that problem.  they may measure good but replace them . under dc bias their capacitance becomes snot

I've got a TDS 744A that seems compressed in the bottom of the vertical but to a less extreme degree, so i'm following along at home lol.
 

I have attached the schematic of the monitor section for a TDS544A although that would be the color version I believe ?

Ooh... I wonder if the monitor section is the same on my 744A...

[nctnico]

Also: I had a TDS540 for a short while too, and its CRT driver board went bad in much the same way. I tried replacing every cap on the board with no luck, and eventually replaced the whole board with a donor. I think the flyback transformer had failed, and they're unobtainium.

In my experience with this line of scopes it usually is the flyback transformer. Getting a working board or do an LCD conversion are the only options.

@Shmidtey: your scope seems to have a power supply problem on the vertical amplifier. This should be reasonably easy to fix.

[snoopy]

Also: I had a TDS540 for a short while too, and its CRT driver board went bad in much the same way. I tried replacing every cap on the board with no luck, and eventually replaced the whole board with a donor. I think the flyback transformer had failed, and they're unobtainium.

In my experience with this line of scopes it usually is the flyback transformer. Getting a working board or do an LCD conversion are the only options.

@Shmidtey: your scope seems to have a power supply problem on the vertical amplifier. This should be reasonably easy to fix.

Or a shorted turn on the horizontal winding of the yoke but more likely the line output transformer.

cheers

[BU508A]

If you are going to repair a CRT, the first things I always check are:

- is the heating working? (orange glow at the socket)
- is the high voltage working? (testing this with the tiny hairs on my forearm near the CRT front. If there is HV there, you can sense this)

If both tests are ok, then I'm going to trace the signals starting with the video output stage.
This can be easily spotted without the schematics at hand.
Checking the powerrail for the video output stage goes along.

If one of the first checks fail, I'll check next the powerrails.
In this case, all these measurements are annoying, but soldering some
wires at the right points should do the trick.

Good luck with the repair.

[free_electron]

check the circled caps , especially the double circled one. the deflection  current goes through that one.
there is image on the screen during startup which means HV is ok and vertical is ok. it is just a very narrow horizontal sliver ,like 1 cm wide. eventually the protection circuits will kick in and turn off the display.

also : check those cermet trimmers , just move it a tiny little bit. it wouldn't be the first time a cermet goes open at its wiper.

[free_electron]

on a sidenote : does anyone know what CAD system was used to make those schematics.
Before you scream OrCad : nope. And not Mentor or Cadence either.

This is a style i have encountered a number of times but never been able to figure out.
I suspect it is either Vanguard (Teradyne) , Daisy/Dazix or Intergraph / Bentley

[nctnico]

someone made an fpga based 'converter from tektronix  to lcd
https://forum.sparkfun.com/viewtopic.php?f=14&t=23147

That would be me. 

[IanMacdonald]

I'd take a stab at a break in the horiz scan coil feed. The S-correction cap (probably that white 1.33uF 200v job) is a prime suspect.  (With reduced scan current the EHT also reduces, giving a faint and distorted display)

[omerd]

The flyback transformer on this scope series is prone to failure. Sometimes it will work OK when cold and when it warms up the image will "compress" or do funny things.
In my case (540B) the CRT driver board also became hungry for current and I noticed the driving transistor and LDO getting toasty after a while...
I decided to replace it with a modern LCD, keeping it B&W for now (maybe one day I will order those video memory chips), and found this one on eBay: https://www.ebay.com/itm/LCD-VGA-2AV-Controllers-with-Remote-Control-7inch-800x600-LCD-Display-A070SN01-/263388878055?hash=item3d533354e7
It was just the right size and supported VGA. Only thing I had to do was to machine a little back panel to mount it on and drop in a little DC-DC converter (the supply voltage for the original board was around 20-24 volts if I am not wrong).
This was more of a machining job, and I was really happy to get this scope up and running again
Photos of the process here: https://www.facebook.com/media/set/?set=a.10210258887748277&type=1&l=97875849c6
Omer

[MT]

on a sidenote : does anyone know what CAD system was used to make those schematics.
Before you scream OrCad : nope. And not Mentor or Cadence either.

This is a style i have encountered a number of times but never been able to figure out.
I suspect it is either Vanguard (Teradyne) , Daisy/Dazix or Intergraph / Bentley

Supermax Ecad (bought by mentor in 2005) i guess any old Unix system cad.

[snoopy]

I still miss equivalent time sampling. Not sure if anyone's doing that any more

There is a lot to be said for 100 GSamples/second or higher to remove all doubt about what the DSO is showing but if I am reading the specifications right, the TDS500D series is real time only and the TDS700D series is needed to support equivalent time sampling.

Some high end DSOs still support equivalent time sampling but it requires extra hardware outside of the vertical signal chain and digitizer which is not amendable to integration unlike faster digitizers so ETS became relatively more expensive as Moore's Law marched on.  Implementing ETS after the ADC is a contradiction.

Update:

The tear-down video seems to show that ETS is supported but the documentation says otherwise.  I wonder what the story is with that.

All of the 500D scopes can do ETS. The one that Dave has can do up to 100 Gs/s whilst real time sampling is limited to 2Gs/s on 1 and 2 channels and 1Gs/s on all 4 channels.

cheers

[nctnico]

AFAIK all of the 500 series can do ETS. The TDS510A I had could do it.

[David Hess]

I have always assumed that all of these oscilloscopes support ETS but the manual implies otherwise.  I only checked it because I was going to calculate the ETS sample rate if it was not directly specified.  My personal experience is with the lower performance TDS400 series which support ETS.

Note that the TDS600 series is more specialized with an independent CCD based digitizer for every channel (like the earlier 2440 series) so the sample rate is not divided between active channels.  CCD based digitizers were competitive with interleaved ADCs for a long time.

Range, Interpolated Waveform Rate (2)(3):

TDS 600C: 10 GSamples/sec to 250 GSamples/sec
TDS 694C: 10 GSamples/sec to 500 GSamples/sec
TDS 520D, 540D, 714L, 724D, and 754D: 1 GSample/sec to 100 GSamples/sec
TDS 580D, 784D, and 794D: 2 GSamples/sec to 250 GSamples/sec

(2) The range of waveform rates for interpolated (or equivalent-time on the TDS 700D) waveform records.

(3) The Waveform Rate (WR) is the equivalent sample rate of a waveform record. For a waveform record acquired by real-time sampling of a single acquisition, the waveform rate is the same as the real-time sample rate; for a waveform created by interpolation of real-time samples from a single acquisition or, on applicable products, the equivalent-time sampling of multiple acquisitions, the waveform rate created is faster than the real time sample rate. For all these cases, the waveform rate is 1/(Waveform Interval) for the waveform record, where the waveform interval (WI) is the time between the samples in the waveform record.