Need Tek TDS520 repair help

[TheD312]

Hi guys,

First of all sorry if I make language mistakes, I`m german 


So rescently I bought myself an used Tektronix TDS520 Oscilloscope sold as defective. I bought that nice device and today the german Post delivered to me.

I switched it on and the "Diagnostic Log" popped up showing all the errors. The things that failed were the "Aquesition Processor Interface" and the "Acuesition" as it was showing the FAIL next to it.

Using good ol`google a day before it came delivered to me my first plan was that the capacitors were old and had leaked out.

I took the cover off the scope and realized that the capacitory were actually replaced by somebody else and were in perfect condition.

So I started to randomly check for Voltages on the A10 aquesition board and found out that there was Voltage missing on like 10 electrolytic capacitors that I marked in the photo with red circles.
I used the beeper on my multimeter to trace where these capacitors were connected to the ribbon cables (Yes all caps were in parallel) and found out that all of them were connected to the power supply with some wires of the ribbon cable.

Then I actually had no schematic or service manual ad all.

So I started to take things apart, disconnected all the Wires of the PCBs and took a look at the PSU board. I googled its part number online (It`s a "zytek 22904701") and found a schematic of it with i could then measure more stuff on the PSU board. I came to the conclusion that nothing with the power supply was wrong.

While actually looking for the PSU schematic I found a service manual for the TDS520 (more below).

With my hope that the PSU was intact I started to put things back together and tried to power the scpe up.

There obviously was the same problem.

So having the service manual I started checking different voltages of J26 and J27 of the A11 board as the voltages were listed in the service manual. All of these voltages were fine, maybe a little bit out but still in the listed tollerance range.

I´m prette sure it is something with those missing voltages.

The little 7 segment display is just showing one bar (also see pictures).

I have all the basic tools like Multimeters, Function gen, PSU, JBC Soldering and Hot air station so I think a mechanical repair should be no problem. I do not have a second oscilloscope. I have the service manual and the PSU schematics.


I would kindly apprechiate if there is some Tek specialist here and can help me with the repair since this to me is still a nice scope.

Thanks for reading this long story 

[TheD312]

Looks like the voltage rails are actually there. My mistake 

However I will still try to replace the caps, maybe it actually are those who are defective.

[Daxxin]

this kind of scope series usually need to recap all board smd capacitors due to leakeage , included the front panel and wash well the boards with aggressive
clearner then dry with oven ..without burn them  , then if you lucky that no corroded or corrupted pcb traces it will work again , look at youtube
expaining how to remove easily old caps without damages ..pinch method.

1) remove all caps
2)wash board with green solution , rinse with water , wash with isopropil then dry
3) check all traces visually and ohm meter
4)recap and try if still not working test all opamps sometimes also them fails

[Armadillo]

Watch this video;



and how to enable the FFT;

[Kilo Tango]

Google is your friend.

A search for "TDS520B service manual inc schematic" brings up http://bee.mif.pg.gda.pl/ciasteczkowypotwor/Tek/ds520b-cm.pdf which is the whole service manual, including the circuit diagram. It is kind of the only complete one out there for the TDS 520 but the other models 540, 6xx models tend to be similar in working.

Read this and have fun.

Best of luck

Ken

[Daxxin]

Cant remember the link but if you google can find also schematic of acquisition board of 540-544

[TheD312]

@Daxxin

I have ordered all the Electrolytic SMD capacitors and soon will replace them. Also the board will be washed with alcohol.
I actually found like 4 lifted or broken/ damaged pads to which the caps were soldered on.
I assume some idiot replaced these capacitors with his harbour freight fixed temperature "hot stick" at 520°C 
I already found some damaged pads and vias which I repaired. The problem still ramins though.
Yes there are schematics for the 540/ 544 but the A10 board in the 540 is completly different than the 520 one.


@Armadillo

I watched these videos about three times 


@Kilo Tango

Thanks for the link, I already found this schematic before and took a peak at it. It looks very promising but I noticed that the A10 Aquisition board of the TDS520B CM is different than mine, the layout and number of chips is different.


One other thing that makes me think is that Dallas SRAM module. I checked the date and time in the Scope and it was at Feb 2 2016 so my guess would be that the Dallas IC stopped working exactly back at that day.

The previous owner told me (He`s a HAM) that someone replaced the caps about one year ago for him, I can`t quite imagine that these caps only lived for one year, exept those were the finest aliexpress has to offer 
But that could indeed be possible if those caps are surrounded by 100°C hot chips.

So let me wait for the caps to come then I will replace those, clea the board and see how to move on.

[Kilo Tango]

Cant remember the link but if you google can find also schematic of acquisition board of 540-544

http://hakanh.com/dl/kits.htm

looks pretty helpful,

Ken

[TheD312]

Thanks, I already saw this Manual link from the TDS540 reair Video on Youtube.

Currently I`m not planning to get rid of 10$ for the schemtaics 

[TheD312]

Ok so I Cleaned and replaced all capacitors on the A10 Aquesition board, but both fails still show up.

Now I really don`t know what to do. Does anyone think replacing the Dallas module could hep?

[Daxxin]

Did you check all voltage at all opamps? just one trace broken can give big trouble ..post some video for better help ..replacing the module does not help with acq board

[TheD312]

Yes, I´ve measured the voltages of all DIP Packages I could find on the A10 board. They are all there and seem to be ok except for one Rail connected to an opamp that was 13,8v instead 15, bit low but I think that shouldn´t matter.

I also found out that there is two tantalum capacitors without any voltages on them (See those 3.3µF 25v Caps?), I am not sure if that is normal...

I also uploaded the diagnostic log results, maybe that could help someone here.

[TheD312]

So now what I have done is I reflowed all DIP SMD ICs on the a10 board (desoldering with hot- air and then soldering back on) looking for and cleaning corrosion.

And this thing really pisses me off. Now its even showing me 3 Errors not 2. I am going to sell this sucker on ebay....

[Daxxin]

Well , not good idea to desolder and resolder better not you did..  i can view only 33uf not 3.3