Tek TDS544A Busted

[lgbeno]

Hi Guys,

I dusted off my TDS 544A Scope which was decommissioned from a company that I used to work for in 2007.  it used to work but now it seems to have some issues. 

At first when I started it up, it asked me something about keeping my old probe configuration, I said no, clear the configuration thinking that I would just run the compensation again once it warmed up.

At boot the diagnostic log says that the Acq/Proc interface, Acquisition and Attn/acq interface tests have failed.

As I'm using the scope, looks like none of the channels are working properly, I test just by attaching the probe to the compensation posts and looking for the +0.5V 1khz suare wave.  I just get a flatline trace.

Might crack it open but just looking for some advice.

-Luke

[AndyC_772]

The early TDS5xx models are very well known for containing dozens of electrolytic capacitors which fail, leak, and deposit harmful electrolyte residue on the PCB. You'll find plenty of people who have had problems with these models over on the Tek forum at:

http://www1.tek.com/forum/viewforum.php?f=5

It's possible with these units to calibrate each channel to the probe's characteristics, but depending on the probe type that may or may not be worth doing. I believe Tek advise against bothering if you're using the P6139A probes that came with it. The fault you're seeing is unlikely to be related, though.

[lgbeno]

No signs of leaking caps, checked most of them and they have reasonable voltage across them.

Just a note, the error log give me the following info:

vertattenshiftreg bit#0fail
extTrigregdiag bit #7 fail
acqmemadata addr=7300000 fail
acqmemdata addr=7320000 fail

There are others as well.

[lgbeno]

Here's some pics of the error log and the post results

[nctnico]

This is a very common problem for these scopes. You'll need to replace all the SMD electrolitic caps on the processor, acquisition, RS232 and front panel. Your scope will probably work after replacing the caps. I just restored a TDS544A and used caps from Farnell (9696849  10uf/35V and 1868414 33uf/35V). Also give the boards a good cleaning with 'simple green' and alcohol using a toothbrush to get rid of the electrolite. The easiest way to remove the caps is using a hot air station like the Atten 858D.

The picture is probably what you'll find on the processor and acquisition board:

[lgbeno]

Did you have similar error codes as well? Sounds like something inexpensive to try.  I will definitely order up some caps.

looking at
http://www.digikey.com/product-detail/en/EDK106M035A9DAA/399-6666-1-ND/3082798

and
http://www.digikey.com/scripts/DKSearch/dksus.dll?Detail&itemSeq=124169466&uq=634924024489448003

-Luke

[nctnico]

Without changing the caps the errors are meaningless. Beware that changing the caps may take up to 8 hours of work. If you are unlucky the electrolite may have eaten away some vias as well.

[lgbeno]

Thanks for the tips, really the board looks pretty pristine inside with a nice matte green solder mask finish , a little dust but that is it.

Man those ac asics get toasty when the board is running.

Just ordered up the caps, will get them in a few days and give it a try.  I have the metcal solder tweezers for removal, should be a snap but yeah take a little time.  It will be worth it either way.

I'll be sure to reply with the result.  Reminds me of when my HP laser printer crapped out and the fix was to reflow the bga solder joints by baking the control board in the oven!

[nctnico]

You could try and remove one cap (be sure to add a little flux from a flux pen). If you smell 'cat pee' then it is leaking electrolite. If you remove the dust with compressed air and look at the board under different light angles you'll see that there are greasy spots around the capacitors.

[casinada]

Make sure you have a fan running if you start the scope with the case open. Some of the chips can overheat without proper cooling.

[daveyk]

This is a very common problem for these scopes. You'll need to replace all the SMD electrolitic caps on the processor, acquisition, RS232 and front panel. Your scope will probably work after replacing the caps. I just restored a TDS544A and used caps from Farnell (9696849  10uf/35V and 1868414 33uf/35V). Also give the boards a good cleaning with 'simple green' and alcohol using a toothbrush to get rid of the electrolite. The easiest way to remove the caps is using a hot air station like the Atten 858D.

The picture is probably what you'll find on the processor and acquisition board:

How well do the via's survive? In your pic it looks like corrosion goes right up to the via.


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[xwarp]

How well do the via's survive? In your pic it looks like corrosion goes right up to the via.


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My TDS460A had this cap issue, and, had instances where the components near the caps had the "frosting" on the exposed tin, but the via's did not have any issues. I suspect that I was either fortunate, or, the coating was good enough to protect them.

[daveyk]

How well do the via's survive? In your pic it looks like corrosion goes right up to the via.


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My TDS460A had this cap issue, and, had instances where the components near the caps had the "frosting" on the exposed tin, but the via's did not have any issues. I suspect that I was either fortunate, or, the coating was good enough to protect them.

I finished replacing all caps and still have same failure. My board does not have a huge amount of corrosion and only about a third of the caps were open or low in value .


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[nctnico]

The boards needs a thourough cleaning using simple green and alcohol. It may also be necessary to resolder/replace some components.

[daveyk]

The boards needs a thourough cleaning using simple green and alcohol. It may also be necessary to resolder/replace some components.

I've used some gentle flux remover and swabs around the caps that did leak. I'm not sure a full cleaning is warranted but I have used simple green on boards from bad environments covered in grease.

What's also weird is that after it's been on a while it power cycles. Across the caps I do read a good 5volts. There is very low resistance across that 5 volt rail. It could be it is drawing too much current and that thermal shutoff in the power supply is triggering. If someone has one apart, I would like a resistance reading across the caps on the 5volt rail.




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[daveyk]

The boards needs a thourough cleaning using simple green and alcohol. It may also be necessary to resolder/replace some components.

I think I knew what's wrong. I think the main -15v power supply is bad. With the acq board unhooked it is only -12.5. It may be around -9 with it hooked.

Now to find schematics. The service manual doesn't have them .

Dave


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[nctnico]

Use the TDS520B component level service manual for a hint on how the PSU works. AFAIK there are various versions of the PSU but the basic concept is the same.

[daveyk]

Use the TDS520B component level service manual for a hint on how the PSU works. AFAIK there are various versions of the PSU but the basic concept is the same.

Thank you! The power supply schematic look exact. Not to try to figure out how to power it up outside of the scope....  The -15v supply is pretty simple but I would want to t/s it properly. There is a 10uf cap at the control pin of the 317 regulator that I can read. Looks like it is not bulged; it may be leaking; hard to say; I'll just have to pull it.

I really want to figure out how to power this supply.


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[skennedy]

Schematics are available for the TDS544A. Down the bottom of this page: http://hakanh.com/dl/kits.htm

[daveyk]

Schematics are available for the TDS544A. Down the bottom of this page: http://hakanh.com/dl/kits.htm

Thank you!!! I searched and searched and couldn't find them.

I got it to boot now without errors, but channel 3and 4 won't compensate or works. It's as if their digitizes are not being switched on or the multiplexing isn't working right.

A lot of the problem was a bad -15 volt power supply. That was a bad and leaking 10uf cap on the main ps.

Then I had to clean the acq board with flux remover and getting the green cap leakage off some ic's. So far all op amps and chips have proper power, so I'm sure where to proceed from here.

Dave


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[andy2000]

It almost always ends up being an open trace, or via, but finding it can be difficult.  Start by checking around areas that only channels 3 and 4 have in common. 

For cleaning these, I've never found spot cleaning to be adequate.  The electrolyte gets everywhere, including under components.  I always wash the entire board thoroughly.  An ultrasonic cleaner would be ideal.

[nctnico]

AFAIK these scopes have components on their boards which cannot be cleaned using an ultrasonic cleaner so be careful!

[daveyk]

The boards needs a thourough cleaning using simple green and alcohol. It may also be necessary to resolder/replace some components.

Question: I scrubbed the acq board really well with Simple green and a toothbrush. I then rinsed it pretty well in the shower and thoroughly blew dry it with my air compressor with a dryer right before the air nozzle.

I do not have alcohol right now (well I do have Bacardi 150 and I ain't using that - lol). How important is the ISO alcohol?

Dave


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[james_s]

I used to use compressed air to dry boards but then I came across something that said to use only low pressure air because high pressure air can cause ESD buildup. I have not verified this to be the case but it seems something worth watching for.

[daveyk]

I used to use compressed air to dry boards but then I came across something that said to use only low pressure air because high pressure air can cause ESD buildup. I have not verified this to be the case but it seems something worth watching for.

Wow, never heard or thought about that.


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[james_s]

Now I forget where I saw the warning, I think it was actually in the service manual for the Tek TDS300 or 400 series. It had never occurred to me either, I'm not aware of having ever damaged a board that way but it's one of those things that would be hard to know for sure.

[daveyk]

It's back together. I had power supply problems before I could test. It was having problems turning on. Spent three hours on that. Finally changed a 220 uf near center of ps board. Now it turns on again. Any who.... with the scrubbed acq board in place. The traces are 300% better. They are all on screen and very close to a zero baseline.

The scope still will not pass signal path compensation. Ch3 looks noisy.

Also before with a signal trigger 50% was near the top of the signal on all channels. With the board scrubbed, 50% trigger is at 50% of the signals on all channels.

Error log still recording a whole bunch of errors while trying to compensate.


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