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TDS540 fault code

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Joined: October 17th, 2008, 12:15 am

TDS540 fault code

Post by Brimar » October 17th, 2008, 12:25 am

Hi all...

I have acquired a scrapped TDS540 which has become a bit of a restoration project.

It originally failed Acq/Proc Interface, Acquisition and Attn/Acq Interface.

Opening it up there was evidence of electrolyte leakage from and subsequent corrosion around the SMT electrolytic caps.

Replacing the SMT electrolytic caps on every board plus a thorough degrease and clean reduced the power on errors to just Fail ++ Acquisition.

The trigger threshold was not adjustable - always triggered at 0V.
This I traced to an open circuit via under a resistor (U1550 Pin 7 to U1552 pin 16)
I have repaired the via and the scope now appears to work correctly but still has the Fail ++ Acquisition fault at power on.

The entry in the error log is as follows:
181890:23:59 ERROR: diagnostic test failure
ctlConfidencDiag, **2.50e-9<=exp<=1.20e-8 actual=4

I know that this scope is no longer supported and that schematics are not available but if anyone here has experience working on them and knows what this error message means I would greatly appreciate a hint as to what function it relates to or which area of the PCB I should be looking at.

Thanks & regards,
Robin Bowden

Carl M
Tektronix Applications
Tektronix Applications
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Country: United States

Re: TDS540 fault code

Post by Carl M » October 17th, 2008, 12:15 pm

If I knew, I'd tell you but I haven't the foggiest. If I find anything I'll let you know.
Tektronix Application Engineer

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Joined: March 16th, 2009, 6:14 pm

Re: TDS540 fault code

Post by philippehobbycanada » March 19th, 2009, 7:07 pm

Hi Did you find any things on the acquisition board, looks like tds520/540 have problem on unreliable weld around some chips, look at every weld with a lens.

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Joined: March 16th, 2009, 6:14 pm

Re: TDS540 fault code

Post by philippehobbycanada » April 5th, 2009, 5:53 am

Hi Robin,

I have similar problem with a very difficult offset close to zero or not working on some channel.

Can you please explain: open circuit via under a resistor (U1550 Pin 7 to U1552 pin 16

Is it a resistor failling? which one?
Is it a via failing? underneath which resistor?

I have made some measurement between the 2 pins called, I get around 100 Ohm in the both direction, is that correct?

When you say I traced how did you do with a multilayer PCB?


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Joined: March 27th, 2009, 4:57 pm

Re: TDS540 fault code

Post by alautre » June 17th, 2009, 4:26 pm

Hi All

I have similar problem with my TDS540:
In first step, I had a huge offset on the traces. After replacing the SMD electrolitic cap I have a residual offset which decrease to about 3mV after few minutes of running. This is acceptable to use the scope, but I would like to understand!
At the startup, a Fail--Acquisition is reported and the log indicates a diagnostic test failure "ctlConfidenceDiag, **2,5e-9 <=exp<= 1,2e-8 actual=3"

I have observed a high sensivity about offset on the IC U943 then I have drawed a partial shematic of the DAC & offset area. it appears that this 4 x Aop circuit is used as a voltage follower for the sample & hold stages which generate reference for offset/pos/(gain) for each of the 4 channels. On my TDS540 this IC is temperature sensitive for mainly ch3 and ch4, I think that this circuit could be degraded in terms of input impedance and this could generate the diagnostic failure due to residual offset.

I would like to share the piece of schematic on the forum in order to allow to improve it by crossing information, but its seems that is would not possible?

I am very surprised that nobody in the world is able to provided schematic, error log meanings, calibration software! I cannot believe that a professionnal manufacturer as Tektronix had burned/destroy/lost... the documents!!!!!!


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Joined: June 11th, 2010, 2:30 am

Re: TDS540 fault code

Post by wonghoi » June 11th, 2010, 2:36 am

I got the exact same problem (same error message too). Has anybody figured out the solution yet?


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Joined: December 18th, 2009, 2:05 pm

Re: TDS540 fault code

Post by tech8 » June 14th, 2010, 5:15 pm

Tek never published schematics for any of the TDS series products, so you are unlikely to find some. The diagnostic message is saying that the CTL confidence diagnostics failed. The CTL is U1001 (the IC with the large record stack heat sink). The part is almost never at fault for any diagnostic failures though. More than likely something got fried or a run opened up in either the main or delayed time interpolator circuits when your caps leaked. Take a close look at all the parts to the left of the CTL, especially around the little op-amps. Also check the supply voltages to the op-amps. Frequently the + or -15v is missing.

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Joined: March 31st, 2011, 5:54 am

Re: TDS540 fault code

Post by Gorgon » March 31st, 2011, 6:11 am

Tech8 - thanks for your information - it has helped move my TDS644A repair along.

I obtained aTDS644A from EBay - the usual problems of leaky capacitors - and reports of acquisition failure.
( Acq fail / Atten fail / Cal fail )

Quick Summary

I replaced all caps on Acq and Processor boards. And cleaned throughly with IPA.
Now the Acq/Atten error was gone.

The unit would still not pass self test ( Cal/Acquisition problems ) - the SPC would get as far as either ctlConfidenceDiag or AC Calibrator Failed.
After some warm up time - I was usually only left with AC Calibrator error.

Checked +/- 15V - on Acq board they seemed good.
I re-worked around U1003/U1651/U1653 - removed these ICs , cleaned the pcb under the devices - cleaned the devices - cleaned the pads - resoldered.
Re-soldered quite a few resistors / caps around the same area ( slightly dull solder - corrosion from leaky caps )
( Around the area of the VCTL test point )

Now the unit seems to solidly get as far as AC Calibrator failure < 66.67Mhz
Removed / cleaned as above U853 , also resoldered resistors around 77.13Mhz oscillator module.
( Basically around the ACCALSIG test point )

Unit passed all self tests except calibration.

Re-ran SPC - now passes !!!

Unfortunately I still get the AC Calibrator failure from cold - but after 15-20 minutes of warm up it goes away - so still tracking that one down.

I thought I would share to help any others.


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Joined: March 31st, 2011, 5:54 am

Re: TDS540 fault code

Post by Gorgon » March 31st, 2011, 12:25 pm

A little update.

I did some more touching up of 'dull' resistor / capacitor joints ( a little fluxpen followed by re-heating ).

The AC calibration error appears to be banished - even on starting from cold.
( My offsets are around 10-120uV across all channels - I assume from what I've read that this is acceptable )

There are only a couple of problems remaining ( that I can see :-) )

1. I get about 30-40mv of 50hz ripple on the 0.5v 1Khz probe compensation signal ( maybe I need to replace the capacitors on the front panel )
( Other external signals do not suffer the problem ).
2. When I connect a 10:1 probe to ground on the scope - I get quite a lot of pulse noise ( 5-20Mhz ) at 30mV peak.

Anyone seen these before ? My guess would be capacitors in the PSU/front panel - but before I start a bombing mission - I thought I should ask.

( Sorry for hi-jacking this old TDS540 thread - but as far as I can see - these units are quite similar )

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Joined: March 31st, 2011, 5:54 am

Re: TDS540 fault code

Post by Gorgon » April 1st, 2011, 2:02 am

I've started looking at the front panel for leaky caps.

I've noticed something rather disturbing .....

On the TDS520CM component manual - C22/C23 appear to be across the +/- 15V supply.

The capacitors are marked 33uF on the schematic ( no voltage specified ) - yet on my front panel they are clearly 33uF 10V rated parts !!!

No real surprise they die and leak ! ( C22/C23 are the worst caps on my front panel board )

This has me concerned about the caps I've replaced on the acquisition board - perhaps I'll need to look carefully and re-place the ones across the 15v supplies :-(

Wouldn't this be classed as a design error ?

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Joined: March 31st, 2011, 5:54 am

Re: TDS540 fault code

Post by Gorgon » April 1st, 2011, 3:13 am

Cleaned up / replaced al caps on Front Panel - and used 47uF/25v for C22/C23.
Every other cap was replaced with 33uF 10V 105C.

Ripple on test signal gone.

Also - I started to get a continuous beep from the beeper - this has now gone.
( Touched up / cleaned all joints around the beeper section ).

More HF noise than I would like - so might look at hunting down any more rogue voltage rated caps.

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Joined: March 31st, 2011, 5:54 am

Re: TDS540 fault code

Post by Gorgon » April 1st, 2011, 3:49 am

Turns out that the A10 Acquisition board did have a couple of rogue under rated caps.

c1693/c1603 were both 33uf 10V - but had 15v across them.

I'll replace these :-) with a correctly rated part.

I didn't find any others - but I only checked the 10v caps

( And yes - they were definitely installed as built - I made a photo of the board before I started work on it - to ensure I installed the same caps as I removed )

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Re: TDS540 fault code

Post by Gorgon » April 1st, 2011, 4:06 am

Replaced the two caps - and the scope is *really* nice now - I almost can't tell the difference between 20/100/Full bandwidth options on the cal signal.

I'll let it run for an hour or so and run SPC and see if all remains well.

Thanks for a great forum - it has really helped.

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Joined: March 31st, 2011, 5:54 am

Re: TDS540 fault code

Post by Gorgon » April 1st, 2011, 9:07 am

SPC worked fine. Scope good even after a few hours.
Approximately 6mV p-p noise noted at full bandwidth looking at the cal/comp signal. ( in sample mode )
2.8mV p-p at 20Mhz bandwidth ( sample mode )
2.2mV with averaging of 4 i full bandwidth.
(10:1 500Mhz 10M probe )

Noise now seems symmetrical (+ve/-ve of cal signal ) - rather than having 50hz ripple on the +ve half.

( I can now see the 120Mhz glitches on the 10Mhz sinewave from my frequency standard )
I think my TDS210 is now my backup scope.

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Joined: April 5th, 2011, 7:27 am

Re: TDS540 fault code

Post by michelle3 » April 5th, 2011, 7:34 am

That is very disturbing! I came upon your comment and would appreciate you posting where it goes from here!
I can't see how this is not a design error but I'm curious because I don't pretend to be the expert

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Joined: March 31st, 2011, 5:54 am

Re: TDS540 fault code

Post by Gorgon » April 6th, 2011, 12:14 am

The scope is still well - there is only one outstanding issue I've noticed.

If the scope has been on for a few minutes - and then is switched off using the on/stby button - there is a faint ticking from the power supply (1-2hz)

There are no obvious visible problems with the (few) capacitors on the supply board - and so for now I turn off the device at the rear ( although I've left it in this condition for several hours with no obvious ill effects ).

Anyone out there with a TDS[5,6]XX who has noted the 'ticking' ( and their unit is functional :-) )

As far as the other capacitor issues - my thoughts on the over voltaged caps would suggest that the PCB did not have the correct pad layout for a suitable capacitor ( I can't locate 'C' footprint 33uF 35V Electrolytic caps ).
The two caps on the acquisition board I suspect are sample and hold - so not that critical in terms of capacitance - but have 15v most of the time - thus would fail.
( A pulse waveform across them - and don't appear to be across the 15V supply )

Perhaps this is why there is a TDS644B :-)

Anyway - I'm happy - this scope was cheaper than a 60Mhz 2Channel Rigol - and obviously a lot better. ( and it's better than my dual channel 60Mhz TDS210 )
If you were to follow the official Tektronix service manual - then this would be a scrap scope - Processor/Acquisition/Front Panel boards all requiring replacement.

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Joined: March 31st, 2011, 5:54 am

Re: TDS540 fault code

Post by Gorgon » April 6th, 2011, 12:40 am

As far as the 'ticking' I think this may be normal when in standby.

Reviewing the TDS520CM manual ( which as far as I can see the description of the circuit operation was written from a different schematic :-) ) the standby power is generated from a flyback circuit. It appears to DC rectify the incoming supply - store the energy in an inductor - and then dump it via a couple of power FETs.

In standby there is so little current required - the 'dumping' occurs at a low frequency ( the 'tick' )
( I think the standby voltage is 12v , and the DC rectified supply is 400V , so as far as I can see 1-2hz would be sensible )
( I hate SMPS )

Anyone wish to correct this view ? ( This is assuming the TDS520B schematic is relevant to the TDS644A - which it does appear generally similar ).

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Joined: January 20th, 2011, 11:19 am

Re: TDS540 fault code

Post by Payload » April 6th, 2011, 11:31 pm

My TDS754D makes a bit of noise in standby too. I'm less worried about it now I hear I'm not the only one! Scope seems fine...

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Joined: April 11th, 2011, 1:42 am

Re: TDS540 fault code

Post by coffee11 » April 11th, 2011, 3:24 am

My TDS540 produces a ticking noise, which I think comes from the power supply. but it functions well apart from that.
I have been worried that I would have to have it looked at.
But as other people report the same problem, I think i will stick it out as I need the device daily.

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Joined: May 1st, 2009, 8:16 pm

Re: TDS540 fault code

Post by cap » April 11th, 2011, 8:53 am


I would look at the power supply and look for heat damage or electrolytic leakage on the power supply board. On color scopes ticking at power on is a pretty good indication the the display driver board is about to go.

In general, if you have not replaced the smd electrolytic caps on this scope already, the power supply is likely strugling to cope with loads that are now well out of design specs. Ticking is not normal.

The power supplies on this family of scopes are pretty solid. The original SMD surface mount capacitors were crap and must be replaced.


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