Tektronix TDS460A strange behavior. - Page 1
Forum section: electronics / repair

author: starter
author_rank: Newbie
timestamp: December 01, 2016, 09:07:40 pm
content: Hi everybody,Recently, I bought a scope from ebay, that was stated for parts or repair, but when it came, there was no issues with it. It worked flawlessly.But yesterday, when i boot it on, a error appears to the self test (picture attached). Not on every boot there is an error, it appears randomly, even most of the time it didn't appear.Addition to the error, there is a new log entry (picture attached).Until then, there is some strange behavior on channels 3 and 4.I want to mention that I have never used channels 3 and 4. I have only tested them when i buy the scope and they was OK.I have started the Attenuator/Acquisition Troubleshooting Procedure, and until now i have only finished Acquisition and Attenuator Power troubleshooting procedure.And on Attenuator Board Power J101 pin 10, where should be 6V to 6.6V, there is 8.08V. Allso U3 (LM317T) is looking ok but outputting 8.08V. I checked the voltages on the Low Voltage Power Supply and everything is OK. On the A04 Acquisition Board Power all voltages are in ranges.I doubt this could be the cause of the problem but for today it's all i have found.I know about the bad electrolytic capacitor reputation on those scopes, but on my unit there is no sign of any leakage or corrosion, and i prefer not to change the caps until it's necessary to. Also there is signs that the boards of the scope have been removed. Is it possible that the issue with the caps could be solved... I'll desolder and test some of them to be sureI see that forward in the Attenuator / Acquisition Troubleshooting Procedure, i'll need another scope to check the signals on the boards, but i don't have another, i'll try to borrow from friend.I'll be thankful if someone have any information or idea about the issue.Here is a service manual for the series:TDS 410A, TDS 420A & TDS 460A Digitizing Oscilloscopes Service ManualThanks.

author: dan3460
author_rank: Frequent Contributor
timestamp: December 01, 2016, 09:43:01 pm
content: The first thing I would do on that type of scope is to make sure that the lytics have been changed. I don't know of this exact model but the lytics that Tektronix used will lick eventually, and when they do they will corrode traces and other components.

author: starter
author_rank: Newbie
timestamp: December 02, 2016, 07:10:37 pm
content: Quote from: dan3460 on December 01, 2016, 09:43:01 pmHi, I'm pretty confident that the caps on my unit has been replaced before me. There are signs intervention, and the caps are pretty well looking.I hope tomorrow I could get another scope from friend to finish the Attenuator/Acquisition Troubleshooting Procedure and isolate the problematic board. Then I'll report again.I'm also successfully connected the scope through serial debugging interface, but still I don't know how to get more info about the error from it.

author: dan3460
author_rank: Frequent Contributor
timestamp: December 02, 2016, 10:13:06 pm
content: Just make sure of that, leaky caps can produce very weird problems. Use a flashlight and inspect all the boards for sign of gummy deposits and wet spots. Use a magnifying glass to look at the pins of the ICs. I repaired a similar scope that had their caps replaced but the board was not cleaned, several years after that the corrosion eat through several pins of different ICs and some of the circuit traces, took me hours to fix as I chased wild gooses with the symptoms that I was seeing.

author: starter
author_rank: Newbie
timestamp: December 03, 2016, 08:22:13 pm
content: The results from the testing today:The problem is clearly in the A05 Attenuator Board.When the scope (channel3) is working properly there is no issue. On all channels the results are identical (photo below).I had to wait until the scope starts to glitch, but after is started, the measurements on channel 3 has changed.I'm not moving the probe. This is the behavior of the channel 3. Rest of the channels are fine.I have also managed to get the log from the debug console. This is the error, and below is the whole boot and log.Code:acqRampDiagAll ................. ***FAIL***..error details:ERRORID: 151 diagnostic test failure runAcqDiagCh3** STRIGA post-trig: exp 1 act 0** ramp amplitude: exp>=0xe0 act=0xdf** CH3 path FAILEDThis is the relation between channel 3 and 4.Code:ch3 can't display probe compensation signalch4 can display the signal but is affected by the ch3 noise.changing only ch3 vertical v/div.Vertical V/divCH3 NoiseCH4 Noise (displaying probe compensation)1.00mVlownearly good2.00mVlowgood5.00mVhighbad10.0mVhighbad20.0mVhighbad50.0mVlowgoodrelay100mVhighbad200mVhighbad500mVlowgoodrelay1.00Vhighbad2.00Vhighbad5.00Vlowgood10.0VlowgoodIt's not a solution for me to get another Attenuator Board. It's hard to find, and the price is not so good.I prefer trying to repair my board.Looking forward for any suggestions.

author: dan3460
author_rank: Frequent Contributor
timestamp: December 03, 2016, 10:51:43 pm
content: It looks like there is not complete set of diagrams out there in the web.If you have to wait for the problem to happen means that whatever is failing fails when is warm. I see 4 sets of ICs with heat sink, i'm guessing there is one for each channel and it should be a handful of chips roughly in the same position around those chips. See if you can see the numbers of those chips and from the web you can get the pins that should have the voltages. Check the voltages on the channel 3 set of chips, which i'm going to guess is the second in from the side of the scope.See if the voltage changes when the problem happens. The other thing that I do when I don't have schematics, is to use "Freeze Spray" to lower the temperature of different components and see if I can identify the faulty one.Could you post pictures of the A05 board.

author: starter
author_rank: Newbie
timestamp: December 05, 2016, 09:21:04 pm
content: Here's a photo You requested.LinkToday's results:There are a bunch of opamps in the attenuation board. Most of them are dual opamps.As i can find, some (or if not all) of them are controlled from device called 'daculator' (IMP I10412-01). U101 for ch1 & 2, U108 for ch3 & 4.If I'm not mistaken, U117 are sullplying attenuator hybrids on channels 1 and 2, and U124 are supplying attenuator hybrids on channels 3 and 4.U117 (LM358M) voltages (controlled by daculator U101):pin1 is 5.51V, pin2 is -0.44V, pin3 is -0.44V, pin4 is -4.99V, pin5 is -0.44V, pin6 is -0.44V, pin7 is 5.51V, pin8 is 14.86VThese are supplying channels 1 and 2.U124 (LM358M) voltages (controlled by daculator U108):pin1 is 4.30V, pin2 is -0.90V, pin3 is -0.90V, pin4 is -4.94V, pin5 is -0.52V, pin6 is -0.52V, pin7 is 5.26V, pin8 is 14.86VThese are supplying channels 3 and 4.These voltages are not changing when the scope glitches.As we see, the voltages on channel 3 are worst.Is it possible that due to the low supply of the channel 3, it glitches from time to time.This discussion point me in that direction.LinkThe problem is not temperature related (i think).Tomorow I will pay more attention on the daculators.This are the daculators according the Tek's documentationA3U35 156–6224–00 IC,CONV:CMOS,D/A;12 BIT,V OUT,16 CHAN,SER IN,DACULATOR;I10412,SO28.300 80009 156–6224–00LM358MMMAD1108PS: I'll be grateful if anyone has information or datasheet about the daculator: IMP I10412-01Thanks.

author: dan3460
author_rank: Frequent Contributor
timestamp: December 06, 2016, 12:27:02 pm
content: You need to isolate where the fault is initiated. I would start from the top at the BNC connection working my way back to the board. You need to see if the problem is in the attenuator or in the board. In the picture at the attenuator board, there are several relay, poke at them with the other scope and see if the signal is ok.After you are sure that it is not the attenuator, then lets check the actual board.The voltages look ok to me, pins 2 and 3 are the inputs for the opamp and they should be the same, the same is true on pins 5 and 6. So the voltages look ok to me.

author: starter
author_rank: Newbie
timestamp: December 06, 2016, 09:06:45 pm
content: Today I couldn't check anything. The scope is working without much issue.I was trying to see what is going on the attenuators output, as You suggested, but it's working pretty stable.It glitches a couple of times, but it was so brief, that I couldn't measure. I hope tomorrow I could do more.

author: abeasley
author_rank: Contributor
timestamp: April 13, 2017, 10:29:01 pm
content: Have you been able to find the solution to this? I'm having a similar issue although mine is consistent.

author: slbender
author_rank: Regular Contributor
timestamp: September 14, 2018, 09:02:20 am
content: Quote from: abeasley on April 13, 2017, 10:29:01 pmMy TDS420A is working but only on Channels 2,3,4 I suspect something in the Attenuator fails - an under powered resistor, etc. I recall someone else repaired it right on the white ceramic hybred just replacing one part, but I can't find that series of repair posts, perhaps it was on the Tek or Keithly sites I visited a few months back. I anyone comes across those details please post the link here. Thanks,Steven