TDS784A acquisition board partial failure - Page 1
Forum section: electronics / repair

author: Tantratron
author_rank: Frequent Contributor
timestamp: March 24, 2025, 06:34:40 am
content: Hi, I'm trying to restore a TDS784A which can boot without any fault (see attached banner picture) but does fail SPC (see attached picture of error log).The symptoms of Ch2 and Ch3 are confirmed by simple tests:trying to change vertical scale on each channel (1 to 4) will always make attentuator relay click audible (1V to 2V) and 100 mV to 200 mVonly for Ch2 and Ch3 there willNOtrace display if vertical is less or equal than 100 mVonly for Ch2 and Ch3 will display 1 KHz square signal if vertical scale is higher or equal to 200 mV but trigger willNEVERworkwether Ch1 or Ch4 will always display traces, good trigger and using Leo Bodnar pulser confirms 1 GHz bandwithBesides the pciture of error log file, here is GPIB extract from the same logCode:ERROR: Internal adjustment range exceeded,(1 @ 200c): .9997 => Balance dac #2 (ID #145)ERROR: Internal\sadjustment\srange\sexceeded,(2 @ 200c): .9997 => Balance dac #3 (ID\s#146)ERROR: Characterized values out of bounds, Ch2 trigger gainERROR: Characterized values out of bounds, Ch3 trigger gainERROR: Characterized values out of bounds, Ch2 trigger gainERROR: Characterized values out of bounds, Ch3 trigger gainERROR: 2260 Calibration failed, TriggersSince half of the input channels work fine, do you think it is pin soldering issue on some chips, which chip (see attached 2 pictures of the board) do you recommend to check first ?Thank you

author: TERRA Operative
author_rank: Super Contributor
timestamp: March 24, 2025, 07:22:57 am
content: How are the relays on the input hybrids? They can and will cause SPC to fail if worn out. Might be worth replacing the full set of 16.I would swap the hybrid modules between channels 1 and 2, and between 3 and 4. Then reset errors and do a full self test and SPC.If the SPC errors follow the hybrid modules, you need new relays.If the errors remain on the same channels, then time to dive deeper..You can also double check contact resistance with a 4-wire multimeter and activating the relays with a bench PSU manually.

author: Tantratron
author_rank: Frequent Contributor
timestamp: March 24, 2025, 07:35:52 am
content: Quote from: TERRA Operative on March 24, 2025, 07:22:57 amIs it possible to test the relays (on off and continuity) without deconstructing the attenuator metal housing module ?What is confusing or intriguing, both Ch2 and Ch3 have same type of failure (no trace signal when vertical selection < 200 mV and no trigger stable trace when vertical signal selection >= 200 mV). There could be a common point failure, why only relays of Ch2 and Ch3 fail then why trigger fail but signal trace is seen on display ?

author: TERRA Operative
author_rank: Super Contributor
timestamp: March 24, 2025, 09:02:01 am
content: You will have to open the metal housing (It's just a bunch of torx screws and it lifts away), then desolder the BNC connection to the hybrid and the hybrid modules carefully lift out.There's no other way to test the relays.Just be super careful that they all line up properly when you retighten it back together so they don't crack. As long as they are seated right, it's fine.Also be sure to replace the thermal paste on the chip, you'll see it when you take it apart.I'm not sure re. the trace failure, I'll have to think about it a bit. Maybe something to do with the interleaving between channels or something that is only affecting those two.The relay test is the easiest to do empirically. It may just be those two channels were used a lot causing more relay actuations which wore out the relay contacts.I've often seen channel 1 and 2 die mostly due to relays wearing out, but it would depend on scope usage patterns.It might not even be the relays, but with the acquisition board on the bench, you're halfway there already.

author: Tantratron
author_rank: Frequent Contributor
timestamp: March 24, 2025, 09:37:24 am
content: P.S. Maybe one explanation could be related upon boot, for a few seconds the 7-segment display shows ".L" as I've mentioned in thisother thread. To my best knowledge, the ".L" is not found in service manual

author: Tantratron
author_rank: Frequent Contributor
timestamp: March 24, 2025, 07:12:45 pm
content: I've run the ConsolePort to see of any possible clue, see the log fileCode:.Bootrom Header Checksum passed..Bootrom Total Checksum passed..BootRom Check Sum passed..Bus Error Timeout test passed.Kernel Diagnostics Complete.Calling SDM (monitor) Routine..Enabling Bus Control register. Value = 0x67.IMR 1 Register test passed..Misc. Register test passed..Timer Interrupt test (Auto-Vector) passed..NVRam DSACK test passed..NVRam Write protected..Flashrom DSACK and JumpCode test passed..Flashrom Checksums passed.Bootrom Diagnostics Complete.Transferring control to the Flashrom.DTesting System Ram.D-> Console Loopback O.K. <-.No ethernet available]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]] ]]]] ]]]]]]]]]] ]] ]]]] TM] ]]]]]]]]] ]]]]]] ]]]]]]]] ]] ]]]]]] ]]]]]]] ]]]]]]]] ]]]]]] ] ]] ]]]]]]] ]]]]] ] ]]] ] ]]]] ]]] ]]]]]]]]] ]]]] ]] ]]]] ]] ]]]]]]]]] ]]] ]] ] ]]] ]] ]]]]] ]]]]]] ]] ]]]]]]] ]]]] ]] ]]]]]]]]] ] ]]]] ]]]]] ]]]]]]]] ]]]] ]] ]]]] ]]]]]]] ]]]]]]]]]] ]]]]] ]]]]]] ] ]]]]] ]]]] ]] ]]]] ]]]]]]]] ]]]]]]]]]]] ]]]]] ] ]]]]]] ] ]]] ]]]] ]] ]]]] ]]]] ]]]] ]]]]]]]]]]]] ]]]]] ]]] ]]]]]]] ] ]]]]]]] ]]]] ]]]] ]]]] ]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]] Development System]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]VxWorks version 5.0.2.Stand Alone Rom Version.Columbia Proc. Brd. Rev COLOR-2!BIGSystem Ram Size = 2097152, Proc ID = 0x31NvRam Size = 131072, Clock Interval = 13108 usecs,Bootline Storage Size = 220 stored @0x4000024Flash ID = 0x4, Flash Wait states = 3 (Misc. reg = 0x1c),KERNEL: WIND version 2.0. GCC COMPILEDCopyright 1991-2001, Tektronix, Inc.made on: Fri Jan 29 13:15:26 PST 1993.Copyright Wind River Systems, Inc., 1984-1990Executing DiagnosticsStart Power-On Diag SequencehwAccountant probe routinesProbe for unexpected pending intsDsp Instr mem sizeDsp D2 mem sizeDsp D1 mem sizeDsy Vect0 mem sizeDsy Vect1 mem sizeDsy Wfm0 mem sizeDsy Wfm1 mem sizeDsy Text0 mem sizeDsy Text1 mem sizeAcq number of digitizersAcq mem sizeCpu timer interval uSecCpu Dram sizeNvRam mem sizeLimit to 1GS/sec presenceOpt Math Package presenceOpt RS232/ Cent presenceOpt 1M presenceAcq Intlv Cal Id presenceOpt TvTrig presenceOpt TvTrig indexDsy color presenceOpt floppy drive presence-> Acq number of user channelsdspForcedBus ................... passcpuDiagD2MiscReg ............... passcpuDiagDSPIntMaskReg ........... passcpuDiagDsyAccess ............... passdsp68kMemTest .................. passcpuDiagFIFOMem ................. passdspRunVerify ................... passdspBusRequestTest .............. passdspImplicitBusAccess ........... passdspTristarMemTest .............. passdsyDiagPM100Reg ................ passdsyPM011RegMem ................. passdsyLCSRamdacRegMem ............. passdsyMonoRamdacRegMem ............ UNTESTEDdsyVGARamdacRegMem ............. passdsyDiagPPRegMem ................ passdsyDiagRasRegMem ............... passdsyDiagRegSelect ............... passdsyDiagAllMem .................. passdsySeqYTModeV0Intens ........... passdsyDiagSeqXYModeV1 ............. passdsyRastModeV0Walk .............. passdsyRastModeV1Attrib ............ passdsyWaitClock ................... passcpuDiagAllInts ................. passnvLibrariansDiag ............... passcalLibrarianDefaultCk .......... passdspForcedBus ................... passacqProcThermistor .............. passtrigGtlRegisterDiag ............ passtrigBtlRegisterDiag ............ passch1LogicTypeComp ............... passch2LogicTypeComp ............... passch3LogicTypeComp ............... passch4LogicTypeComp ............... passch1EdgeTrigDiag ................ passlineTrigDiag ................... passdlyTrigDBETrigAfter ............ passdlyTrigDBTRunsAfter ............ passslewrateTrigDiag ............... passtrigAttenSerialReg ............. passtrigPreampSerialReg ............ passtrigDTCSerialReg ............... passtrigExtlSerialReg .............. passtrigDacSerialReg ............... passTICountersDiag ................. passgtlBigCountersDiag ............. passtrigBtlConfidenceDiag .......... passtrigGtlCompRamDiag ............. passdigRegisterConf ................ passdigSpecialRegisterConf ......... passdigPhaseLockLoopDiag ........... passdigSpeedMemConf ................ passdigAcqMemPatConf ............... passdigDataFormatConf .............. passdigAcqSubSampleConf ............ passdigAcqPeakDetectConf ........... passdigAcqHiResConf ................ passdigAcqMemDataConf .............. passdigAcqMemAddrConf .............. passdigInterruptConf ............... passdigA2DConnectsConf ............. passdigHFStepConf .................. passdigDspConf ..................... passglitchTrigDiag ................. passpulseWidthDiag ................. passdigPixelatorConf ............... passtrigNibDiag .................... passfpDiagConf ..................... passoptDiagPM110Reg ................ passoptDiagFloppyCacheMem .......... passoptDiagFloppyControllerIO ...... passoptDiagFloppyDrive ............. passoptRS232DuartIO ................ UNTESTEDoptRS232DuartIntLoop ........... UNTESTEDoptCentronCntrlReg ............. UNTESTEDoptTv8bitRegDiag ............... UNTESTEDoptTvShiftRegDiag .............. UNTESTEDoptTvXparentCodes .............. UNTESTEDoptTvDontCareCodes ............. UNTESTEDoptTvEdgeAndLevel .............. UNTESTEDoptTvSyncLevels ................ UNTESTEDExecuting SmalltalkhwAccountant probe routinesProbe for unexpected pending intsDsp Instr mem sizeDsp D2 mem sizeDsp D1 mem sizeDsy Vect0 mem sizeDsy Vect1 mem sizeDsy Wfm0 mem sizeDsy Wfm1 mem sizeDsy Text0 mem sizeDsy Text1 mem sizeAcq number of digitizersAcq mem sizeCpu timer interval uSecCpu Dram sizeNvRam mem sizeLimit to 1GS/sec presenceOpt Math Package presenceOpt RS232/ Cent presenceOpt 1M presenceAcq Intlv Cal Id presenceOpt TvTrig presenceOpt TvTrig indexDsy color presenceOpt floppy drive presenceAcq number of user channelsSmalltalk/V Sun Version 1.12Copyright (C) 1990 Object Technology International Inc.One question I wonder, it is the first time I see part of this Console output reportingNo Ethernet availableandDevelopment SystemOtherwise most test seems to have pass or untested so no clue about why SPC fails with 2 failures...

author: TERRA Operative
author_rank: Super Contributor
timestamp: March 25, 2025, 01:04:49 am
content: The Ethernet was a function that was apparently planned but unfortunately never implemented in the shipped product.Apparentlyinstalling an Ethernet card into the PCMCIA version of the HDD option would be somewhat functional, besides no UI or direct OS support etc, so I think you can safely ignore that message.It would be cool to get more info to try to get it working with an installable driver (way beyond my abilities though!).I'm not sure about the Development System message though, I am currently making a debug adapter, so I haven't yet played with this part of these scopes yet.I'll have something in my hands soon though (I'll make an adapter to use the Option 13 serial port, followed by completing mu Debug-USB adapter later) so I can help compare debug outputs then.My money is still on shuffling the hybrid modules to eliminate that as an issue.

author: Tantratron
author_rank: Frequent Contributor
timestamp: March 25, 2025, 04:58:00 am
content: Quote from: Tantratron on March 24, 2025, 09:37:24 amI think we can rule out the 2 seconds display of ".L" because if connecting a USB-RS232 console port, the ".L" never appears. In fact, the dynamic boot 7-segment display is bit different wether connecting or not connecting a console port on J40.

author: Tantratron
author_rank: Frequent Contributor
timestamp: March 25, 2025, 02:02:09 pm
content: Since I've a spare processor board from a TDS540C (this board runs with 68040 under firmware 5.2e), I've installed it in replacement from the TDS784A processor board (runs with 68020 under firmware 4.2.1e). The idea of this test is to eliminate any root cause from processor board to confirm the issue is in the TDS784A acquisition board.Everything will boot OK except there will be a new error regarding the EEPROM (crc and other) which is known error because 2 reasons. The first reason is if U1052 and U1055 chips storing EEPROM cal constant to be failed then processor board can read these calibrated values. The second reason is because tektronix has progressively the data structure and crc method of the EEPROM depending firmware and acquisition board design. Since TDS784A seems to have same ASIC as all TDSxxC except firmware of U600 chip, indeed I can still see and trigger the signals so in tehory TDS784A could be transformed into a TDS784C.N.B. As of today, I do not know the correct math description of EEPROM content versus firmware to transform its data but my test shows again the same SPC error message regarding Ch2 and Ch3.So for sure now, the acquisition board is partially failed with 2 channels not working correctly wether under firmware 4.2.1e or 5.2e.By the way, since I had a spare D1 databus board, I've tried but it is not the culprit.

author: Tantratron
author_rank: Frequent Contributor
timestamp: March 27, 2025, 08:45:58 am
content: Quote from: TERRA Operative on March 25, 2025, 01:04:49 amI think to have found as spares some hybrid modules from a TDS520D.Do you know if hybrid version from TDS784A would be same and compatible versus hybrid from TDS520D (made much later design) ?How about the AUX1 and AUX2 hybrid versus Ch3 and Ch4 hybrid, are they actually the same so they could re-used or swap ?Generally speaking, is it possible to inter-mix hybrids within A serie B serie C serie and D serie ?Thank you, Albert

author: TERRA Operative
author_rank: Super Contributor
timestamp: March 27, 2025, 11:23:53 am
content: There are two types of hybrid module, smaller and larger types.IIRC, if the hybrids are mounted separately to the smaller acquisition board as on some TDS500 series, they are the smaller type hybrid.If they are mounted directly onto the larger type acquisition board (as you have), they are the large type hybrid.There are a few revisions of the hybrids (H, G, etc, it's printed on the hybrid board itself) but all large hybrid modules are cross compatible with each other (and all small hybrids are cross compatible with themselves too), regardless of what the channel type or scope model number is. But you can't swap a large for small and vice-versa.

author: Tantratron
author_rank: Frequent Contributor
timestamp: May 06, 2025, 01:19:39 pm
content: Ok here is a status where first, for sure the hybrids of this TDS784A are large model similar to C serie or D serie.I've removed the 20 screws to extract the hybrids block where as a reminder, only CH 2 and Ch 3 are failed for some reason. Not sure if this is normal but i've noticed that 3 hybrid are revision G3 and the last one is revision J. From this Oscope, there was not thermal paste trace then since I've a spared TDS520D, I've decided to remove as well its hybrid block. Again so trace of any thermal paste. This time, I see all hybrids are revision J so uniform use of hybrid models.Question 1: do you confirm they should be always thermal paste and if yes, where exactly having in mind I did not desolder the BNC tap so all hybrids are still in place inside the metal housingQuestion 2: do you think one issue could be related to mixed hybrid version G3 with version IQuestion 3: on each hybrid, there is a black IC with always marked Maxtek 165-2462-12 (all from TDS520D) whereas the ones from TDS784A are not the same (165-2462-02 and 165-2000-00)I'm attaching single close view picture of both hybrid blocks (top is from TDS520D and botton from partially failed TDS784A)My plan is now to install the attenuator hybrids block from TDS520D into the TDS784A acquisition board to see if it solves the Ch 2 and Ch 3 failure.

author: TERRA Operative
author_rank: Super Contributor
timestamp: May 06, 2025, 03:17:29 pm
content: If the hybrids physically fit, they are cross compatible. The letter denotes the revision (i.e., J is a later revision than G). Mixing hybrids will be no problem (assuming the hybrids are not faulty, or more commonly, the relays aren't worn out)The heatsink compound is put on the other side of the hybrid, on the opposite side to the chip where it contacts the metal block. You'll see it if you remove a hybrid.BeVERYcareful when reinstalling to make sure all the hybrids are seated properly. If they are not properly seated in the metal block, they WILL crack!The best way is to place the hybrid assembly as per your photo and lower the PCB down from above.

author: Tantratron
author_rank: Frequent Contributor
timestamp: May 06, 2025, 04:45:13 pm
content: Ok many thanks TERRA, so now I've installed the complete hybrid module from spare TDS520D inside the TDS784A. So far, I do not feel enough confident to remove each hybrid so basically I've swapped in one time the 4 hybrids to see if this solves the initial double failure.Indeed as you said, the J serie hybrid seem fully compatible or interchangeable, the TDS784A was able to run fine and this time, I could see the 2 traces (Ch 2 and Ch 3) when voltage division was less than 200 mV. I could not still trigger a 1 KHz signal on Ch 2 and Ch 3 but then after some warm up, I run the SPC which fixed that other problem.So clearly the initial hybrid 2 and hybrid 3 are failed, probably relay thanks to this macro-test using a spare complete TDS520D attenuator.Another question: do you know that if changing one hybrid or more requires then to re-calibrate the oscilloscope or the interchange of attenuator hybride does not really imapct the initial calibration ?Last question: have you repaired any hybrid with faulty relay, is it easy to de-solder one failed relay then re-solder a new relay without breaking the ceramic substrate or do you rather recommend to replace faulty hybrid with one valid hybrid (new or from spare stock) ?Thank you, Albert

author: TERRA Operative
author_rank: Super Contributor
timestamp: May 06, 2025, 10:30:06 pm
content: Swapping out hybrids doesn't require a recalibration, the SPC can take care of it. No problems there.As for replacing the relays, I made a video!Just be quick with the soldering iron (some extra flux can help, just be sure to wash it away after) and silver loaded solder might help too maybe? But I always just used standard solder with no problems that I could see.

author: Tantratron
author_rank: Frequent Contributor
timestamp: May 08, 2025, 10:58:12 am
content: Ok thank, I'll first remove both Ch 2 and Ch 3 hybrid from my partial failed TDS784A attenuator housing (only 2 de-soldering action).Now I still wonder another question, please see again my new attached picture where I've TEXT commented in RED the channel numbers and put a YELLOW arrow pointing at a strange marking of CH 1 chip.It is clear that whoever repaired this oscilloscope or maybe at factory production in the 90's, they decided to inter-mix hybrid versions (H2462J versus H2462G3). Let's assume it works (inter-mix or interchangeable) even though this is bit surprising but do you think it is normal that the MAXTEK US chip firmware marking of CH1 is upside down silk written ?My assumption would be that if this black chip hosting the local firmware to digitally interface with the oscilloscope commanding relays, if soldered correctly then the printed white label should be all aligned the same. However the CH 1 or left one is different orientation... any thoughts ?

author: TERRA Operative
author_rank: Super Contributor
timestamp: May 08, 2025, 03:05:00 pm
content: I've never taken notice of the chip. I just swap hybrids as I need to, I don't really care much about the revision either...It doesn't have firmware, it's pretty simple inside. You can see roughly how it works from the schematics available on Tekwiki for the TDS500 series scopes.There is no real way to rework the chip, as it is bonded to the ceramic substrate in a way that removal would be destructive, so how it is is how it was manufactured.The hybrid wouldn't have left the factory, let alone be in service for decades if it didn't work...

author: Tantratron
author_rank: Frequent Contributor
timestamp: May 08, 2025, 04:48:08 pm
content: OK but then depending on the hybrid revision, chip version, how can you tell if it can run up to 500 MHz or 1 GHz. Each hybrid besides the attenuator commanded by relays must have some specific filter or amplifier. So depending on the year of design, the TDSxxx serie (suffix A, B, C, D) not all models would run up to 1 GHz. Just curious to understand why they would be inter-mix or interchangeable on any TDSxxx/A/B/C/D

author: TERRA Operative
author_rank: Super Contributor
timestamp: May 08, 2025, 10:56:37 pm
content: All TDS600 and TDS700 series scopes are essentially capable of 1GHz, the limiting factors are personality identification resistors on the acquisition board that tell the processor board what it is, and a capacitor on each input diff pair on the 500MHz model, located between the hybrids and acquisition chips that act as a roughly 500MHz low-pass filter.All hybrids are capable of 1GHz, and essentially all 500MHz acquisition boards that have a 1GHz equivalent can be modded to 1GHz (This may not be true for the early non A, B, C, D models, I haven't played with them enough to check).I.e., any TDS754A, B, C, D can be upgraded to a fully functional TDS784 of the same letter type, 'simply' by swapping the ID resistors, removing the bandwidth limiting capacitors, and then running a full cal/adjustment procedure. (500MHz scopes are not calibrated from factory to 1GHz. The scope will work at 1GHz, but it won't be calibrated beyond 500MHz).