Tektronix TDS 520 repair

Update: Added a link the Debug Interface schematics (pdf)

The Tektronix TDS 520 is a digitizing oscilloscope from the early 90s. It offers 500 MHz analog bandwidth, 500 MS/s sampling rate, 2 full-featured channels and 2 auxiliary channels with a limited vertical sensivity. Although being at age nowadays, this oscilloscope has perfect properties for a prominent place in the shack at home.

My TDS 520 was saved from the scrapheap.

The problem

Bad SMT capacitors! Although it looks like capacitor plague, I don’t think it actually is. The well known capacitor plague from the early 00’s, probably has it’s origin in some sort of industrial espionage. The capacitors used in this generaration of Tektronix equipment are just not that good. Combine that with many operational hours on a lab bench and high temperatures inside the case and you’ll end up electrolyte all over the place.

That leaking electrolyte eats pcb and components for breakfast. The copper simply dissolves, the pcb delaminates and resistors decompose. This causes the infamous fails during the power-on self test (POST):

Note: My TDS failed on ‘Acq/Proc Interface’ and ‘Acquisition’. The picture above shows a different screenshot.

Fix #1: Replacing SMT electrolytic capacitors

First thing I did is replacing the capacitors on both the processor board (A11) and the acquisition board (A10). Before doing this, make sure you clean the boards thoroughly with something like IPA. Then do it again, again and again. Once more the make sure all the electrolyte is gone. There are 2 type of electrolytic capacitors on PSU the board: 10uF/35V and 33uF/10V. Make sure you replace the correct ones.

(click to enlarge)

In the picture above the red dots indicate a 33uF/10V cap and the blue ones indicate 10uF/35V capacitors.

Cleaning the boards and replacing the caps will solve your problems in some cases. In my case, it did not.

Further research

For more information I used the Tektronix TDS debug interface (see below). Both ‘Acq/Proc Interface’ and ‘Acquisition’ faults still exist. The output of the debug interface shows this:

DDStart Power-On Diag Sequence
 hwAccountant probe routines
 Probe for unexpected pending ints
 Dsp Instr mem size
 Dsp D2 mem size
 Dsp D1 mem size
 Dsy Vect0 mem size
 Dsy Vect1 mem size
 Dsy Wfm0 mem size
 Dsy Wfm1 mem size
 Dsy Text0 mem size
 Dsy Text1 mem size
 Acq number of digitizers
 Acq mem size
 Cpu timer interval uSec
 Cpu Dram size
 NvRam mem size
 Opt RS232/ Cent presence
 dspForcedBus ................... pass
 cpuDiagD2MiscReg ............... pass
 cpuDiagDSPIntMaskReg ........... pass
 cpuDiagDsyAccess ............... pass
 dsp68kMemTest .................. pass
 cpuDiagFIFOMem ................. pass
 dspRunVerify ................... pass
 dspBusRequestTest .............. pass
 dspImplicitBusAccess ........... pass
 dspTristarMemTest .............. pass
 dsyDiagResetReg ................ pass
 dsyDiagVscReg .................. pass
 dsyDiagPPRegMem ................ pass
 dsyDiagRasRegMem ............... pass
 dsyDiagRegSelect ............... pass
 dsyDiagRamdacRegMem ............ pass
 dsyDiagAllMem .................. pass
 dsySeqYTModeV0Intens ........... pass
 dsyDiagSeqXYModeV1 ............. pass
 dsyRastModeV0Walk .............. pass
 dsyRastModeV1Attrib ............ pass
 dsyWaitClock ................... pass
 cpuDiagAllInts ................. pass
 optRS232DuartIO ................ UNTESTED
 nvLibrariansDiag ............... pass
 calLibrarianDefaultCk .......... pass
 dspForcedBus ................... pass
 acqProcThermistor .............. pass
 digDiagD1Conf .................. ***FAIL***
 ..error details:
 digDiagConf .................... ***FAIL***
 ..error details:
 vertAttenStrobeDiag ............ pass
 vertAttenShiftReg .............. pass
 dacRangeDiag ................... pass
 fpDiagConf ..................... pass
 hwAccountant probe routines
 Probe for unexpected pending ints
 Dsp Instr mem size
 Dsp D2 mem size
 Dsp D1 mem size
 Dsy Vect0 mem size
 Dsy Vect1 mem size
 Dsy Wfm0 mem size
 Dsy Wfm1 mem size
 Dsy Text0 mem size
 Dsy Text1 mem size
 Acq number of digitizers
 Acq mem size
 Cpu timer interval uSec
 Cpu Dram size
 NvRam mem size
 Opt RS232/ Cent presence

Hmmm… Not very helpful…

I took out the boards again for a detailed optical check with a microscope. This showed some disturbances around memory chip U213. Desoldering this chip reveals the damage done by the leaking caps: broken tracks!

(click to enlarge)

As you can see, the pad for pin 23 is gone and the pad for pin 19 isn’t connected anymore to the track going right.

Fix #2: Fixing broken tracks

With these multilayer boards, checking connections can be confusing. Tracks that go into one of the inners layers are hard to follow. Looking at the other memory chips gave me all the information I needed.

Each channel has eight 84K x 8 memory chips. The address-buses of all chips are wired in parallel. The data-buses are wired separately to some ASIC, which acts (beside other functions) as memory controller.

<< insert table with memory connections >>

With 2 kynar wires soldered to fix the tracks, the scope works fine again!

<< insert picture >>

Debugger output:

DDStart Power-On Diag Sequence
 hwAccountant probe routines
 Probe for unexpected pending ints
 Dsp Instr mem size
 Dsp D2 mem size
 Dsp D1 mem size
 Dsy Vect0 mem size
 Dsy Vect1 mem size
 Dsy Wfm0 mem size
 Dsy Wfm1 mem size
 Dsy Text0 mem size
 Dsy Text1 mem size
 Acq number of digitizers
 Acq mem size
 Cpu timer interval uSec
 Cpu Dram size
 NvRam mem size
 Opt RS232/ Cent presence
 dspForcedBus ................... pass
 cpuDiagD2MiscReg ............... pass
 cpuDiagDSPIntMaskReg ........... pass
 cpuDiagDsyAccess ............... pass
 dsp68kMemTest .................. pass
 cpuDiagFIFOMem ................. pass
 dspRunVerify ................... pass
 dspBusRequestTest .............. pass
 dspImplicitBusAccess ........... pass
 dspTristarMemTest .............. pass
 dsyDiagResetReg ................ pass
 dsyDiagVscReg .................. pass
 dsyDiagPPRegMem ................ pass
 dsyDiagRasRegMem ............... pass
 dsyDiagRegSelect ............... pass
 dsyDiagRamdacRegMem ............ pass
 dsyDiagAllMem .................. pass
 dsySeqYTModeV0Intens ........... pass
 dsyDiagSeqXYModeV1 ............. pass
 dsyRastModeV0Walk .............. pass
 dsyRastModeV1Attrib ............ pass
 dsyWaitClock ................... pass
 cpuDiagAllInts ................. pass
 optRS232DuartIO ................ UNTESTED
 nvLibrariansDiag ............... pass
 calLibrarianDefaultCk .......... pass
 dspForcedBus ................... pass
 acqProcThermistor .............. pass
 digDiagD1Conf .................. pass
 digDiagConf .................... pass
 extTrigRegDiag ................. pass
 vertRegDiag .................... pass
 cvrRegDiag ..................... pass
 trigSrcRegDiag ................. pass
 lstlDiag ....................... pass
 ctlDiag ........................ pass
 trigAcqRstDiag ................. pass
 logicTrigConfDiag .............. pass
 alphaEdgeTypeComp .............. pass
 lineTrigDiag ................... pass
 dlyTrigDBETrigAfter ............ pass
 dlyTrigDBTRunsAfter ............ pass
 glitchTrigDiag ................. pass
 pulseWidthDiag ................. pass
 vertAttenStrobeDiag ............ pass
 vertAttenShiftReg .............. pass
 dacRangeDiag ................... pass
 fpDiagConf ..................... pass
 hwAccountant probe routines
 Probe for unexpected pending ints
 Dsp Instr mem size
 Dsp D2 mem size
 Dsp D1 mem size
 Dsy Vect0 mem size
 Dsy Vect1 mem size
 Dsy Wfm0 mem size
 Dsy Wfm1 mem size
 Dsy Text0 mem size
 Dsy Text1 mem size
 Acq number of digitizers
 Acq mem size
 Cpu timer interval uSec
 Cpu Dram size
 NvRam mem size
 Opt RS232/ Cent presence

Debug interface

Update. I had quite a nice post on my old (crashed) site about the debug interface. As you can read in the comments, I’m asked for the schematics so now and then. Here it is: TDS debug interface

Information sources

– Tektronix TDS 520 Operating manual

– Tektronix TDS 520 Service manual

– IDT 7164s25 datasheet

– Tektronix TDS520B Digitizing Oscilloscope Component Manual (with schematics!)