Fixing a 50GHz Sampling Scope (Tektronix CSA803)

About a month ago I found a rather nice-looking Dell laptop being thrown away on the Stata loading dock, through which the majority of MIT’s tech trash passes. Knowing me to be a scope junky (I currently own 10), a friend quickly offered to trade me an old Tektronix sampling mainframe he’d found that was failing a POST test for my new laptop (in unknown condition), so of course I accepted. Who needs computers when there’s interesting test equipment to be had!

Upon closer examination, the scope (Tektronix CSA803) came with an SD-26 dual-20GHz sampling head installed and was throwing error E5322 on power up. Here’s the unhelpful documentation of the error from Tektronix (translation: “we don’t support this product, we don’t publish a service manual with detailed troubleshooting instructions like we used to, and we want you to buy a new Tek scope”), and here’s a great Google Group thread that has a real solution. It turns out that error E5322 indicates an issue communicating with a pair of battery-backed RAMs on the timebase board, and replacement parts are available from Digikey for about 15 bucks a piece! Win.

A few days later, after the parts had arrived, I pulled the thing apart to get a closer look:

The replacement RAMs are the tall ST chips visible in the top right, U500 and U511.

The replacement RAMs are the tall ST chips visible in the top right, U500 and U511.

Beautiful!

Probing between the power pins on the RAMs confirmed that the battery backups were dead. After replacing them with the new parts, the scope powered up and passed POST. Here’s a measurement of the 1ns risetime of the output clock waveform (this scope has both a calibrator output and internal clock output):

CSA803 Self Cal Test

With the scope working, I decided to buy an SD-24 dual-20GHz sampling head with TDR, extending the scope to 4 channels at 20GHz each, and adding a pair of TDR inputs/outputs. The TDR outputs are capable of generating risetimes as fast as 17.5ps, the idea is that instead of using a network analyzer (which I don’t have) to characterize some system, I can hook up one TDR channel to port 1 and the other to port 2, and then record the step responses on both channels to a step applied to either channel (that’s four measurements, each corresponding to one of the four 2-port S-parameters).

Here’s a shot of the reflected signal from an open termination (nothing connected to the TDR pulse output channel). Looks about right!

Yes, that 7000-series Tek Cart was *definitely* intended for this purpose.

Yes, that 7000-series Tek Cart was *definitely* intended for this purpose.

Translating to frequency domain should then be fairly straightforward: send the data to a computer over RS-232, import into MATLAB, take the derivative to find the impulse response, and then calculate the Fourier transform and scale it appropriately.

Next up, building some high speed probes to make this thing useful.

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3 thoughts on “Fixing a 50GHz Sampling Scope (Tektronix CSA803)

  1. Great find, 11800 / CSA800 series Tek scopes are impressive. I have a couple of 11801A scopes with similar timebase error which I haven’t worked on yet. However, I was wondering if the battery backed RAM contents included factory set cal constants which will recover to “default cal constants” resulting in degraded instrument performance despite passing self-test. Do you know if that is true or not. If so, did you arrive at a way to improve the default cal constants?

    Thank you for the post, hands on info on these Tek scopes is very rare.

  2. Hello,
    Really interesting doc.
    I got a CSA803A and the POST failed at E5322. U500 and 511 are Sony SRAM (CXK58257P) are on top of a Dallas RTC + battery.
    I replaced the stack by the ST M48Z35. The E5322 error is cleared but I have now a problem of Battery with error T1331. But I can still exit the POST and use the scope.
    Do you have any idea ?

    Thanks a lot.

    • Finally, after skipping the test once I have no more error when restarting. I guess the remaining error is linked to the “nvram” emptyness.

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