During one of my experiments with the TDS 340 oscilloscope made by Tektronix, I suddenly noticed that the AUTOSET button does not set the correct parameters for most signals, including the 1kHz calibration signal. Although it worked fine for a long time, the button now sets the oscilloscope to 5V/div, 25ms/div with wrong trigger settings, obviously not optimal to display a 1kHz square wave:
Failed self-calibration and diagnostics test
Thinking that the oscilloscope is out of calibration, I ran a self-calibration using the UTILITY menu, only to find out that things have gotten worse. The calibration process failed after 4 minutes and the oscilloscope reported problems upon power on:
WIth all input signals removed, I ran a diagnostics test from the UTILITY menu and sure enough, acquisition and calibration errors were reported:
The error log provided some more details – the calibration issues may have been due to the acquisition problems affecting certain tests during the calibration process. Problems with trigger and signal path of Channel 1 (error codes diagAcq_ch1Trigger, diagAcq_ch1SigPath and diagAcq_holdoff) were reported:
No errors were reported with channel 2. Are these errors related to my autoset problems? To answer this, I performed a simple test by turning of channel 1 waveform display, feeding a signal to channel 2 and press AUTOSET. Surprisingly, autoset worked just fine and selected the correct settings to display a stable waveform on channel 2:
The suspect: faulty acquisition board
So some problems with channel 1 prevented the oscilloscope from detecting the correct settings in autoset mode. Determined to find the issue, I opened up the oscilloscope and located the acquisition board:
To my disappointment, unlike other Tektronix oscilloscope models, this acquisition board is also the mainboard of the oscilloscope and is very compact with mostly surface-mounted components. There are no through-hole electrolytic capacitors on the board which may cause problem as the capacitors become dry and fail with time. Although I did find some posts here and here referring to a problem similar to mine, my limited time and the nature of the problem prevented me from putting in more efforts to fix the issue. Except for the autoset feature, the oscilloscope still seems to be stable and works well even at high frequencies. I decided to live with the issue and manually select the correct settings for each input signal until some other more serious problems occur.
UPDATE (Jan 2018): You may find the service manual, schematics, and various other interesting technical documents for the TDS 520 and other similar oscilloscopes by Tektronix here. Thanks to PrecisionAnalytic for providing the files.