Tek TDS7404 triggering issue

[geshka]

Hello there.

I've been offered TDS7404 for fair price with one issue that I am observing with this scope - triggering is  funky. To be exact  - I am able to synchronize with test signal only on "rant"  mode.  External signals are difficult to synchronize and sometimes impossible at all. Is there definitive test for synchronization/triggering ? Would it be hard to fix if problem exist? what scope with in that condition would be worth ?

Thanks.

[nctnico]

I'd apply a signal (from the probe calibrator output) set the scope back to default and use the auto setup. If that fails to get a stable trigger with a regular then it is safe to assume the oscilloscope has a serious problem. If you don't have a donor oscilloscope with a known good acquisition board I wouldn't spend a lot of money on it because an oscilloscope which can't trigger is basically worthless.

[geshka]

Thanks for quick response, Nctnico.

This is exactly an issue... test signal (compensation meander in autoset) is jerky but not to the point when it is running away all the time - it is sometimes jumping left or right and  it is surely not rock solid like on my old trusty analog scope.  I am really confused and undecided.  and is that repairable if this is a problem?

[KE5FX]

One tricky aspect of fast DSOs is that if the trigger circuit is running at full bandwidth, it may see ringing and ground bounce on a low-frequency square wave that the acquisition system is hiding from you at slow sweep speeds. 

I'm not familiar with the TDS7404, but see if there's a lowpass filter option for the trigger (may be called 'HF REJECT' or something similar.)  In general, try switching to a much faster timebase than you would ordinarily use, and see if you see a lot of ringing/undershoot appear.  If there's an envelope or peak-detect acquisition mode, this may also give you a clue as to what the trigger circuit is actually doing.