Tektronix TDS694C triggering chips mystery solved?

[flyte]

Hi all,

I'm currently working on a TDS694C repair.

It is a well known fact this scope has its trigger chips burnt out, sooner or later. Now, about everywhere you read that you can swap two of the ACQ board's chips by the defective ones, and it indeed works again. Supposedly Tektronix included "free spare chips" on the board.

Because of the repair job, I've been analyzing the triggering circuits and my prediction would be that is a false statement.

The trigger input signals are being fed into the 44-pin PLCC ASICs. There are two of them and each one has two direct connections from the input ASICs. So one chip is CH1/CH2, the other one is CH3/CH4. The PLCC ASICs are the level comparators in my understanding.

Both PLCC ASICs have two outputs. Next, the 4 signals, in differential setup so 2 lines per signal, are fed into GaAs SPDT switches made by M/A COM. Then they pass through the infamous "burner" chips, and then again through a series of SPDT switches. This stage seems to be some kind of signal conditioning/amplifying part. So the "burner" chips must be some kind of (controllable?) amplifiers. This corresponds to the setup and the pads layout, clearly with lots of power connections.

After the last stage you get 4 differential signals which are fed into the main logic ASIC (big one in the front).

In the usual trigger mode, each of the PLCC ASICs switches between either CH1/CH2 or CH3/CH4, depending on what you select, and then obviously always feeds the selected channel into the same amplifier stage lines. Otherwise it could never work with only two of those "burner" chips.

Great if you only use that. But one of the triggering modes in the TDS694C is called logic triggering combining logic state of up to four channels.

And here my hypothesis comes: a logic trigger based on four channels will no longer work as expected on units which had the two "spares" removed.

Now, as I'm not really willing to remove these chips just for testing my hypothesis, it would be great if someone with such a unit could test this. Then we all know it works but there may be a trade-off, and Tektronix didn't include free spare chips.

Greetz to all,
flyte