Unusual use for a Tektronix scope?


Last summer I purchased a 38 year-old sailboat with a depth sounder that was only partially working. It looked like the unit basically worked, but the LCD display was completely dead, so you didn’t know how deep the water was. I had a user’s manual for this 40 year-old unit, but there was no schematic. I figured that I might be able to replace the LCD, but held little hope of finding a direct replacement for it; I didn’t even have a part number for the display. I purchased a generic 3 ½ digit direct-drive LCD for about $6 and proceeded to figure out how to map the pin-out from the dead display to the new one.
The obvious approach was to get the unit under repair attached to a transducer and put it in various depths of water between 4 feet and 400 feet. Well the only transducer I had is permanently mounted in the boat which is out of the water for the winter, so I had to figure out how to simulate the return echoes that the depth sounder would see. I traced as much of the circuitry on the circuit board as I could before I went nuts trying to identify the pin-outs of the various transistors. I found a point that looked like it generated a pulse to the processor from the return echo. I build a little circuit to invert the pulse and then soldered it to my candidate circuit location.
I took my 2465B and attached one channel to the output of the depth sounder that went to the transducer (which I was on the boat and not available) and I could see the several hundred volt pp waveform generated as the transmit pulse and I triggered the scope on that. I then took the B-sweep trigger pulse off of the back of the scope and used that to drive the inverter attached to the return echo detector output to simulate the pulse that normally would be generated from the echo. Putting the sweep in “B start after delay” I could simulate any depth of water using the delayed sweep, from 1 ft, to 399 ft, the limit of the depth sounder.
Then I attached the input pins of one digit of the new LCD, which I obviously had the pin-out for, to the dead LCD, looking for activity on the new LCD to isolate which pins on the old LCD corresponded to the segments for the least significant digit. After many observations simulating various depths using the B-delay, and going from digit to digit, I was able to ascertain which pins on the old LCD corresponded to the appropriate pins on the new LCD.
If I had found a drop-in replacement for the malfunctioning depth sounder, I probably wouldn’t have done this. It took any hours to do. Now for the hard part, figuring out how to mount the new LCD in the unit and wire it up.
An unusual application for a Tek scope: simulating the depth of water in the lake?


On Tue, May 11, 2021 at 9:48 AM Matt <mhofmann@sbcglobal.net> wrote:

An unusual application for a Tek scope: simulating the depth of water in
the lake?
That's ingenious, using the scope as a delay pulse generator.


Would it be possible to use a pulse generator as a scope?