Hi,
I have acquired this obviously non properly functioning TDR with the intention of fixing the display.
The LCD screen is a 256x128 pixels panel from the beginning of LCDs with greenish high voltage backlight and heating system for cold temperatures (as the instrument is supposed to be used in the field).
According to the service manual (easily found on the Internet; the one for the 1502C includes the full schematics, while the 1502B's does not), the 'image bitmap' to be displayed is simply written regularly by the main CPU into a static 4096x8 bit RAM located on the display board. The display processor on its side reads this memory to drive the LCD panel.
My plan was to sample the write operations (Address + Data) to this RAM with a micro-controller and then to drive a simple small color LCD display I have (used in black & white). Then, I could find a bigger display to fit into the instrument.
The first part of the project was OK, and I managed to have a functioning display with some very simple glue logic and a few program lines. The TDR was functioning OK, with a clean trace and the ability to measure cable length was OK (picture below is after the incident):
Then, while probing some connector to search for power supply to power the micro-controller, I think (but I'm not sure) that I accidentally shorted two power pins together.
As a result the TDR then powered-up with an error message:
Also the displayed trace was flat with a lot of noise.
The error message is undocumented, but by using the troubleshooting charts and a USB logic analyser, I identified a defective 74ALS161 counter that I replaced (with a 74LS161, not ALS).
This brought back some life to the instrument: the pulse can be seen, but the trace is very noisy.
When the trace is moved vertically, at some positions, the line is less noisy.
The trace is digitized internally by sampling a 'video signal' with an ADC (top right):
The video signal at the input of the ADC seems OK, so I think the ADC is damaged
The part seems to be available, but quite expensive (around $20), so I want to be sure that my diagnosis is OK...
Now the restoration project has switched to a repair project!
Michel.