From what you have said, the camera produces a normal thermal image display via its composite video output connector. This is good news as that shows that most of the camera is operational.
From memory the camera drives the LCD display from a dedicated video processor and the composite video comes from the same IC. This would suggest that either the IC developed a fault that only effects the digital video output to the LCD or that one or more important digital signal connections between the video processor IC and the LCD panel have failed in some way.
We need to consider what potentially caused the broken LCD panel before you bought the unit. These cameras are pretty robust and by far the most common accident is a fall event where the camera is dropped onto a hard surface from a height. A fall event serious enough to break the LCD panel is also capable of breaking surface mount solder connections on heavy components, IC’s and ribbon connectors. Such solder joint failures can be very hard to see so testing with test equipment is needed.
There is another ugly possibility that we must also consider. Some companies destroy equipment before disposal as a matter of corporate policy. If equipment suffers a fault and is to be disposed of, further damage is inflicted to put it beyond reuse. With equipment that contains a display, this is commonly smashed before disposal. In this scenario you camera could have suffered a video processor IC fault and the smashed screen was not the original fault
. Something to keep in mind.
OK, so how to proceed…….
1. Where able, reverse engineer the connections from the LCD panel PCB connector to the video processor IC to see whether there are any in-line buffer IC’s or filters that could cause loss of data line integrity.
2. Check each LCD display to Video processor connection for continuity. One way to do this is to monitor the connections to the LCD panel PCB connector to check that all expected supplies and data lines are present and active. You have bought replacement LCD panels so there should be a data sheet available to show the various signals needed by the display. Look for any that are inactive when they should be carrying data. If monitoring of the data lines is not possible due to lack of a suitable oscilloscope, it may also be possible to test for continuity between the video processor, any intermediary components, and the PCB connector using a multimeter set to diode test mode. Power is removed from the camera and the LCD panel disconnected. The multimeter probes are placed between 0V and the pin of interest to see whether the protection diode on an IC’s I/O pin can be detected. Where accessible, standard resistance measurements of data and supply lines may be carried out. Any connection between the video processor IC and the LCD panel connector that appears to be ‘floating’ should be further investigated as there may be a loss of continuity.
3. If all expected data, sync and power lines to the LCD are present and active, you could potentially have a failed video processor that is sending garbage to the LCD panel processor (unlikely as you have good composite video) or there could be a firmware issue that has misconfigured the video processor digital output such that it no longer matches the needs of the LCD panel that is in use. Firmware corruption can occur if someone tried to modify the cameras configuration and messed with something that they should not have touched.
Hope this helps
Fraser