Another TDS700 series scope lives again!
So, this TDS714L I just fixed, it had no video on the internal CRT, but did boot fine and would output video on the rear VGA connector.
On closer inspection, I found that the CRT was cracked (This unit was obviously dropped, but not too much external damage beyond cracked feet etc), so that was replaced with one from a junker scope that had a good CRT.
Too bad, so sad, no video still.... Poking around I found that the main video processor isn't outputting a video signal, but the separate chip doing the rear panel VGA output is fine.
The video processor has an 80MHz crystal that is a known failure point, and mine turned out to be busted (I bet from the shock of that drop), so that was swapped with a replacement from a junk board. Still sadness and no video...
I poked a few things more, swapped the main video processor (and now got memory errors due to an unexpected data value, could be an ID thing from different versions etc) but still no video, so it's looking more and more like a job for probing at the PCB with another scope and doing some signal tracing... Meh, I'm getting tired now, lets take a look at my spare processor boards....
Turns out that internally the TDS714L looks
very much like a TDS7xxD, just with a different ID resistor combination to make it the TDS714L, it even uses the same firmware, but I don't have a spare 'D' version processor board. However, I do have a spare 'C' version board that looks
identical to the TDS714L. Could be that this spare one I have is a later version 'C' board?.. I'm not sure, but it is running v6.x firmware, typical for 'C' boards.
I stuck it in anyway and it booted most of the way (showing the boot sequence video!
a small win), but ultimately failed and hung on the final acquisition board checks... hmmm....
Well, how about I just try flashing it with the 7.xx firmware that's found on the 'D' series boards and in the TDS714L too, just for shits and giggles?
Time to connect the GPIB adapter, flash v7.4e firmware and also a copy of the original TDS714L NVRAM (for all the options and settings and stuff, just in case).
Bingo, bango, bongo! It
JUST FREAKING WORKS!!! So that's interesting.... I know there are a number of revisions of the processor boards and this processor board will work with this acquisition board but not that one, and that processor board won't work with this acquisition board but will work with that one, etc etc, but I wonder if the real limiting factor is that the firmware version in the processor board needs to match the acquisition board version and the processor board will just 'go along for the ride'...?....
Hmmm, it seems it could be so with my tests so far, a 'C' version processor board works with a 'D/L' acquisition board only when firmware is updated to 'D' version firmware.
Also, I repaired a TDS620 with a TDS700 series processor board (with the relevant video chips swapped over to convert it from colour to monochrome) and also that all important firmware change to match the TDS620 and it boots fine (and feels more responsive, maybe a faster CPU? The presence of the extra video RAM?).
Anyway, that's another TDS scope fixed and on the 'to sell' pile, 6 scopes now ready to sell, and one last one to fix, once I figure out the golden ticket to get the machine reading and writing cal data to those damn EEPROM's.....