Fellow Tek Tweakers,
The adventure is over! After ending up with a 'bricked' TDS784D, I was able to recover it to full operation this evening with the aid of some very smart people over on the Tektronix forum. For the entire story, see this thread:
https://forum.tek.com/viewtopic.php?f=568&t=140017Just in case that link ceases to work, I'll repeat much of what I mentioned here.
First, the programming tool. I'm sorry to say I had only limited success with tekfwtool. I was able to read my 'scope's guts with it, and make backups of both NVRAM and firmware, but trying to write back to it (at least with a PCI-based GPIB card) resulted in corrupted firmware and a bricked unit.
What did finally work was using the original tool (tektool_0.exe) from the file package mentioned in that thread referenced above (it should be downloadable by anyone... If not, I have it on my public FTP archive), in combination with an ISA-based GPIB card (specifically, a AT-GPIB from NI). In retrospect, I have to wonder if a PCI-based setup simply runs too fast in some way or other...?
Some notes on the O-scopes themselves: It seems there are differences in the version of CPU and Acquisition board used on the TDS7xxD series, and NOT all firmware loads are compatible with them. I would strongly recommend checking both the serial number AND doing an actual visual check of the part number labels on the boards before you even think of messing with firmware updates.
Specifically: Serial numbers LOWER THAN B04xxxx may have CPU board part number 679-4172-00. This board is NOT compatible with anything later than firmware version 6.6e, nor is the acquisition board which it pairs with. It also uses Intel 28F016 FLASH chips.
Serial numbers of B04xxxx and higher will likely have CPU P/N 679-4349-00. This board, and its matching acquisition board, need firmware version 7.4e to work (other versions in the 7.x series may work as well -- That would be a question for Tek support). It uses Intel 28F160 chips, and may require a different version of tektool (or, perhaps, tekfwtool) to support.
There's a couple of other things I learned during the recovery process. First, that seven-segment display on the CPU board can convey quite a bit of information. Under normal conditions, with a 'scope which is booted and running normally, the top and bottom segments (a and d, if I remember correctly) should be alternately flashing at about a 2Hz rate.
When the 'scope is in 'Unprotected' mode, the display will count up to numeric 8 and remain there. Parts of it will likely flicker if you start reading or writing processes with tektool.
When you have a 'scope with a mismatched CPU and Acquisition board (Example: 679-4349-00 CPU paired with a pre-B04xxxx acquisition board), you'll get as far as the 'Digital Phosphor Oscilloscope' splash screen, after which it will blank and the alternating flash on the seven-segment display will freeze with one or the other segments dimly lit. This is the TDS equivalent of a Microsoft 'Blue Screen of Death.'
On using tektool or tekfwtool itself: If the tekfwtool -e or tektool -e commands do not show you a series of strings of zeroes when you execute them, your flash is NOT being erased and the program will likely spit back an error code after about a minute. This is a sign that your GPIB setup may not be optimal, and you should try a different arrangement (in my case, I have every reason to believe the switch from a PCI to an ISA-based card is what did the trick).
Keep experimenting, and post your own results (especially if they differ from mine). Hopefully, between this thread and the one on the Tek forums, anyone else who's struggling with a TDS7xx series will be able to come out of it a winner.
Now, if you'll excuse me, I just have to put the outer case back on my unit and return it to its spot on the bench. My thanks to all who replied, here and on the Tek forum.
73 de KC7GR