Finally made progress on fixing the real TDS 380. [Funny how that happens immediately after I upgrade a "For Parts" TDS 340 to a TDS 360.]
There is (hopefully) some useful information here for others.
For example, in the photos you will see that I replaced the DS1644 with a DS1744. I can confirm that this works. (I could not find an answer to that question when I started.) I expect it will also work in the TDS 340 and TDS 360 in place of the DS1644.
Another important fact is that assuming your scope is not damaged (and thus can be recalibrated) you can install a blank DS1744 (or blank DS1644) into the TDS 380 and start the calibration. You don't need to save the data from the old NVRAM. I would prefer to save the data myself but the TDS 380 arrived in
terrible condition. (See the last photo for what I received from eBay!) Note that attenuator damage (such as burned resistors on the ceramic hybrid or over voltage damage to the MAXTEK preamp chip on the ceramic hybrid) will cause calibration attempts to fail.
Next I tried to build a number of voltage calibrators and the Calibration Voltage Reference always failed. (Error message "Cal error, CVR source gain".)
Here are some hints to make it work: Listen CAREFULLY and observe the timing between the individual voltage calibration steps. If it quickly moves to the next step then you know that it has already failed. Make a note of that step and improve your calibration voltage source for that voltage step! Otherwise you will go through all eight steps and not know where the failure occurred. In my case it passed with a battery powered home-made calibrator with a battery powered meter attached. (Not necessarily a good idea to leave the meter leads (antenna) attached but it worked in this case.)
Now you can build a relatively low-cost calibrator. I finally got it to pass when I used a battery (3x18650) and op-amp buffers to drive the 50 Ohm terminator. Each op-amp buffer has an RC filter at the input using a 0.47uF polypropylene capacitor. Note do not use electrolytics for the noise filters due to leakage current. The final step is 15.0mV which isn't very much.
I did not have the ideal rail to rail op-amps available. I really needed low offset voltage and low input bias current precision op-amps that are rail to rail at least to the negative rail on input and output. I did not have those so I had to be extra careful about using low resistance dividers due to the problems cause by the input bias currents. I don't recommend that you use the op-amps I used. If you must build your own calibrator try to make your life easier with a good precision reference and good quality precision rail to rail op-amps that are low input bias current. I recommend battery power.
I used 3x18650 followed by LM7805 followed by a TL431 to generate 2.5V Vref. (The TL431 is not great but that is what I had. Try to get something better.)
Then I used a resistive divider, RC filter and op-amp buffer to generate 1.5V from the 2.5V Vref.
Then I did the same with a resistive divider, RC filter and op-amp buffer to generate 0.3V from the 2.5V Vref.
I could not successfully generate a sufficiently stable 80mV and 15mV from the 2.5V Vref.
So I used the buffered 0.3V with a low impedance resistive divider, RC filter and op-amp buffer to generate 80.0mV.
Finally I used the buffered 80.0mV with a low impedance resistive divider, RC filter and op-amp buffer to generate 15.0mV.
I could not get it to work with relatively low noise HP supplies, resistive dividers and RC filters. I suspect it was a combination of going to battery power and op-amp buffers (with RC noise filters) which made it finally work.
I am not saying my calibrator is the way to do it. Just that it worked and I was able to build it without buying anything. I am sure there are much better ways to do this, especially if you are willing to buy more expensive parts and/or equipment.
I have attached pictures. Note the last two pictures. The second to last is this TDS 380 as it is today. The last picture is the TDS 380 as it arrived from eBay! (With apocalyptic damage done inside by someone else.)
I placed this here in this thread because I believe it might possibly help others calibrate a TDS 340 upgraded to TDS 360, for example. Such as the "For Parts" TDS 340 that I just upgraded to TDS 360.