2) Doesn't the label above the CRT tell you whether it is a PM3394A or PM3394B. My "B" model is clearly marked, and besides, the "B" model also says
"25 GS/s repetitive sampling," unlike the "A" model.
The model number is printed on the front panel and is shown on the status screen as demonstrated in the photo HighVoltage posted above.
There is no ambiguity - if there is just a plain model number without "A" or "B" then it is one of the original series, if it is a newer 'scope the model number will end in "A" or "B"
That should be the case, unless the scope has been upgraded by some user. PM3394, PM3394A and PM3394B have clearly different labels.
The two model numbers could be different if someone has replaced the CPU card with one from a later 'scope - this won't "upgrade" the 'scope as such - except that there were supposedly bugs in the CPL interpreter which made some things not work over RS-232 which weren't fixed until the "B" version. I don't have the exact details of what the bugs were.
I'm not actually sure what "upgrades" are possible or meaningful for this series of instruments. They are actually fairly tolerant of swapping boards between the series although there were minor changes in the schematics over time. In particular the A/B version of the CPU card is quite a bit different to the original and the EEPROM's are soldered in so it isn't possible for the user to change the firmware. There were rumours and hints in the manual that a PC program existed for firmware updates but I have never seen even the slightest clue as to a copy on the 'net.
There were also minor front panel changes between the original and A/B series so I would only really suggest putting a "B" CPU card into an "A" chassis to get the firmware updates.
They are also pretty tolerant of swapping the A8 board between 'scopes. If you put a 100MHz A8 board in a 200MHz 'scope you get a warning (and vice-versa) but it works. In theory the A8 board only differs in a few components between the two but the only attempt that I made to "upgrade" a 100MHz A8 board to a 200MHz one ended in disaster. I think I know what I did wrong but don't have an inexhaustible supply of boards to wreck getting it right. In any case if you change the ID resistors it will go in fine without any error and can be calibrated to the 200MHz spec so there isn't all that much point.
It isn't feasible to upgrade a 100MHz chassis to 200MHz bandwidth - the front end has a relatively small number of component differences (but a lot more than the A8 board) but the CRT, delay line and final X/Y amplifiers differ physically.
Swapping the CPU card to gain the maths package or GPIB would be reasonable but the supply of faulty 'scopes has dried up (at least on eBay) so in practice unlikely. You can't just populate the empty GPIB hardware on the CPU card as the firmware on a non-GPIB version will not know how to drive it (as well as the hardware interface here is an entire command interpreter as well).
The only other upgrade which is feasible in practice would be to increase the amount of DSP/acquisition RAM on the A8 card but, again, I tried this and the card was unstable afterwards. Not sure why.
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