1159A differential probe mod/update

[Someone]

The older active probes from Agilent are no longer supported on their newer scopes, and plugging a 1159A differential probe into a 3000X the scope pops up an "unsupported probe" message requiring the probe to be removed before proceeding. Having already checked that the probe would be electrically compatible (not using the variable 3-6V supply pins) it was a little disappointing so it was time to add some modifications to get the probe operating.

Inside the inline amplifier there is a small PCB stack with a recognisable manufacturers mark:


Preamble Instruments being well known for their differential amplifiers and supplying some of the differential magic here, the probe connected to the right with two coax runs going to the hand piece. Flipping over to the other side of the stack:


A small CPU (under the thermal pad is a DAC) but more interestingly an array of LEDs and two buttons fitted among space for possibly more. And although the 2 wire communications are run from the scope up to the amplifier assembly, the scope still identifies the probe when they are disconnected. So it was off to the BNC shell for a peek:


Here there is a 24C021 identification EEPROM and more importantly the ID strapping resistors off to the right. The lower resistor can be left as 10k ohm, but the blue resistor at the J2 marking needs to be changed from 47k to 15k so the probe will identify as a 10:1 probe into 50 ohm termination. I also pulled pins 4 and 6 out of their sockets in the shell so that the 2 wire interface is disconnected.

Now that the probe configures the scope and operates, what of those mysterious buttons? The N1025A version of this probe for spectrum analysers has manual controls. The good news is they're still functional as the offset control, so a few holes in the case later and I've got a mostly complete probe:


Although its stuck with the 10:1 attenuator as a fixed ratio as the auto probe ID cannot be overridden on the scope the result is still far more useful than anticipated.

[TheSteve]

Thanks for posting this, I have an 1159A sitting here that might get modded(or it will be sold).
I wonder if we can reprogram the 24C021 to a compatible model like has been done with other probes.

More then anything I think we should all be asking Keysight why they failed to add support for the probe in the 3000x series.
Perhaps we should all send a PM to Daniel and ask why it isn't supported, maybe there is a legit reason.

[Someone]

Thanks for posting this, I have an 1159A sitting here that might get modded(or it will be sold).
I wonder if we can reprogram the 24C021 to a compatible model like has been done with other probes.

More then anything I think we should all be asking Keysight why they failed to add support for the probe in the 3000x series.
Perhaps we should all send a PM to Daniel and ask why it isn't supported, maybe there is a legit reason.

Its understandable that they didn't want to test/QA all the probes in the catalog going back to dot for the windows CE platform and I'm not sure if the 1159A runs on the recent windows scopes as its not mentioned at all in modern documents (lecroy don't have this problem by only having the single platform?). I find it annoying that updates and new options are coming along for the 3000T but they aren't being offered for the 3000X, so it looks like their software support timeline is very limited (as it has been historically) despite moving more towards a recurring revenue (software sales) model.

Even if we put in a custom EEPROM image it will still only recognise a single attenuation and termination setting, the magic happens when the scope asks from the amplifier CPU what attenuator is attached and automatically changes the attenuation. Or requests the auto DC balance (offset) direct from the scope, but that still seems easier with buttons!