Emulating Old Tektronix Coded Probes

Question

I've got an older Tektronix 2430 100MHz oscilloscope. The older probes are "coded" and according to the manual are supposed to automatically select the proper scale based on the probe type. My last original probe just broke, so I ordered some inexpensive generic replacements. These have a 1x/10x switch on them to select the attenuation built into the probe. Obviously, the 'scope doesn't know anything about the generic probes and always defaults to 1x mode.

Thus, when trying to use the 400mV P-P 1 MHz square wave for calibration it shows up on the display as 40mV P-P (10% of actual value).

Normally, there is an attenuation setting on the 'scope itself to get the scaling right for display, but this this one is either automatic-only or there is a really cryptic setting that I can't find... Sure, I could always do the 1:10 conversion in my head, but it would be more fun to build something that fakes the 'scope out.

Question: Does anybody know how this "coding" works so I can fake it out and make the 'scope think that the probe is a 10x probe all of the time?

Images below for your reference. Thanks for your help!

Connection on 'scope:

Genuine Tek probe (P6131)

Generic replacement probe

Answer 1

According to the manual of my Tek 465B scope, the coupling ring is connected through a resistor to the base of a transistor that controls the illumination of the 1X and 10X LED indicators. A typical probe (P6105) has an 11k resistor connected between the coupling pin on the probe body and ground. Thus when the probe is connected to the scope BNC connector, the transistor is connected to ground through this resistor. This changes its state which then turns off the 1X indicator and turns on the 10X indicator. If you could take the Tek probe BNC (with the coupling pin) and substitute it for the generic probe BNC, you might be able to make the probe sensing work.

Answer 2

I experienced the following on my 2425A scope for the probe coding, with Rp being the resistance between the coupling ring and ground:

Rp               scaling
------------------------
>10k5            x1
8k5 < Rp < 10k5  x10
5k5 < Rp < 8k5   x100
0k5 < Rp < 5k5   x1000

Confirmed on a 2445

Answer 3

If I recall correctly, the pin on the coupling ring shown in your picture of the Tek probe grounds the insulated ring around the connector on the scope to indicate that the probe is set to 10 X. If your probe is sufficiently intact, you could check this with an ohmmeter...

I don't think you could easily make a generic probe switch the indication to match the probe setting, but with a little ingenuity you may be able to make the generic probe force the scope to always indicate the 10X scale. If you don't mind making a small mod to the scope, you could add switches to the scope to change the scale indication, by grounding (or not) the sensing ring.