Mystery (to me) bnc probe ends

[super7800]

I recieved (a bunch, 50+) of these probes with a local purchase of two tek 7904a oscilloscopes. I would have just assumed they where custom to an application, as searching the part number turned up nothing, but I did recieve one awhile back with a previous purchase, so perhaps they are not uncommon, and I am simply unaware of them. There seems to be two varieties, one of which seems to inject what I assume to be -15v into the signal through a resistor.

My question is has anyone seen these before or know of their uses? Perhaps to probe points on a custom pcb?

[vk6zgo]

I think it might be something to do with the "pre-charge" function of the 7A19 plugin.

https://w140.com/tekwiki/index.php/7A19

[super7800]

Interesting thought, but I'm unsure as to how the connector shown relates to the precharge function. Are you referring to the "yellow connector" on one of the types of probes? Thanks

[vk6zgo]

Interesting thought, but I'm unsure as to how the connector shown relates to the precharge function. Are you referring to the "yellow connector" on one of the types of probes? Thanks

I was thinking more about your comment re "injecting -12v into the signal."

[David Hess]

That looks like a homemade low-z offset probe, which is something that would definitely be used with a 7904.  Low-z probes are simple and high bandwidth, but their low input impedance can upset the DC operating point of some circuits.  An offset probe prevents this by presenting a high DC impedance at a voltage other than ground.  They were commonly used for ECL.

https://w140.com/tekwiki/wiki/P6230

An offset probe can be as simple as injecting a current into the dividing junction of a low-Z probe.