I went back to the original unit and am trying to repair it. When I push the degauss button I see signals on the inputs and outputs of the first black rectangular custom IC but only on the inputs to the second black custom IC. The outputs are supposed to be at zero volt baseline and are positive instead. So I am suspecting this IC but have a bit more things to look at.
To your question about the probe itself, I just hooked up a resistor to a DC power supply for a 100ma load and can see this through the probe amplifiers, plus the DC level works. The manual discusses using a special input cable to a signal generator of some sort. There's not a lot to the probe itself so I'd think most of these are good unless the ferrite got cracked from dropping it.
If you debug the degauss circuit you validate a lot of the circuits. The gold wipers on the range switching needed cleaning on both units. Take a thin strip of paper, put it under the wiper, spray a bit of contact cleaner on it and pull it as you press the wiper down on it. You'll see little dark stripes caused by the crud you cleaned off the contact points.
I'm at a point now that I need to compare signals between the two units and look for differences. The original has a jump in DC level with the range switch that it probably shouldn't have (not sure). After the unit's been on awhile a few of the transistor pairs etc. get kinda hot. Wondering if that's normal.
I tried using two blue LEDs for the range switch lamps but one gets biased on a bit when it is supposed to be off. I don't really like the blue, will switch to white leds or back to lamps.