[ronbryan]

Here is a picture of the underside of the probe pcb, which I have removed from the probe body. The FET is the small white round device at the sharp end. The circuit is slightly different to the published one, as the FET bias pot R152 has been replaced with a fixed 2k resistor, probably selected for the correct source voltage (0.025V to 0.050V).

The FET is shorted as expected, but also some of the bias resistors in the BJT part of the circuit have changed value significantly, particularly the 160 ohm R170 (270 ohms) and the 510 ohm R160 (460 ohms). The 10M gate resistor R140 has also gone up to 12M. I had fun trying to test the FET out of circuit as it kept flipping about - eventually I pushed it into a bit of Blu-Tak to anchor it!

The resistors are tiny Allan-Bradley BB 1/8W 5% hot moulded carbon types about 3.5mm long. I've ordered some old stock replacements, but 510 ohms is still outstanding, so if anyone has a 510 ohm BB resistor I would be pleased to make an offer for it.

It remains to be seen if the BB hot moulded carbon replacements I have ordered are still in tolerance. Alternative 1/8W metal film resistors have a spiral cut to trim the resistance value. I would appreciate opinions as to whether this extra inductance would make a significant difference to the probe performance.

I have made up series-parallel trees of Electrosil C3 1/8W resistors using values that were to hand, to replace the faulty ones so I can test the circuit, with a temporary 2N3819 FET in the front end. The probe pcb seems to work with a 10MHz input, so not much more can be done until the parts arrive. It looks as if a MMBF4416 SOT-23 package will fit on the pcb, provided that source and drain are exchanged and the drain leadout is extended a little, so the challenge will be keeping it in the right place while I solder it in.

Ron