Hi All:
I saw Mike Garrett's post about the sympson 260 meter modification and
thought I might help.  The chopper circuit used by SFB in its 260
modification used a unijunction transistor for the oscillator and a chopper
transistor who's number I don't remember at the moment.  It also used a 4
lead audio amplifier made by motorola.  I Think all of these parts are
obselete at this time, but the circuit could gbe modified using modern
parts.  A fet llike the mpf102 could be used for the chopper and either a
transistor oscillator or even the 555 timer would work for the oscillator.
The lm386 would work well for the audio amplifier.  I think I have the
circuit for the old meter reader, so I'll try to dig it up and put it on the
list.
In the sympson, the meter was removed and it was replaced with a resistor.
I think the simpson used a 50 microamp meter, and it was removed and a 1500
ohm resistor was put in its place.  The chopper circuit was connected across
this resistor.
The chopper circuit was mounted in the area formerly occupied by the visual
meter.
Another method that was used was to leave the visual meter and build the
chopper circuit in a separate box.  Then the chopper circuit was connected
across the terminals the terminals of the visual meter.
I dont remember which issue of btp the article was in.

Tony McCloskey, WA3CAO