I never questioned the quality or the user friendliness of the Hickock units, just that their patented approach means that its not as straightforward as a pure DC tester.
Work was started on an electrical prototype for the grid regulator and current measurement unit, the functionality of the grid regulator already being verified in a different prototype.
The other protorype was unsatisfactory, in therms of leakage, power busses ran parallel to the grid current sense circuit, so to en order to avoid allot of problems with leakage a new prototype was needed.
When final electrical testing is complete the last step is to simulate a shorts from G2 to G1 to see if the circuit survives reverse polarity.
One goal was to get the entire bandpass filter, grid regulator oscillator and current sense circuit on one PCB, considering the limited space of a 100x160mm PCB SMD becomes ever more tempting.
The floating current measurement uses an 1k resistor that gives an burden voltage of 200mV full scale, i think i need to take a good look at the RC filter on the input, to avoid the µA meter being influenced by the grid signal applied by the transconductance measurement 1400Hz signal.
Heres some photo's of the first and second prototype, and mechanical prototype.
The mechanical proto has a 10 turn pot for setting the grid voltage, 10 turn pot for calibrating the Gm measurement. One display displays the grid voltage, the other displays either grid current or GM. the aluminium was botched slightly.
The switch switches the bandpass filter between CAL and measurement mode. Because experience shows that contacs become bad after some years all the measurement circuits will use Reed relays for the signals.
The BNC connector is for feeding the measurement signal into the module, this to avoid running long traces to the rear connector, Keeping all the delicate circuits on a module that produces nearly no heat makes sense from a stability standpoint, as styroflex capacitors have a temperature coefficient of around 100ppm
A friend of mine is prototyping a 0-400v flyback module that operates from a 24V rail, perhaps a good idea to see if i can get a PCB to experiment with.