-Pat, I agree with you and think I may go with the original layout, those guys at HP really did know there stuff, I think I would be kidding myself if I could improve it. the miniKits heater can pump out about 3 watts or so quite well, the SMD chip is roughly like a 3/4 TO-220 and takes quite a bit of soldering to get it to flow properly, I made a little "resistor oven" for some crude resistor temperature co-efficient measurements on some low temp co vishays, it worked well there. The photo below shows a resistor (leads sticking out with Kelvin clips) held in a brass U shaped 'oven' soldered onto the back of the MiniKIts. The resistor is held in place with a bit of foam and a plastic clothes peg (very technical!) The brown cable is a temperature probe.
- I haven't cleaned the gunk off and retested the resistor yet, I think it is a carbon type, I wonder if the 'short' may be the foam plus also some capacitor 'goo', the PCB had some corrosion in that vicinity. I am going to replace the lot with I believe equivalent parts.
-Gyro, thanks for the comments re the thermistor, I have not used them before , so the comments appreciated.
I was handling the crystal very carefully so only recently turned it over to find the label , the crystal / oven needs to be at 65.5C (written on the crystal as was previously suggested
)