Update – I tried adding the 40MHz range using a small tapped coil mounted directly on the range switch, but found that the maximum frequency obtainable was some 25MHz due to self-inductance of the leads from the circuit board to the range switch. In order to reach higher frequencies I think you would have to replace one of the coils on the circuit board and use very short leads (or hard-wire the board for one range), or alternatively, build a single-range unit. While I had the box open I did undertake a few modifications to increase the span of each range slightly and to improve tuning precision, stability and repeatability:
– replaced R4 (22k) tuning pot with 20k high-quaility 10-turn pot
– replaced R3 (1k) fine-tuning pot with a high-quality 220ohm pot
– replaced R2(4.7k) with a 3.3k resistor
– replaced R5 (33k) with 15k resistor
– rewired the on-off switch such that both the wobbulator and the DFM switch on together (I found I often left one of the units on and ran the batteries down! – also, it was a nuisance having the wobbulator on-off switch on the tuning pot as the frequency had to be re-adjusted every time I used the unit.