I managed to complete the Mini Mod project with much practical assistance, advice and encouragement from a very kind Forum member!
The capacitors were sourced from RS – the task was made much easier with the product codes to hand kindly posted by Nick. I had the polystyrene ones in stock although they are readily available from ebay. The other components were sourced from Spectrum, very fast and helpful service from Jean.
I was conscious of the potential problem with the BF256 which David has highlighted and was ready to swap over the connections should the unit fail to oscillate. However I was in luck, after connecting a frequency counter to the output I was greeted with a signal. I then connected an audio signal generator to the input and tuned in the radio. Although I was rewarded with a healthy signal unfortunately it landed on a powerful station so I had to adjust L1 to find a clear spot on the dial. I then adjusted L2 using an oscilloscope to find the peak in the signal. The cores are fairly stiff to adjust and I had some difficulty finding a suitable non metallic tool. Eventually I filed and shaped a dental flossing stick and managed to make the adjustments. It is vital to use something with a very square profile, any slight taper will not engage properly with the slots. However as the cores are fairly tight this should mean that they are unlikely to move.
I did note some discussion about frequency drift. Out of interest I connected the frequency counter and observed the readings over time. The results were:-
1.02164 Mc/s at switch on
1.02125 Mc/s after 10 minutes
1.02149 Mc/s after 30 minutes
1.02165 Mc/s after 60 minutes
So that does seem very stable. The unit does give good results although I found it necessary to place it quite close to the receiver. I’ll have an experiment with the length of aerial and possibly an earth connection.
All in all a very neat little unit which gives a good account of itself giving a good balance of component count and performance.