Quote:
Originally Posted by Bazz4CQJ
I've been trying to find the explanation why Ian decided to leave the 2N3819 for the BF256 but haven't found anything. Can anyone (perhaps Ian) advise? I've got genuine 3819's sitting in the junk box as a well as quite a few 2SK19, which were popular in FM tuners and have served me well as a general purpose J-FET.
Thanks
B
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I explained it in one of the threads, but can't remember which one now.
The reason I changed the 2N3819 for a BF256 was because after making about 30, I had one or two that gave poor modulation. I tracked this down to insufficient drive to Q2. It seems the performance characteristics of the 2N3819 are quite wide, so some just weren't up to the job. I expect J310s would do but I haven't tried them.
I also found that changing Q2's base resistor from 10k to about 3k3 also slightly improved things using the BF256.
Like most circuits the design is generally fairly tolerant of component values (+/- 20% shouldn't be a problem) except perhaps C1 and C13. Any transistor equivalents should work, but be very mindful of the lead configurations!
I did find that mica or good quality ceramic capacitors used at C1, C2 and C13 gave a bit more output than polystyrene ones. (Cheap disc ceramics used for C1 or C2 might introduce temperature-dependent frequency drift.) See this post:
https://www.vintage-radio.net/forum/...&postcount=150
Also remember to earth one side of the unused winding of L1 to aid frequency stability. The boards supplied by Peter take care of that.
After the ones I made and sent out, I wonder how many have been built in total?
For anyone not familiar with the simple MiniMod initial setting up, I attach again the instructions I sent out with the ones I built.
Ian
PS: I'm glad LEDs now have low enough current not to have any significant battery drain. I think in 2011 when I published the design such LEDs were a bit pricey so I didn't bother.