[mictester]

My Minimod - built using a J301 FET and a 2N3904 for the output transistor, using the coils from Spectrum Comms - drifted about 400Hz in 10 minutes from switch-on. I thought that I could try some means of stabilisation to eliminate the little bit of FM, too.

I built a "huff & puff" CMOS stabiliser that locks in 1kHz steps. It worked OK, but used three CMOS ICs and a crystal oscillator module, which seems like overkill for a little, simple AM modulator! It was weird tuning the VFO, as it went in jumps (usually bigger than 1 kHz, since it's difficult to tune that slowly).

My next effort - and the one I'm using now - used an MC145106 PLL IC (scavenged from an old, broken CB radio) and a 4608 kHz crystal for the reference. It can now be tuned in 9 kHz steps over a small range - from about 970 kHz to 1170 kHz - using a green LED as the varicap. Frequency is set with an 8-way DIP switch.

The other addition I've made is a quad op-amp (TL074) and a FET to limit the audio bandwidth and provide compression and clipping (to get rid of over-modulation). The little box now accepts pretty much any audio source without twiddling the levels!

The whole thing is powered from a "10V" DC adaptor, which actually gives well over 12V off load, through a 7809 regulator. The useful range is about 15m through two walls, using about 2m of dangly wire as the antenna!

I suppose that I've gone away from the original - rather nifty - concept of a cheap and simple AM modulator to feed old receivers, but it's been fun adding bits to it to improve its performance!