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Old 12th Sep 2018, 10:34 am   #16
Ian - G4JQT
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Join Date: Jun 2004
Location: Reading/Fakenham, UK.
Posts: 1,320
Default Re: Building the 'Minimod' AM modulator-questions!

Looks good Nick. Layout is not particularly critical. The coils are screened (or should be if you earth the covers!), but the LM386 can get a bit 'feisty' if the decoupling caps are not located close to the pins.

As the article describes (found at first link in post one above) there is a small amount of FMing, mostly due to there being no VFO buffer or VFO voltage regulation, just to keep the circuit simple. In any case it is completely undetectable in normal AM use. However, it is detectable on strong modulation peaks if you listen on an SSB receiver.

To keep things simple, I suggest the fet to use is the BF256. Spectrum Communications is still operating (I think some one in this thread recently bought the coils from them), so they can supply the 90uH TOKO 10mm coils and also the BF256B fets. They sell the majority of the other parts, including the LM386 and BC109 (although any equivalent silicon NPN transistor will do).

I have no commercial connection with Spectrum Communications, but the proprietor sells lots of commonly used components at very reasonable cost. His website components list is dated 5 August 2018. Here: http://www.spectrumcomms.co.uk/Components.htm

If you look on this forum there is at least one other MiniMod thread. The longest one is here: https://www.vintage-radio.net/forum/...hlight=minimod

Regards,

Ian
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