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Sunday, October 20, 2013

BITX Build Update #15 -- Necessary Mic Amp Mod

I was having trouble with the mic amp on my new Manhattan Discrete BITX 17.  I had it wired as per the schematic, but it just wasn't working right -- I was getting very little DSB out of the balanced nodulator, but everything worked fine if I 1) unbalanced the bal mod or 2) injected audio (from a sig generator) directly into the audio in port of the balancec modulator.  Clearly something was wrong in the mic amp circuit.  I noticed the collector voltage seemed quite low.

Some quick Googling revealed that others had struggled a bit with this problem also.  Nicolae's note was especially illuminating and useful: 

As Nicolae noted, the 10K value may have worked with lower gain transistors.  We must remember that BITX was (very admirably) designed for minimal cost and maximum use of scrounged parts.

I swapped the 10K resistor for a 39K and all is right with the world.  The band is not yet open here, but I bravely called a European aeronautical mobile station, hoping that he would be my first QSO on this rig.  I think he heard me, but no QSO (yet!).

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