[TenTec] 1220 Audio and PL Fix.
Mon, 05 Jun 2000 19:51:24 -0400
My 1220 2 mwter transceiver was plagued with an excessively loud PL
(CTCSS) tone and somewhat weak transmit audio. I had tried extra grounds
on the power amplifier assembly and some extra grounding on the VCO
shield can. I even added extra shielding and grounding here and there
in the form of foil tape. These measures helpd a little bit but didn't
really correct the problem.
I reasoned that the CTCSS tone was getting into the VCO by some path
other than the intended one. In playing around with bypass capacitors,
I discovered that I could improve the transmit audio AND reduce the
CTCSS tone amplitude by increasing the value of C36, which bypasses the
collector of Q14 to ground near the VCO. This fix is best accomplished
by soldering a .01 uf cap across the C36 pads on the underside (foil
side) of the board. Make the ground lead a little longer so that it
reaches from the ground pad to the bottom VCO shield to provide another
ground point for the shield.
By the way, the 1220 works extremely well on packet if the receive
audio is taken from the detector via the DIN connector and not the
speaker line. I had to modify my KAM TNC to make it more sensitive, but
this mod is provided in the QRZ CD ROM. After switching to detector
output, I was able to work packet nodes reliably that I could never
even connect to in the past.
73, Bob WB2VUF
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