On 6/2/2011 9:48 PM, James Ewen wrote:
> If you feed the audio into the mic input on pin 2, you won't be able
> to get enough drive level, even if if you short out the resistor on
> the OT2 output circuit. I have another digipeater running with the
> audio fed into pin 2, and the levels maxed out. It only does about 2
> kHz deviation, but I can decode it about 45 km away. Maybe with a bit
> more drive I would get a better copy, but I need to go out to the site
> and change the pin and settings. It's only about 3 hours east of here,
> so it will be a couple days before I get a chance to get out there
> These OT2 units are sure nice!
Before I left for Dayton (I STILL haven't gotten back home yet!) I ran
into this exact same problem with a Motorola Spectra and an OT1+. I
batted my head against the wall trying to fix it. When I'd crank up the
OT1 tx audio level, the distortion on the scope was horrible, and I
still had only about 2 khz of deviation. The twist was all screwed up also.
I finally got a Velleman K1803 "Universal Mono Pre Amplifier" kit for
around 12 bucks. It's just a simple 1 IC mono preamp that could be
breadboarded in a few minutes, but the kit eliminated the hassles of
rounding up parts, making a board, etc.
I put it between the OT1 and the Spectra Mic In, cranked down the OT1
audio output, which greatly improved the distortion, and set the
deviation with the preamp. I now have plenty of drive, and the twist is
The Motorola mic inputs seem to demand a LOT of audio drive, I guess
that's a relic of the carbon mic days.