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PRC-103 Bias Setting




 
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KA3EKH
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« on: September 10, 2018, 09:21:48 AM »

The common assumption is that I should know this but looks like I got no clue. I am working on a little back pack transceiver that develops around tem watts of AM or 20 watts of SSB for operation on both 80 and 60 meters. I use this radio a lot for field operations. The radio has two outputs, one 50 Ohm output that works well with inverted V and stuff like that and provisions for a whip antenna attached to the radio. The 50 Ohm output connects to an input for the radios internal antenna tuner that has ten presets (one for each channel) and that connects to a seven foot vertical antenna attached to the side of the radio. I have been trying to get the antenna tuner set for the channels I have installed in the radio but in the process have done damage to the PA deck. Ok, so I repaired the PA deck and its working again but this is the question that I have. The PA deck has two stages with the first being a driver in push pull and the second being a power amplifier in push pull. Both stages have a bias pot that allows a small level of DC, maybe under 2 volts to ground depending on the setting of the pot to forward bias the amplifiers. I thought this may affect gain of the stages but apparently has almost no effect on the output, maybe a watt at the most. What are these bias pots for? How do I set them? Were they intended to set idle current for the amplifiers? Are they used for adjust linearity? Or distortion?
This radio was built by Sunair back in the seventies for the USCG to operate on HF AM/SSB before VHF took over everything, at least in portable operation. All runs from one twelve volt gel cell and have been using it for a while now on the 60 Meter USB M&S Net and want to use it at a lot of the Military vehicle shows on 3.885 AM so that why I am doing all this damage to it to get it working with a whip.




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KD6VXI
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« Reply #1 on: September 17, 2018, 09:04:14 PM »

Set the pots for lowest crossover distortion on a single or two tone test.

That will correspond to around   .7 volts on the base.

--Shane
KD6VXI
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