Solid State PA Bias pot

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KA3EKH:
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.

DMOD:
If you can please point to a link for a schematic some here might be able to help but crystal balls are few and far between.  :)

For example, Sunair model what?


Phil - AC0OB

WBear2GCR:
It's either a bias pot or a balance pot.

In the case of the bias pot, it will set the quiescent current through the output transistors, setting
them for lowest distortion, probably. Depends entirely on what class of operation it is in. IF it is AB
(which seems likely) then it would be setting a minimum current to prevent crossover distortion. I'd
look at what transistors are in there, and look at the data sheet, there's almost certainly a current
level suggested for "typical" operation. Set it there.

                          _-_-

KA3EKH:
Ok, this is a part of the schematic. The Amplifier has two pots R1107 for the driver and R1108 on the PA Looking at them I am assuming they are bias and not balance? but you tell me. was going to play around with them and see how they affect splatter with modulation and if its them that drive the amplifiers into Class AB. But that's why I am  asking here to see what the "Greater Minds" think. working with solid state amplifiers in AM/SSB is a new experience for me.

DMOD:
Both pots adjust BIAS and are not balance pots.

I would think the manual has a procedure and or a voltage chart for adjusting those to a specific bias voltage at the bases of the transistors Q1101 through Q1104.

It appears this circuit is a linear amplifier so proper adjustment is critical for least distortion.

Upon closer inspection it appears Q1101 and Q1102 require 0.7 Volts on those bases and Q1103 and Q1104 require 0.6 volts on those bases.

I would only adjust those pots if those voltages are off spec as measured with a good DMM.


Phil- AC0OB

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