[Elecraft] XV50 PA Bias

[Don Wilhelm]

Dave,

The PA bias control usually has much greater range than is needed, and 
the PA bias input should draw very little current.
Check the resistance of R39.  Is there a chance that you swapped R39 and 
R10 during the assembly?

73,
Don W3FPR

On 10/24/2014 6:46 AM, Dave Sergeant wrote:
> I am aligning my new XV50, latest build using the RA30H0608M module and
> having issues with setting the quiescent bias of the pa as per the
> manual. I see others have had this same problem but I can find no
> answers.
>
> The manaul calls for R39 to be adjusted for 20mV on the DMM across
> TP3/4 (across the 20mohm current sense resistor R17). With R39 at its
> maximum I get 16mV corresponding to a total current of 800mA as against
> the required 1A. This is confirmed by the digital current meter on the
> 12V psu powering it and my K2, which increases from 0.5A to 1.3A when I
> enable transmit on the XV50.
>
> Before anybody asks, I DO have link W1 fitted, and in fact I have the
> correct bias voltage on pin 2 of the output module. However there is
> where I suspect the problem as the maximum voltage achievable there is
> 4.7V.
>
> I found a datasheet of the RA30H0608M, albeit a preliminary one which
> has a few sheets 'in preparation'. It does not give any figures for
> standing bias versus the bias volts but suggests it must be somewhere
> between 4.5 and 5V (there is no power output below 4.5V). The voltage
> at the R29/R39 junction is limited by the current flowing through R28
> and the module bias current (typically 1mA). So I am wondering whether
> some rf modules need a bit more than 4.7V to get the correct bias
> current of around 1A. Maybe reducing the value of R39 (56 ohms) is the
> answer. Or is mine near enough for good linearity?
>
>