[Elecraft] XV50 PA Bias

[Dave Sergeant]

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?

Any help welcome.

73 Dave G3YMC

http://davesergeant.com