Ericsson RS 2061
modification to use the UHF amplifier in class AB with SSB
The Ericsson RS 2061 UHF amplifier was originally operational in the analog NMT 450 cellular networks. It is a PA which was used to provide 50 Watt rf, but is capable of delivering more than 100 Watt on 70 cm. (class C)
After modification to class AB in order to allow the use of SSB the useful undistorted output lies around 75 Watt.
The modification described here was "invented" by Ron, DL3BPC / PA3BPC. Ron provided the various pictures as well.
Steps for modification :
The modification of the driver stage is quite simple. Remove the driver stage completely, including the PC board and replace the driver with a M57716 module. The M 57716 is mounted within the box containing the PA transistors.
A picture of the modified PA. Notice the M 57716 in the top-middle. One of the bias circuits can be seen left of it.
A closer look on the M 57716 and one of the bias circuits.
The bias circuit of the other PA transistor.
The schematics of PA-bias and M57716 power supply are given here.
MeasurementsThe gain of the M57716 is approx. 23 dB [linear] and a "clean" 18 W is obtained. [and approx. 24 Watt in compression]
The idle current of the PA transistors is set to 800 mA each. During warm-up of the PA this idle current decreases slowly.
At full power the collector currents of both PA transistors are not equal [ 3BPC measured : 8.5 A + 7.5 A]
The power-added-efficiency of the PA is [at full drive] approx. 35%.
The PA is capable of delivering 100 Watt in compression. For use as a SSB PA a range of 70 - 80 Watt output should be taken as maximum output.
The 14V power supply is quite capable of delivering the necessary power. No voltage drop could be measured under the full load.of 19 A [3A driver + 16A PA]