Dave Robinson WW2R
Introduction
These amplifiers are still seen today on the surplus market. Officially they cover 5925 to 6425MHz but have no trouble producing power at 5760MHz. . My sample was bought at Dayton in 1996, but has been unused since that time. It appeared as a good amplifier for dxpeditioning, but the requirement for a 28V supply soon deterred me. Recent pressures by other roving operations at Microwave update 2004 have forced me to investigate a 13.8V power regulation scheme, and this is what is presented here. No attempt has been made to reduce the size of the amplifier. The following shows the unmodified amplifier internals.
Power Supply
My amplifier was marked as needing 28V Positive ground. Measuring the supply current showed that it took 2.5A at 28.0V. After removing the covers it showed that there was a DC to DC converter module powering the devices. Looking at the markings on the PSU PCB showed that +10.5V and -12V were provided to the amplifier module, although it was measured at +10.5 and -11.8V so that is what the new PSU will be designed for. The currents taken were +3A from the +10.5V supply and 75mA from the -11.8V supply. The same configuration as the power supply for the Qualcom 10GHz Omnitrak Amp was used, with the voltage programming resistors adjusted for the new voltages. Notice that IC2, is uprated to a LT1083CP 7.5A device from the 1 Amp LT1086 used in the 10G version. To give some room for the voltage drop along the supply cables, the unit must be powered from 13.8V. The Power output is unaffected by the slight voltage change. Figure 1 shows the circuit, Figure 2 shows the PCB layout, Figure 3 shows the component overlay. Notice the components are mounted on the track side of the board. Table 1 shows the component values, the voltage programming resistors may need "selection on test" to achieve the correct voltages. Figure 4 shows a closeup of the new regulator circuitry. A pdf of the circuit provided by K2BM is available awp-64100ps.pdf
The existing power supply board was completely removed from its enclosure, removing the 6 screws and the glue holding the transformer in place. Half of the orange tape lining the base of the enclosure was removed by soaking it with acetone. An insulating washer is needed under the LT1083CP. The P package "TO-3P" was chosen for better heat transfer characteristics. The majority of components are SMT. Pin 7 is chopped off IC1, it is not soldered to the track underneath it on the PCB.
The potentiometer which sets the output level actually provides a forward bias current to a diode immediately after the input isolator via the first feedthrough capacitor from the input. Maximum gain occurs when the pin is grounded. This information may allow the user to remove the potentiometer for space saving considerations.
Figure 1: Circuit
