Communication Concepts AN779L - Manual (Page 2)

Revised 07/15/05
2
Install R7 (47 ohms, ¼ watt, yellow-violet-black). Place an approximate ¾” length of
insulating tubing over each end of the resistor before inserting through T2. After inserting
the resistor leads through T2, place the beads over the leads and insert the leads into their
proper holes on the PC board and solder.
Install R12 (130 ohms, ¼ watt, brown-orange-brown). Place an approximate ¾” length of
insulating tubing over each end of the resistor before inserting through T3. After inserting
the resistor leads through T3, place the beads over the leads and insert the leads into their
proper holes on the PC board and solder.
Install RFC5 (VK200 19/4B)
Install C1 (39pf silver mica)
Install C2 and C3 (680pf ceramic disc)
Install C4 and C10 (220uf, 4 volt, tantalum teardrop) The black bar is the positive lead.
Install C5, C7, C11 and C13 (0.1uf ceramic disc)
Install C6 (56pf silver mica)
Install C8 and C9 (1200pf ceramic disc)
Install C12 and C14 (10uf, 25 volt tantalum teardrop) The black bar is the positive lead.
Install Q1 and Q2 (MRF476). Q1 and Q2 should be matched to prevent unbalance and
overheating. Prepare the MRF476’s for mounting by trimming off the center (collector)
lead close to the body of the transistor. The mounting tab will provide the electrical
connection for the collector. Mount the transistors using the small heatsinks and 3/8” 4-40
screws and nuts. Before tightening the mounting screws, ensure the heatsinks are
pressed against the diode D1 to provide thermal contact. Better thermal contact can be
achieved by using thermal compound or silicon grease at these junctions.
Install Q3 and Q4 (MRF475). Q3 and Q4 should be matched to prevent unbalance and
overheating. Prepare the MRF475’s for mounting by trimming off the center (collector)
lead close to the body of the transistor. The mounting tab will provide the electrical
connection for the collector. Mount the transistors using the small heatsinks and 3/8” 4-40
screws and nuts. Before tightening the mounting screws, ensure the heatsinks are
pressed against the diode D2 to provide thermal contact. Better thermal contact can be
achieved by using thermal compound or silicon grease at these junctions.
The AN779L amplifier should be powered by a regulated 12.5-volt power supply for best
bias stability.
The amplifier will run hot when running CW or heavy SSB. The maximum output in the CW
mode should be limited to 8 to 9 watts. A small 12-volt fan mounted at the end of the
amplifier so that the air is blown across the heatsinks will help cool the amplifier and
increase the life of the transistors.