Instruction Manual

Revised 07/15/05

CONSTRUCTION HINTS FOR THE MOTOROLA AN779L DRIVER

These instructions are only meant as guidelines in constructing the AN779L amplifier. They are

meant to be used with the Motorola Application Note 779.

Install R1 and R4 (10 ohms, ¼ watt brown-black-black)

Install R2 and R3 (30 ohms, ¼ watt, orange-black-black)

Install R5 (240 ohms, 3-watt nominal) and R6 (100 ohms, 3-watt nominal). Mount the resistors

approximately 3/16” above the PC board, as these resistors will run hot. These values may have

to be adjusted to allow for the differences in the voltage drops in D1 and D2.

NOTE: The recommended values of 82 ohms for R5 and R6 in the application note

tend to increase the bias voltage for the RF transistors. This causes the RF

transistors to run hot at idle. If they get too hot, they may enter thermal runaway and

destroy the transistors. Values of 240 ohms for R5 and 100 ohms for R6 appear to

eliminate this problem. The supply voltage should also be reduced to 12.5 volts. No

noticeable change in the amplifier's performance was noted except for the reduced

power output caused by reducing the DC power supply from 13.6 to 12.5 volts.

Install R8 and R11 (6.8 ohms, ¼ watt, blue-gray-gold)

Install R9 and R10 (15 ohms, ¼ watt, brown-green-black)

Install RFC1, RFC2, RFC3, and RFC4 (10uH)

Install D1 and D2 (1N4001). Mount the diodes 1/16” above the PC board.

Install the two sets of double beads, which mount in the DC power lines. The application

note says to mount the beads under the transformers T1 and T2. However, there is not

adequate space under the transformers unless one files one side of the beads flat. An

alternate method is to mount the beads on the bottom of the board. The PC board is then

mounted on spacers high enough to clear the beads.

Install T1 by soldering the ends of the transformer to the PC board in four places. Use just

enough solder to ensure that the transformer is mounted solidly to the PC board. Then

install the transformer windings leads in their proper holes and solder. HINT – mount the

transformer T1 centered on the PC board foil pads.

Install T2 by soldering the ends of the transformer t the PC board in four places. Use just

enough solder to ensure that the transformer is mounted solidly to the PC board. When

positioning the transformer for soldering, insert the center tap lead into its proper hole but

do not solder until after the transformer has been soldered into place. Install the

remaining transformer winding leads into their proper holes and solder all leads including

the center tap. HINT – mount the transformer T2 offset on the edge of the PC board foil

pads toward the RF Output side of the board. This will facilitate the mounting of R7 and

the output transistor heatsinks later.

Install transformer T3 by soldering the transformer in four places to the PC board. Use

just enough solder to ensure that the transformer is mounted solidly to the PC board.

Then install the remaining transformer windings leads in their proper holes and solder.

HINT – mount the transformer T3 offset on the edge of the PC board foil pads toward the

RF Input side of the board. This will facilitate the mounting of R12 and the output

transistor heatsinks later.

PAGE 1
Revised 07/15/05 CONSTRUCTION HINTS FOR THE MOTOROLA AN779L DRIVER These instructions are only meant as guidelines in constructing the AN779L amplifier. They are meant to be used with the Motorola Application Note 779. Install R1 and R4 (10 ohms, ¼ watt brown-black-black) Install R2 and R3 (30 ohms, ¼ watt, orange-black-black) Install R5 (240 ohms, 3-watt nominal) and R6 (100 ohms, 3-watt nominal). Mount the resistors approximately 3/16” above the PC board, as these resistors will run hot.
PAGE 2
Revised 07/15/05 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.