2008-03-10

70 MHz 25 W Power Amplifier Components

Resistors

Resistor Value Delivered Colour code
R1, R3 100 Ω 100 Ω Brown black black black brown red
R2 68 Ω 68,1 Ω Blue grey brown yellow brown
R4 390 Ω 383 Ω Orange grey orange black brown
R5 10 kΩ 10 kΩ Brown black black red brown
VR1 100 Ω, trim pot 100 Ω, trim pot -
VR2 10 kΩ, trim pot 10 kΩ, trim pot -

All resistors are 0,25 W/0,4 W and metalfilm type.

You have to select the value of R1, R2 and R3 according to your application. Under no circumstances may the input power level to the module exceed 100 mW. If used together with the 70 MHz transverter a 10 dB attenuation is required.

Attenuation R1, R3 [Ω] R2 [Ω]
0 Do not mount Short
1 820 5,6
2 470 12
3 270 18
4 220 22
5 180 33
6 150 39
7 120 47
8 120 56
9 100 56
10 100 68

Capacitors

Capacitor Value Marking
C1, C2, C7, C8 1 nF 1n0, small yellow
C3, C4, C6, C9, C10, C18 10 nF 103
C5 10 µF, 25 V, electrolytic 10µF
C11 1 mF, 25 V, electrolytic 1000µ/25V
C12 4,7 pF 4P7
C13 15 pF 15p
C14 27 pF 27 CH
C15 56 pF 56p
C16 18 pF 18p
C17 1 pF 1, black top

All capacitors are ceramic types unless otherwise stated.

Semiconductors

Component Description Delivered
D1, D2 1N4148 or equivalent 1N4148, red
D3, D4 1N5711, HP5082-2800 or other schottky diodes BAT45, blue
IC1 78L05 78L05
IC2 RA30H0608M RA30H0608M
Q1 BC177 or similar BC177

Inductors

Inductor Description
L1 Ferrite bead, small
L2 Ferrite bead, big
L3 122 nH, 5 turns, 6 mm inner diameter, 1 mm enamelled wire, 12 mm long
L4 85 nH, 5 turns, 6 mm inner diameter, 1 mm enamelled wire, 13 mm long

Miscellaneous

Component Description
Metal sheet box 111 x 74 x 30 (or 50) mm3. Schubert Weissblechgehäuse type. Do not use bottom lid.
BNC sockets Two x UG290/U or UG447/U
Copper bands Two pieces each 10 x 30 x 0,5 mm3
Copper foil Mounted under the power module in the 90 µm gap and soldered to the PCB ground
Heat sink Find one yourself suitable for the need
Screws Four M4 8 mm long, use washers on IC2
Feed through capacitors Four x 1 nF, solderable into a 3,2 mm hole

Copper foil

Cut out the foil to the right size leaving a small strip that can be bent and soldered to the PCB between module pin 2 and pin 3. This will improve cooling and grounding.

Changes

On a handful of the modules sold before May 2006 intermittent operation has been seen. This is caused by a crack in the supply track to the first stage. Fortunately it is fairly easy to fix the problem and it is safe too.

  1. Gently remove the plastic cap of the module.
  2. Solder an insulated wire to the 13,8 V DC input pad (right side) as shown on the picture below.
  3. Solder the other end to the pad (left side) as shown on the picture below.
  4. Put the plastic cap back on. No gluing is needed.

Some soldering heat might have to be applied as the heatsink is absorbing a lot of the heat. A 50 W soldering iron is adequate.

Replacement wire between voltage input terminal and first stage. Source: Uffe, PA5DD.

Mitsubishi has somewhat acknowledged the problem, doc1 doc2. But the statement that the surface of the heatsink is to crude is nonsense. The problem is the bimetal bending and design of the track.

I have not experienced the problem myself despite a lot of hour-long meteor scatter QSOs and no copper foil under the module either.

Bo, OZ2M, www.rudius.net/oz2m