WhitakerAudio J C Whitaker 20 W Stereo Audio Amplifier, Manual

Page: 52 / 194

52
WhitakerAudio
4 Construction Techniques
Most of the construction work on this amplifier involves populating the PWBs, and then
connecting the chassis-mounted devices to the various circuit boards.
There is no mystery to populating a PWB. Having said that, some general guidelines are
worth noting:
• Proceed in a logical manner, usually beginning with the smallest components; these are
typically resistors. Begin at R1 and move through to R x .
• As each component is installed, check it off the list before moving on.
• Confirm the value of each component before it is installed on the PWB. While rare,
incorrect packaging or labeling of components can occur. Therefore, verify the marked
resistor and capacitor values before installation. If you are a bit rusty on the resistor color
codes, check the resistors with an ohmmeter. Capacitors can be checked with a
capacitance measurement bridge. While this is perhaps overkill, it is instructive to see the
variation in component values, which measurement before installation will reveal. The
resistor color code is shown in Figure 4.1.
• Observe proper polarity of devices such as electrolytic capacitors and diodes. A distinctive
pad usually denotes the positive terminal, in the case of an electrolytic. The silk screen
legend may also provide guidance.
• For devices that may dissipate some amount of heat, such as 5 W resistors, allow extra
lead space between the device and the board so as to keep the device above the board. This
will minimize heat-caused damage to the PWB and provide for better cooling of the
device. For resistors in the 2–5 W range, 1/4-inch should be sufficient. For higher-power
devices, more clearance should be provided.
Figures Multiplier Tolerance Temp. Coeff.
(
Ω
)
(0.10 /K)
-6
1
0
2
3
4
5
6
7
8
9
0.01
0.1
1
10
100
1 k
10 k
100 k
1 M
10 M 10%
5%
1%
2%
0.5%
0.25%
0.1%
200
100
50
15
25
10
5
1
Silver
Gold
Black
Brown
Red
Orange
Yellow
Green
Blue
Violet
Grey
White Figure 4.1 Resistor color code.