Diodes:
Diodes D5 and D6, see Figure 5, Page 6, are located on
the black finned sinks with the output transistors. Special
care is required to insulate these diodes from the heat
sink while providing good thermal transfer from sink to
diode. Heat shrinkable tubing or “spaghetti” should be
placed over the diode and connecting leads; the diode
should be securely clamped to the heat sink with the clamp
provided.
Check Transistors and Diodes:
Defective transistors and diodes may be located by use
of an ohmmeter. Polarity of the ohmmeter must be verified
before these checks are made.
With a known diode orientation, measure the diode re-
sistance in the forward and reverse directions. The lowest
meter reading will establish the probe at the cathode end
(schematic symbol arrow points to cathode) as the “minus”
probe while the other probe will be “plus.” Some ohm-
meters are not polarized in this manner with relation to
“volts plus probe” and “volts minus probe.”
To check transistors, the ohmmeter should be set to the
100-ohm or 1,000-ohm scale. Small signal transistors (Q1
through Q3) must be removed from the circuit before test-
ing. Transistors mounted with screws (Q4 through Q9) may
be tested in place; however, the base and emitter leads to
these transistors must be removed.
If all conditions in the above table are met, the tran-
sistor may be considered free of any gross defect; if any
of the following conditions are not met, the transistor
should be replaced. See lower right corner of circuit dia-
gram, Figure 7, Page 9 for transistor terminal code,
With the ohmmeter “plus” probe on the anode end of a
diode, and the “minus” probe on the cathode end, the
ohmmeter should read approximately 2,000 ohms or less.
With the meter probes reversed, a reading of about 10,000
ohms or more should occur. If either of these conditions
is not met, the diode should be replaced.
Power Drain Resistor (R
SEL
):
The following condition may occur after replacement
of driver transistors Q4 and Q5, output transistors Q6
through Q9, or diodes D5 and D6. If the unit appears to
operate at an excessively high temperature or thermally
recycles after about 10 minutes with no signal input, the
“cold” standby power drain may be excessive (up to 20
watts higher than normal; a higher power drain indicates
further circuitry problems). The nominal power drain under
these conditions should be 40 watts; if the measured figure
exceeds 55 watts, insert a resistor (R
SEL
) in parallel with the
56-ohm resistor between the base of transistor Q4 and the
junction of diodes D5, D7 and D8 (figure 7, page 9). R
SEL
(½ W, 10%) should be either 180 ohms to decrease the
power consumption by 20 watts, or 82 ohms to decrease the
power consumption by 30 watts.
Ohmmeter Connections
-
-
-
“Plus” Lead
“Minus” Lead
Collector
Emitter
Emitter
Collector
Collector
Base
Emitter
Base
Base
Collector
Base
Emitter
Ohmmeter Reading
N.P.N.
P.N.P.
Transistor
Transistor
High
High
High
High
High
Low
Low
-
Low
High
Low
-
Lamp Replacement:
The lamp 12, Figure 6, Page 7, which provides illumina-
tion of the front panel controls, is soldered and epoxied
into the molded plastic lamp socket in order to meet Un-
derwriters’ Laboratories requirements. The assembly may
be removed by unsoldering the lamp leads at the terminal
strip and removing the screw that holds the socket to the
chassis.
FIGURE 6.
7
