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In servicing the equipment, the first step, of course, is to check for obviois defec ts such as
lack of B plus, defective tubes and so forth. After these preliminary tests h;ve teen made, the
Heathkit RF Signal Generator is used as an audio signal soui ce by pluggingthe output cable into
the "AF-OUT" jack.
The level of the audio tone obtained is adjusted by the ' AF IN-OUT" con¬
trol.
The MODULATION control should be set in the "INT. " position.
Check the operation of the output stages by connecting the ground lead of the autpat cable to the
chassis of the amplifier or circuit ground and touching the ' hot" lead of the output cable to the
grid of first one output tube Point Ain Figure 16 then to the ather output tube Point B in Figure
16.
All of these tests must be made through a .05 pfd condenser. In this fat hion the operation
of each output stage is checked individually.
As each grid is touched, a clear audio tone should
be heard in the loudspeaker.
Next, the operation of the phase inverter stage can be checked by transferrii g the hot lead (to¬
gether with a .05 pfd condenser) to the input of this stage (Point Cin Figure 16). Again, a clear
signal should be heard from the loudspeaker. The tone should be louder than l efore. If tcoloud,
the volume can be reduced, using the "AF IN-OUT" control.
The second audio stage is now checked by transferring the hot signal generator lead to the input
of this stage, Point D in Figure 13. If this stage is dead, no more tone will be heard in the loud¬
speaker. Having isolated the trouble to a specific stage, it is now a simple math r to check DC
operating voltages and parts until the defective component is found.
As each amplifier stage is checked, work back towards the input stage (first auc io), a definite
increase in volume should have been noted. It may even be necessary to reduce th ; volume using
the control mentioned.
This, then, provides an additional test technique. A weak stage can be
identified if little or no increase in volume is heard as the "hot" lead is transferred past the
stage.
Be careful when interpreting the resulls of this test, for overloading a stage may cause
apparent loss in volume even when the stage itself is in good condition.
Th are! are, when at¬
tempting to isolate a "weak" stage, be sure to keep the signal level at a reas >nal le point.
AN OPEN COUPLING CONDENSER may be ist latedusing this signal injection te< hnique. Refer
to Figure 17, which is part of a conventional resistance coupled amplifier. I coupling conden¬
ser C-l should open, little or no signal would Retransferred between stages. To :heck tie con¬
dition of the coupling condenser, it is first necessary to connect a small bloi kin
condenser in
series with the "hot" lead of the signal generator. The condenser may have : va ue of .05 pfd.
The exact value is not at all critical. The block¬
ing condenser permits the "hot" lead to be placed
on the plate of a tube without danger of the DC
voltage present injuring circuits within the gen¬
erator.
If a clear tone is heard when the "hot" lead is
touched to Point E but not when touched to Point
F, we are reasonably sure Condenser C-l is
open.
B +
RADIO RECEIVER
Referring to the block diagram for a typical small AM receiver, Figure 18, again the technique
of signal injection, is essentially the same as that used in the case of the audio amplifier. That
is, a substitute signal is injected into each stai ;e startinj at a convenient point and working back
towards the "front" of the equipment.
In the < ase of thi audio amplifier, we work towards the
input jack of the amplifiei.
In the case of the radio receiver, we work towai ds he anteni a.
There is one important dil erence between using the signal injection technique )n n audio ai lpli-
fier and using the same technique on a receiver. In the case of the amplifier as we have ; een,
the audio output signal ob lined from the signal generator was used exclusive ly; however, vhen
trouble-shooting a receiver, it becomes necessary to switch to an RFaignalwl en< hecking sfages
Page 21
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