B&K 1076, Instruction Manual

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capacitor (See Figure 5). The intermittent condition again appears, placing
the blame for intermittent loss of signal to the coupling capacitor C-4 which
is opening up intermittently.
(If it were the first video amplifier tube itself, screen bypass capacitor C-1,
plate load resistor R-2, peaking coil L-4, or any other component in the first
video amplifier circuit, the trouble would not have appeared with the signal
injected at point (H) in the circuit. However, if the signal was injected at
point (K) on the plate of the first video amplifier and the same intermittent
indication appeared, then both the peaking coil L-3 and the coupling capacitor
C-4 would be suspected).
G
H
C-4
R-2
-
B+
2ND VIDEO
lsr.VIDEO
Figure &-Video section of typical TV receiver
It should be clear at this point that the signal injection procedure can
actually begin at either end of the circuit. That is, we could have started at
video and worked toward the R.F. Tuner, or start at the Tuner and work
toward the Video. Each individual technician will develop his own technique
for use of the instrument. Where the trouble is an intermittent one, then it
is sometimes preferable to start at the tuner because we are looking for a "drop
out or loss" of signal for short intervals.
Where the trouble is in the circuit constantly, it may be preferable to start
at the video end and work toward the R.F. tuner. Also it is evident that as
the technician becomes more familiar with the instrument he will skip stages
for quick analysis and take complete sections at one time (I.F., R.F., Video).
After determining which section is at fault, he could then· signal inject to find
the one stage or component which was at fault.
Let us take another example of a receiver which comes into the shop
having intermittent sync. The sync circuit of this receiver is shown in Fig. 6.
Let us trouble shoot by signal injection from the rear of the signal path. We
inject sync signal from our Television Analyst to the plate of the 6BF6 sync
amplifier. We would now observe the raster of the television set and would see
that it was in sync.
If the picture now remains in sync, this would tell us that from the plate
of the sync amplifier tube into the oscillators there was no trouble. We would
now shift our point of sync injection to the grid of the 6BF6 sync amplifier.
At this point we would have to reverse the phase of the sync signal to allow
for the phase inversion of the sync amplifier tube and also reduce the level
of the sync signal to take into account the gain of this stage. Again we
would observe the picture on the face of the tube and watch for the inter­
mittent trouble to occur. If it did not occur, we would know that there was
no difficulty from the grid of this amplifier tube all the way through to
the oscillators.
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