B&K 1076, Инструкция по эксплуатации

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Since the picture displayed in Fig. 11 could be caused by overload of the
I.F. stage it might be wise at this point to determine the signal handling
capabilities of these stages. By reducing the setting of the R.F.-I.F. Attenuator
on the Television Analyst we are capable of lowering the level of the I.F.
output to very low levels. As the amount of I.F. is reduced we reach a point
where the picture on the customer's receiver no longer appears negative and
is in sync. The picture will very likely be snowy as the signal is very weak
at this point. We have now proved that the receiver is capable of handling
a very weak signal but is certainly not able to handle a strong signal. This
definitely points to an A.G.C. defect. The next step is to see if the I.F. strip
will accommodate a strong signal if an external bias voltage is used to control
the gain. With the I.F. signal injected at point C we connect a test lead from
the bias supply of the Television Analyst to point D which is the A.G.C.
feed point. A ground lead from the Television Analyst must also be connected.
With the Negative Bias control set to -50 we see that the picture has com­
pletely disappeared. This is due to the fact that with this large bias we have
cut-off the I.F. amplifier. As the bias voltage is reduced we will observe
that a picture will begin to appear at around 6 to 12 volts. At about 3 or 4
volts we observe a normal picture and as the bias is reduced even further
the sync begins to get unstab'le and then finally turns negative. These tests
conclusively prove that this I.F. stage in the receiver is capable of handling
without overload a wide range of signals running from the very weak to the
very strong only if external bias control is exerted on the I.F. stage. This
proves conclusively the trouble must be related to the A.G.C. circuits. Remove
the negative bias lead.
The receiver under test employs a keyed A.G.C. system, which depends
for its operation on a high level pulse which is derived from the horizontal
deflection system. The absence of this pulse will render the A.G.C. in­
operative, and cause the receiver to overload. We now disconnect capacitor
C-1 at point E. Connect a test lead from the A.G.C. Keying Pulse jack on
the Television Analyst and insert this high level pulse at the plate of the
A.G.C. tube (E). We now !lee that the picture on the receiver screen has
been restored to normal operation when I.F. signal is injected at point C,
the grid of the 2nd I.F. amplifier tube. The trouble could be a defective C-1
or an open winding on the coil supplying the A.G.C. pulse. Reconnect C-1 and
try inserting the A.G.C. pulse at point F. If proper operation results then C-1
is O.K. A resistance reading in this case will immediately show that the pulse
coil is the defective component.
Another example of an A.G.C. trouble is as follows. A receiver having the
same symptoms as the previous example is to be repaired. Signal injection
has isolated the trouble to the 2nd video I.F. stage. We find that by reducing
the I.F. Attenuator control until the signal is at a very low level that a normal
picture results. We now connect the negative bias supply to point D in Fig. 12
which is the A.G.C. bias buss. We find that adjustment of the Bias Control
will not result in a normal picture at any setting of the control, with signal
injection at point C at the I.F. frequency. These two tests indicate· that the
I.F. strip will amolify, as indicated by its ability to show a picture when the
signal is very weak. However when the signal is very strong, a control voltage
placed upon the A.G.C. buss has no effect on the stage as indicated by the
use of the negative bias supply. This would indicate that if the A.G.C. system
were operating normally it would have no effect on. the stage gain and therefore
the trouble must be peculiar to the 2nd video I.F. amplifier and must not relate
to the control voltage that the A.G.C. system exercises. This points to the
Grid circuit. A few voltage and resistance checks indicate a slight positive
voltage on the grid of the 2nd I.F. amplifier. A positive voltage was found on
the grid of the stage and a resistance check showed a leaky C-2.
In summary the receiver I.F. system must pass two tests to establish that
the trouble is specifically in the A.G.C. First it must be capable of amplifying
a very weak signal without distortion. Secondly, with a strong signal input
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