Dick Smith Electronics In Circuit Lopt Fbt Tester Assembly Manual

Text and illustrations courtesy of Electronics Australia
I
f youre reading this, then chances are
that youre a TV and/or computer mon-
itor repair technician - who doesnt need
to be told that horizontal output stage
faults cause more than their fair share of
headaches! Operating at high voltages,
frequencies and power levels, many
components in this part of the circuit are
highly stressed, and failures are not only
common but their cause is often hard to
identify.
The usual symptom of a major hori-...

horizontal output stage will greatly
increase the losses in the LOPTs
primary circuit. That is, they lower
the Q. 
We chose the principle of `ring
testing as the basis for this instru-
ment because its easy to implement
with relatively simple circuitry and
common components, and produces
predictable results with no need for
calibration.
`Ring testing gets its name from
the fact that when a fast pulse is
applied to the primary winding of
the LOPT, the total inductance and
capacitance in the circuit will...

until the ringing amplitude has decayed
down to about 15% of its initial value.
This square wave is connected straight to
the clock inputs of shift registers IC2a
and IC2b.
3. The LED bargraph display:IC2
consists of a pair of identical four-bit
serial-in/parallel-out shift registers, con-
nected to form a single eight-bit unit,
with each output driving one LED in the
`bargraph display via resistors R17 to
R24. The serial data input of the first
stage (pin 15) is permanently connected
to the positive...

collector, or excessively high EHT result-
ing in HV shut-down.
Because this tester uses impulses of
only 650mV to minimize the forward
biasing of semiconductors, such defects
will not be reflected in the ring count.
Under these circumstances, I check for
measurable leakage resistance between
the EHT cap and the other LOPT pins. It
should be unmeasurable, otherwise the
LOPT is defective.
If I have gone through the above tests
and have these symptoms and a normal
ring count on the tester, the diagnosis...

the LEDs off the board for now. Take
care with the orientation of the polarised
components, including the IC sockets.
With everything but the LEDs installed
on the PCB, once again illuminate it
from the top, and check for and correct
any solder bridges or other problems.
Now turn your attention to the front
panel, mounting the banana sockets and
the power switch in their respective
holes.
Attach the tapped spacers to the cor-
ners of the board using plain 3mm
screws, and solder long component lead...

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Text and illustrations courtesy of Electronics AustraliaDick Smith Electronics © ZA8738-2
adhesive tape to stick the battery holder
into the bottom of the case, with the cells
aligned in a `north-south direction for
easiest access. All that remains to bedone now is to screw the front panel into
place and try out your tester on some
LOPTs and their associated circuitry.
Finally, our sincere...