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Dick Smith Electronics In Circuit Lopt Fbt Tester Assembly Manual

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    Page 1 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-...
    Page 2 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...
    Page 3 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...
    Page 4 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...
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