Build your own TV

Installment Nine

Testing the monitor:

Useful Equipment:

Volt/ohmmeter, oscilloscope, audio signal generator.

How you might go about testing the mechanical television monitor will depend a lot on your past experience working with electronics and to a lesser extent, electro-mechanical assemblies.

One of the important techniques for simplifying the testing process is to consider the monitor a series interconnected simple circuits. In this case, they might be considered to be the following:

1) low voltage power supplies and regulators.

2) Audio amplifier

3) DC Restorer and video amplifier

4) Sync pulse clipper

5) Disk sync pulser

6) Motor control

We will go through these, one at a time.

1) Testing the power supplies/ low voltage circuits:

The basic circuit is shown here on the right. In my monitor, there are three of these, two of which share the same transformer. This is not an important point. All three could have be connected to the same transformer as well, if it were large enough to support the total load. Only one rectifier circuit could have been used too, if the other components had be sized to carry the total load. I elected to do it in the manner that I did because of the available parts on hand.

The circuit consists of a transformer supplying 16 to 17 volts to a bridge rectifier and capacitor filter. This provides approximately 21 volts DC to an integrated circuit type voltage regulator (type 78XX). The last two digits indicate the voltage output of the device. The audio amplifier uses a type 7812 device to supply a regulated 12 volts to the amplifier and a 7815 is used to supply 15 volts for the remaining circuits.

Testing consists of first being absolutely certain that all of the parts are connected in the circuit properly. This includes that all of the polarized components, such as electrolytic capacitors and diodes are properly connected too. Then an ohmmeter test on the power supply output is in order. Readings under 100 ohms should concern you, before you ever turn on the power. If this is the case, do not apply power until you have established that there are no wiring errors.

Keep in mind that the circuits used here are well proven and will work if good parts, are wired as shown in the schematics. Printed circuit boards are recommended because they tend to minimize the chances of having wiring errors. Printed circuit board layouts for the two boards along with other pertinent information about the monitor are published in recent issues of "Electric Pictures", the quarterly bulletin for the Experimental Television Society. All members receive this bulletin.

The power supply should also be checked with an ohmmeter for isolation from the primary power circuit, (the AC line). A test from each prong of the AC plug to the ground circuits of the monitor (those circuit connections marked "return" or RTN or GND) should be made. Any readings of less than 10 megohms indicates that a problem exists and needs to be corrected before power is applied. If you don't know how to correct this sort of problem...get help!

Assuming everything is all right so far....

If you are not experienced in working with electronics operating from the AC line, be aware that the voltages present in the AC line circuits can be deadly and are able to cause serious injury and/or death. During the following tests, do not connect any test probes to circuit wiring that forms a part of the transformer primary circuits.

When first applying AC power to the circuits, connect the test leads of a voltmeter (negative probe to RTN or GND) to the output terminals of the 12 or 15 volt supplies. Observe the meter reading as AC power is applied. Within 2 seconds, the meter reading should reach 12 or 15 volts, depending on the supply under test. A reading even 1 volt low suggests that a problem exists.

Disconnect the circuit loads from the supply under test and test the voltage level from the supply again. If now correct, the load circuit is likely to be drawing excessive current and those circuits should be checked for correctness.

If the power supply voltages are correct with the loads attached, observe if any parts appear to be overheating or giving off an unusual odor or visible smoke. These conditions must be corrected immediately.

If the monitor can at this point have AC power applied, without the problems described in the previous paragraph, The power supplies are likely to be operating properly and testing can continue.

2) Testing the Audio Amplifier:

Testing the amplifier consists of applying an audio input signal from some sort of audio source with an output in the range of 300 to 3000 hertz, such as an audio oscillator/signal generator and listening to the speaker output. Since the amplifier has a gain of 100, very little input signal is necessary to provide full output to the speaker. Also check that the volume control is able to smoothly control the output level.

3) Testing the DCR and Video Amplifier:

The video amplifier consists of an emitter follower, a DC restorer (DCR) and an output amplifier stage made up of an FET and a an operational amplifier. Video input passes through the emitter follower to a the input of the DCR (Ic1a, pn2) and to the input of Ic2, pn 3, the video amplifier. The emitter follower also applies signals to the sync separator which will be discussed later. The DCR is a form of peak detector that provides a varying bias voltage to the video amplifier ( and to the sync separator) proportional to the video input signal level. This stage reinserts the missing DC level of the video signal, lost due to capacitive coupling in the previous stages of video amplification. The output of Ic2 is amplified by a FET, working with Ic2 as a voltage to current converter providing high available current levels to the LEDs. The two diodes in the source connection of the FET provide for a level of gamma correction in the amplifier.

Amplifier Ic2 has an adjustable trim pot (set black level) to set the output of the amplifier such that the LEDs are just ready to turn on when the input signal is at the level corresponding to black.

Here are two examples of oscilloscope photos, taken at the input of the video amplifier, across the video gain or contrast control, on a working system.

The time base on both photos is .5ms/cm. The sync pulse ( about .2ms wide) is the most negative level and white is the most positive. Each photo shows approximately two scan lines at 32 lines per frame and 12.5 frames per second. The signal levels here are about 300 mv peak to peak. With the oscilloscope, you can check the signal as it progresses through Ic2, to the FET and on to the LEDs.

4) The Sync Pulse Clipper:

The input video signal includes the sync pulses necessary to control the motor driving the scanning disk. The input video signal passes through the emitter follower and on the the DCR, the video amplifier and to Ic1b, the sync separator or clipper. The DCR keeps the clipper stage threshold set to its optimum setting for correct sync separation, in spite of changing signal levels. The trim pot "sync slice level" is adjusted to provide a sync output of about 20% to 30% of the video level. The oscilloscope is able to show the sync pulses as the pass through the circuits making trouble shooting very easy.

5) The Disk sync Pulser:

The scanning disk contains a circle of 32 small holes inside the image spiral, that in conjunction with an IR LED and a special IR sensor, produces 32 evenly spaced, constant amplitude pulses per revolution of the scanning disk. These pulses are then compared with the sync pulse that are a part of the input signal. Since the IR LED does not give off any visible light, one must be sure that its light does in fact pass through the holes in the scanning disk and on to the sensor.Careful placement of the LED and the sensor will assure proper operation. When installing the Sync fork, which supports the LED and sensor, keep in mind that a small change in its location may be necessary to achieve proper framing of the image. This photo shows the pulse waveform developed by the disk pulser. The time base here is .5ms/cm. The amplitude is .5 v/cm. The pulse width is a function of the hole size in the disk.

The Motor control circuit:

The motor control circuit compares the timing of the sync pulses from the input signal to the pulses from the disk pulser circuit. This comparison is accomplished in an integrated circuit device known as a phase sensitive detector (PSD). this is a digital device that actually compares the timing an produces an output that can control the speed of the motor driving the disk. This is a photo of the actual sync pulse that is applied to the PSD. It is supplied by the sync separator Ic2.

Note that the pulse width is a bout half the width of the sync pulse from the disk, but this is of no consequence as the PSD only looks at the pulse edges.

The output of the PSD is also in the form of pulses, with a varying duty cycle. This is filtered by an RC filter and applied to a FET able to support the motor load currents.

R3, the 100 Kohm resistor connected to the center terminal of the speed control can sometimes be increased in value (150 to 200 Kohms) for greater locking ranges when larger motors or a tighter coupling is used between the motor and the scanning disk.

It is hoped that the basic testing information presented here will give you the confidence to make the necessary tests to determine is your monitor has a chance of becoming a working unit. I would suggest that in order to acquire a signal source for your monitor quickly and easily, contact and join the NBTVA . As a member, you will be eligible to purchase one of their CDs with the 32 line video and sound recorded during one of their recent conferences. A standard CD player then will provide you with the video and audio signals you need to operate your monitor. And if you do join the NBTVA , you are in for an adventure in a fascinating activity with a great group of folks. We are all having a great time involved in this hobby. Join us to see for yourself.

Peter Yanczer

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