Build your own TV
Testing the monitor:
Volt/ohmmeter, oscilloscope, audio signal generator.
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
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
We will go through these, one at a time.
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
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.
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!
is all right so far....
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
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.
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.
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.
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
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.
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.
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.
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
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.
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.