Richtige Fernseher haben Röhren!
In Brief: On this site you will find pictures and information about some of the electronic, electrical and electrotechnical technology relics that the Frank Sharp Private museum has accumulated over the years .
Premise: There are lots of vintage electrical and electronic items that have not survived well or even completely disappeared and forgotten.
Or are not being collected nowadays in proportion to their significance or prevalence in their heyday, this is bad and the main part of the death land. The heavy, ugly sarcophagus; models with few endearing qualities, devices that have some over-riding disadvantage to ownership such as heavy weight,toxicity or inflated value when dismantled, tend to be under-represented by all but the most comprehensive collections and museums. They get relegated to the bottom of the wants list, derided as 'more trouble than they are worth', or just forgotten entirely. As a result, I started to notice gaps in the current representation of the history of electronic and electrical technology to the interested member of the public.
Following this idea around a bit, convinced me that a collection of the peculiar alone could not hope to survive on its own merits, but a museum that gave equal display space to the popular and the unpopular, would bring things to the attention of the average person that he has previously passed by or been shielded from. It's a matter of culture. From this, the Obsolete Technology Tellye Web Museum concept developed and all my other things too. It's an open platform for all electrical Electronic TV technology to have its few, but NOT last, moments of fame in a working, hand-on environment. We'll never own Colossus or Faraday's first transformer, but I can show things that you can't see at the Science Museum, and let you play with things that the Smithsonian can't allow people to touch, because my remit is different.
There was a society once that was the polar opposite of our disposable, junk society. A whole nation was built on the idea of placing quality before quantity in all things. The goal was not “more and newer,” but “better and higher" .This attitude was reflected not only in the manufacturing of material goods, but also in the realms of art and architecture, as well as in the social fabric of everyday life. The goal was for each new cohort of children to stand on a higher level than the preceding cohort: they were to be healthier, stronger, more intelligent, and more vibrant in every way.
The society that prioritized human, social and material quality is a Winner. Truly, it is the high point of all Western civilization. Consequently, its defeat meant the defeat of civilization itself.
Today, the West is headed for the abyss. For the ultimate fate of our disposable society is for that society itself to be disposed of. And this will happen sooner, rather than later.How to use the site:
OLD, but ORIGINAL, Well made, Funny, Not remotely controlled............. and not Made in CHINA.
- If you landed here via any Search Engine, you will get what you searched for and you can search more using the search this blog feature provided by Google. You can visit more posts scrolling the left blog archive of all posts of the month/year,
or you can click on the main photo-page to start from the main page. Doing so it starts from the most recent post to the older post simple clicking on the Older Post button on the bottom of each page after reading , post after post.
You can even visit all posts, time to time, when reaching the bottom end of each page and click on the Older Post button.
- If you arrived here at the main page via bookmark you can visit all the site scrolling the left blog archive of all posts of the month/year pointing were you want , or more simple You can even visit all blog posts, from newer to older, clicking at the end of each bottom page on the Older Post button.
So you can see all the blog/site content surfing all pages in it.
- The search this blog feature provided by Google is a real search engine. If you're pointing particular things it will search IT for you; or you can place a brand name in the search query at your choice and visit all results page by page. It's useful since the content of the site is very large.
Note that if you don't find what you searched for, try it after a period of time; the site is a never ending job !
Every CRT Television saved let revive knowledge, thoughts, moments of the past life which will never return again.........
Many contemporary "televisions" (more correctly named as displays) would not have this level of staying power, many would ware out or require major services within just five years or less and of course, there is that perennial bug bear of planned obsolescence where components are deliberately designed to fail and, or manufactured with limited edition specificities..... and without considering........picture......sound........quality........
..............The bitterness of poor quality is remembered long after the sweetness of todays funny gadgets low price has faded from memory........ . . . . . .....Don't forget the past, the end of the world is upon us! Pretty soon it will all turn to dust!
Have big FUN ! !
©2010, 2011, 2012, 2013, 2014 Frank Sharp - You do not have permission to copy photos and words from this blog, and any content may be never used it for auctions or commercial purposes, however feel free to post anything you see here with a courtesy link back, btw a link to the original post here , is mandatory.
All sets and apparates appearing here are property of
Engineer Frank Sharp. NOTHING HERE IS FOR SALE !
Sunday, December 26, 2010
SELECO 17SM470 " WALKIE 17 " CHASSIS BS700 INTERNAL VIEW.
This Tellye is equipped with the CHASSIS ZANUSSI BS700 which was mounted basically in all sets from 15 Inches to 20 Inches with little variants in that era (1987 - 1990).
The design is based on MOTOROLA AND PHILIPS ASIC TDA3301 + TDA4504 + TDA8191.
Supply is based on the well TDA4601 (Siemens)
It was very durable and gave not big problems except for some dry joint and leaked Electrolytic caps.
The set here in collection has big hour count of use (over 65000) and was never serviced before.
* The TDA4601 device is a single in line, 9 pin chip. Its predecessor was the TDA4600
device, the TDA4601 however has improved switching, better protection and cooler running.
The (SIEMENS) TDA4601 power supply is a fairly standard parallel chopper switch mode type,
which operates on the same basic principle as a line output stage. It is turned on and off by a
square wave drive pulse, when switched on energy is stored in the chopper transformer
primary winding in the form of a magnetic flux; when the chopper is turned off the magnetic
flux collapses, causing a large back emf to be produced. At the secondary side of the chopper
transformer this is rectified and smoothed for H.T. supply purposes.
The advantage of this type of supply is that the high chopping frequency (20 to 70 KHz
according to load) allows the use of relatively small H.T. smoothing capacitors making
smoothing easier. Also should the chopper device go short circuit there is no H.T. output.
In order to start up the TDA4601 I.C. an initial supply of 9v is required at pin 9, this voltage
is sourced via R818 and D805 from the AC side of the bridge rectifier D801, also pin 5
requires a +Ve bias for the internal logic block. (On some sets pin 5 is used for standby
switching). Once the power supply is up and running, the voltage on pin 9 is increased to 16v
and maintained at this level by D807 and C820 acting as a half wave rectifier and smoothing
Pin 1 This is a 4v reference produced within the I.C.
Pin 2 This pin detects the exact point at which energy stored in the chopper transformer
collapses to zero via R824 and R825, and allows Q1 to deliver drive volts to the
chopper transistor. It also opens the switch at pin 4 allowing the external capacitor
C813 to charge from its external feed resistor R810.
Pin 3 H.T. control/feedback via photo coupler D830.
The voltage at this pin controls the on time of the chopper transistor and hence the
output voltage. Normally it runs at Approximately 2v and regulates H.T. by sensing a
proportion of the +4v reference at pin 1, offset by conduction of the photo coupler
D830 which acts like a variable resistor. An increase in the conduction of transistor
D830 and therefor a reduction of its resistance will cause a corresponding reduction
of the positive voltage at Pin 3. A decrease in this voltage will result in a shorter
on time for the chopper transistor and therefor a lowering of the output voltage and
vice versa, oscillation frequency also varies according to load, the higher the load the
lower the frequency etc. should the voltage at pin 3 exceed 2.3v an internal flip
flop is triggered causing the chopper drive mark space ratio to extend to 244 (off
time) to 1 (on time), the chip is now in over volts trip condition.
Pin 4 At this pin a sawtooth waveform is generated which simulates chopper current, it is
produced by a time constant network R810 and C813. C813 charges when the
chopper is on and is discharged when the chopper is off, by an internal switch
strapping pin 4 to the internal +2v reference, see Fig 2.
The amplitude of the ramp is proportional to chopper drive. In an overload
condition it reaches 4v amplitude at which point chopper drive is reduced to a
mark-space ratio of 13 to 1, the chip is then in over current trip.
The I.C. can easily withstand a short circuit on the H.T. rail and in such a case the
power supply simply squegs quietly. Pin 4 is protected by internal protection
components which limit the maximum voltage at this pin to 6.5v.
Should a fault occur in either of the time constant components, then the chopper
transistor will probably be destroyed.
Pin 5 This pin can be used for remote control on/off switching of the power supply, it is
normally held at about +7v and will cause the chip to enter standby mode if it falls
Pin 6 Ground.
Pin 7 Chopper switch off pin. This pin clamps the chopper drive voltage to 1.6v in order to
switch off the chopper.
Pin 8 Chopper base current output drive pin.
Pin 9 L.T. pin, approximately 9v under start-up conditions and 16v during normal running,
Current consumption of the I.C. is typically 135mA. The voltage at this pin must
reach 6.7v in order for the chip to start-up.
- VIDEO CHROMA PROCESSING WITH TDA3300 (MOTOROLA)
TDA3300 3301 TV COLOR PROCESSOR
This device will accept a PAL or NTSC composite video signal and output the
three color signals, needing only a simple driver amplifier to interface to the pic-
ture tube. The provision of high bandwidth on-screen display inputs makes it
suitable for text display, TV games, cameras, etc. The TDA3301 B has user con»
trol laws, and also a phase shift control which operates in PAL, as well as NTSC.
0 Automatic Black Level Setup
0 Beam Current Limiting
0 Uses Inexpensive 4.43 MHZ to 3.58 MHz Crystal
0 No Oscillator Adjustment Required
0 Three OSD Inputs Plus Fast Blanking Input
0 Four DC, High Impedance User Controls
0 lnterlaces with TDA33030B SECAM Adaptor
0 Single 12 V Supply
0 Low Dissipation, Typically 600 mW
The brilliance control operates by adding a pedestal to the output
signals. The amplitude of the pedestal is controlled by Pin 30.
During CRT beam current sampling a standard pedestal is
substituted, its value being equivalent tothe value given by V30 Nom
Brightness at black level with V30 Nom is given by the sum of three gun
currents at the sampling level, i.e. 3x20 |.1A with 100 k reference
resistors on Pins 16, 19, and 22.
During picture blanking the brilliance pedestal is zero; therefore, the
output voltage during blanking is always the minimum brilliance black
level (Note: Signal channels are also gain blanked).
The chrominance decoder section of the TDA3301 B
consists of the following blocks:
Phase-locked reference oscillator;
Phase-locked 90 degree servo loop;
U and V axis decoders
ACC detector and identification detector; .
Identification circuits and PAL bistable; .
Color difference filters and matrixes with fast blanking
The major design considerations apart from optimum
o A minimum number of factory adjustments,
o A minimum number of external components,
0 Compatibility with SECAM adapter TDA3030B,
0 Low dissipation,
0 Use of a standard 4.433618 Mhz crystal rather
than a 2.0 fc crystal with a divider.
The crystal VCO is of the phase shift variety in which the
frequency is controlled by varying the phase of the feedback.
A great deal of care was taken to ensure that the oscillator loop
gain and the crystal loading impedance were held constant in
order to ensure that the circuit functions well with low grade
crystal (crystals having high magnitude spurious responses
can cause bad phase jitter). lt is also necessary to ensure that
the gain at third harmonic is low enough to ensure absence of
oscillation at this frequency.
It can be seen that the
necessary 1 45°C phase shift is obtained by variable addition
ol two currents I1 and I2 which are then fed into the load
resistance of the crystal tuned circuit R1. Feedback is taken
from the crystal load capacitance which gives a voltage of VF
lagging the crystal current by 90°.
The RC network in the T1 collector causes I1 to lag the
collector current of T1 by 45°.
For SECAM operation, the currents I1 and I2 are added
together in a fixed ratio giving a frequency close to nominal.
When decoding PAL there are two departures from normal
chroma reference regeneration practice:
a) The loop is locked to the burst entering from the PAL
delay line matrix U channel and hence there is no
alternating component. A small improvement in signal
noise ratio is gained but more important is that the loop
filter is not compromised by the 7.8 kHz component
normally required at this point for PAL identification
b) The H/2 switching of the oscillator phase is carried out
before the phase detector. This implies any error signal
from the phase detector is a signal at 7.8 kHz and not dc.
A commutator at the phase detector output also driven
from the PAL bistable coverts this ac signal to a dc prior
to the loop filter. The purpose ot this is that constant
offsets in the phase detector are converted by the
commutator to a signal at 7.8 kHz which is integrated to
zero and does not give a phase error.
When used for decoding NTSC the bistable is inhibited, and
slightly less accurate phasing is achieved; however, as a hue
control is used on NTSC this cannot be considered to be a
90° Reference Generation
To generate the U axis reference a variable all-pass network
is utilized in a servo loop. The output of the all-pass network
is compared with the oscillator output with a phase detector of
which the output is filtered and corrects the operating point of
the variable all»pass network .
As with the reference loop the oscillator signal is taken after
the H/2 phase switch and a commutator inserted before the
filter so that constant phase detector errors are cancelled.
For SECAM operation the loop filter is grounded causing
near zero phase shift so that the two synchronous detectors
work in phase and not in quadralure.
The use of a 4.4 MHz oscillator and a servo loop to generate
the required 90° reference signal allows the use of a standard,
high volume, low cost crystal and gives an extremely accurate
90° which may be easily switched to 0° for decoding AM
SECAM generated by the TDA3030B adapter.
ACC and Identification Detectors
During burst gate time the output components of the U and
also the V demodulators are steered into PNP emitters. One
collector current of each PNP pair is mirrored and balanced
against its twin giving push-pull current sources for driving the
ACC and the identification filter capacitors.
The identification detector is given an internal offset by
making the NPN current mirror emitter resistors unequal. The
resistors are offset by 5% such that the identification detector
pulls up on its filter capacitor with zero signal.
See Figure 11 for definitions.
Monochrome I1 > I2
PAL ldent. OK I1 < lg
PAL ldent_ X l1 > I2
NTSC I3 > I2
Only for correctly identified PAL signal is the capacitor
voltage held low since I2 is then greater than I1.
For monochrome and incorrectly identified PAL signals l1>l2
hence voltage VC rises with each burst gate pulse.
When V,ef1 is exceeded by 0.7 V Latch 1 is made to conduct
which increases the rate of voltage rise on C. Maximum
current is limited by R1.
When Vref2 is exceeded by 0.7 V then Latch 2 is made to
conduct until C is completely discharged and the current drops
to a value insufficient to hold on Latch 2.
As Latch 2 turns on Latch 1 must turn off.
Latch 2 turning on gives extra trigger pulse to bistable to
The inhibit line on Latch 2 restricts its conduction to alternate
lines as controlled by the bistable. This function allows the
SECAM switching line to inhibit the bistable operation by firing
Latch 2 in the correct phase for SECAM. For NTSC, Latch 2
is fired by a current injected on Pin 6.
lf the voltage on C is greater than 1.4 V, then the saturation
is held down. Only for SECAM/NTSC with Latch 2 on, or
correctly identified PAL, can the saturation control be
anywhere but minimum.
NTSC operation is selected when current (I3) is injected into
Pin 6. On the TDA33O1 B this current must be derived
externally by connecting Pin 6 to +12 V via a 27 k resistor (as
on TDA33OOB). For normal PAL operation Pin 40 should be
connected to +12 V and Pin 6 to the filter capacitor.
4 Color Difference Matrixing, Color Killing,
and Chroma Blanking
During picture time the two demodulators feed simple RC
filters with emitter follower outputs. Color killing and blanking
is performed by lifting these outputs to a voltage above the
maximum value that the color difference signal could supply.
The color difference matrixing is performed by two
differential amplifiers, each with one side split to give the
correct values of the -(B-Y) and -(Ft-Y) signals. These are
added to give the (G-Y) signal.
The three color difference signals are then taken to the
virtual grounds of the video output stages together with
The TDA3301B may be used with a two level sandcastle
and a separate frame pulse to Pin 28, or with only a three level
(super) sandcastle. In the latter case, a resistor of 1.0 MQ is
necessary from + 12 V to Pin 28 and a 70 pF capacitor from
Pin 28 to ground.
Timing Counter for Sample Control
In order to control beam current sampling at the beginning
of each frame scan, two edge triggered flip-flops are used.
The output K ofthe first flip-flop A is used to clock the second
tlip-flop B. Clocking of A by the burst gate is inhibited by a count
The count sequence can only be initiated by the trailing
edge of the frame pulse. ln order to provide control signals for:
Beam current sampling
On-screen display blanking
The appropriate flip-flop outputs ar matrixed with sandcastle
and frame signals by an emitter-follower matrix.
Video Output Sections
Each video output stage consists of a feedback amplifier in A further drive current is used to control the DC operating
which the input signal is a current drive to the virtual earth from point; this is derived from the sample and hold stage which
the luminance, color difference and on-screen display stages. samples the beam current after frame flyback.
TDA8191 TV SOUND CHANNEL
The TDA8191 is a monolithic integrated circuit that
includes all the functionsneeded fora completeTV
sound channel.TheTDA8191 is assembled in a 20
pin dual in line power package.
.DC VOLUME CONTROL
.TURN-ON AND TURN-OFF MUTING
TDA4504B Small signal combination for multistandard colour TV.
Having the capability to demodulate IF signals with either
positive or negative-going video information, the
TDA4504B (Fig.1) is contained within a 32 pin
encapsulation. It includes a three-stage vision IF amplifier,
mute circuit, AFC and AGC circuitry, fully synchronised
horizontal and vertical timebases with drive circuits and
integral three-level sandcastle pulse generator.
A functional colour tv receiver can thus be realized with the
addition of a tuner, audio demodulator and amplifier,
chroma decoder and respective line and field deflection
· Gain controlled vision IF amplifier
· Synchronous demodulator for negative and positive
· AGC detector operating on peak sync amplitude for
negative demodulation and on peak white level for
· Tuner AGC
· AFC circuit with two control polarities and on/off-switch
· Video preamplifier
· Video switch to select either the internal video signal or
an external video signal
· Horizontal oscillator and synchronization circuit with two
· Vertical synchronization (divider system), ramp
generator and driver with automatic amplitude
adjustment for 50 and 60 Hz
· Transmitter identification (mute)
· Sandcastle pulse generation
· VCR/auto VCR switch
· Start-up circuit
· Vertical guard.
Vision IF amplifier, demodulator
and video amplifier
Each of the three AC-coupled IF
stages permit the omission of DC
feedback and possess a control
range in excess of 20 dB.
The IF amplifier, which is completely
symmetrical, is followed by a passive
synchronous demodulator providing a
regenerated carrier signal. This is
limited by a logarithmic limiter circuit
prior to its application to the
A noise clamp circuit is provided at
the video input (pin 16) to limit
interference pulses below the sync tip
level and is more efficient than a
noise inverter in providing improved
picture stability during the presence of
The video amplifier has good linearity
and bandwidth figures.
Obtaining the AFC reference signal
from the demodulator tuned circuit
presents the advantage of utilizing a
single tuned circuit and one
adjustment. However, since the
frequency spectrum of the signal
applied to the demodulator is
determined by the characteristic of
the SAW filter, the resultant
asymmetrical spectrum with respect
to the vision carrier causes the AFC
output voltage to be dependent upon
the video signal. The TDA4504B thus
contains a sample-and-hold circuit.
With negative-going vision signals the
AFC is active only during the sync
pulse period. When positive-going
signals are applied to the device,
however, the AFC is continuously
active but filtered to ensure only a
small by-pass current is present in the
With weak input signals the drive
signal will contain considerable noise
which also possesses an
asymmetrical frequency spectrum
and could create an offset in the AFC
output voltage. The inclusion of a
notch in the demodulator tuned circuit
minimises this effect.
The sample-and-hold circuit is
followed by a high impedance output
amplifier. Thus the AFC control
gradient depends upon the load
The AFC polarity switch is combined
with the start circuit (pin 12). It has a
negative slope when pin 12 is open or
connected to the main supply and a
positive slope when pin 12 is
grounded. The AFC is disabled when
the sample connection (pin 22) is
For signals employing negative modulation the AGC detector operates on peak sync level but upon peak white content
with those having positive modulation. Selection is facilitated by the system switch (pin 32):
The AGC detector currents are:
With a 6.8 mF AGC capacitor, the video tilt will be < 10% for positively modulated signals and < 2% for negative
To obtain a rapid AGC action when executing a search tuning operation with the circuit set for peak white AGC, the
charge current is held at 55 mA until the detection of a transmitted signal.
The transmitter identification
A mute signal is generated to disable the audio preamplifier of an audio demodulator during the absence of a
transmission signal. When the video switch is in the internal mode, the identification of a transmitted signal is derived
from the coincidence detector.
In the external mode the IF part of the circuit has its own identification system. The system relies upon the detection of
sync. pulses on the incoming IF signal. The separated horizontal sync pulse charges the capacitor on pin 25 which drives
the mute output (pin 14).
The connection of a 1 MW resistor between pin 25 and VCC results in the mute information being overruled by the
50/60 Hz information derived from the internal vertical divider section.
Flywheel horizontal synchronization is desirable when receiving weak signals marred by noise but is usually unnecessary
when receiving stronger off-air signals unless certain types of interference or multipath reception are apparent. Due to
the inherent instability of VCR signals, however, the horizontal time constant should be shorter to prevent loss of
horizontal synchronization in the early part of the scan. Provision is therefore incorporated to automatically switch the
short time constant such that a strong signal instigates the 'VCR' mode and a weak signal triggers the 'TV' mode.
The connection of a switch to pin 17 provides for this to be accomplished manually and may take the form of an auxiliary
switching function associated with a designated program selector button.
The TDA4504B has a separate pin (pin 17) for the VCR switch:
Video output from the demodulator is filtered to remove the audio carrier and DC-coupled to pin 16. If AC-coupling is
employed the internal noise clamp will operate on sync. tips.
The TDA4504B provides the opportunity for a direct video connection (e.g. via a peritel connector) to be made to the
device at pin 13. Selection between internal and external video is made by applying a switching potential to pin 18.
To prevent crosstalk between the IF stages and the horizontal oscillator when the device is operated in its external video
mode with no RF input, the TDA4504B incorporates an option to reduce IF gain by 20 dB. This is accomplished by
connecting a 39 kW resistor between pin 17 and ground. Omission of this component results in the IF amplifier remaining
at full gain.
In the internal video mode the resistor must be disconnected to achieve the auto-VCR mode.
50 Hz 60 Hz None Don’t care Don’t care Don’t care
Pin 25 9.5 9.5 0.3 9.5 9.5 9.5
Pin 28 50 Hz 60 Hz None 50 Hz 60 Hz None
Pin 18 LOW LOW LOW HIGH HIGH HIGH
Pin 14 12 9 0.3 12 9 12
pin 17 HIGH: VCR mode fast time constant; ungated
pin 17 n.c.: auto VCR mode
pin 17 LOW: TV mode slow time constant; gated
pin 18 LOW: internal video
pin 18 HIGH: external video
The horizontal synchronization circuit
of the TDA4504B has been designed
· The retrace of the horizontal
oscillator occurs during the
horizontal retrace and not during
the scan period. This has the
advantage that no interference will
be visible on the screen when
receiving weak input signals. Video
crosstalk will not disturb the phase
of the horizontal locking.
· Reduced frequency shift of the
horizontal oscillator due to noise
since the horizontal phase detector
reference signal is more
symmetrical and independent of
the supply voltage and
· The phase detector current ratio for
strong and weak signals is
increased to obtain a better
performance during both VCR
playback and weak signal
reception. The switching level is
also independent of temperature
and supply voltage.
Generation of the vertical sawtooth
(pin 3) is accomplished by a divider
that permits the production of a
vertical frequency of either 50 Hz or
60 Hz with freedom from adjustment,
amplitude correction and maximum
A discriminator window checks the
vertical trigger pulse. When the
trigger pulse occurs before count 576,
the divider system operates in the
60 Hz mode otherwise the 50 Hz
mode is selected. (2 clock pulses
equal one horizontal line).
The divider section operates with
different reset windows. These
windows are activated via an up/down
counter. This increases its count by 1
for each occasion the separated
vertical sync pulse is within the
selected window. On each occasion
the vertical sync. pulse is not within
the selected window, the count is
reduced by 1.
LARGE (SEARCH) WINDOW; DIVIDER
RATIO BETWEEN 488 - 722
This mode is valid for the following
1 divider locking to another
2 divider ratio found, not within
the narrow window limits
3 up/down counter value of the
divider system operating in
narrow window mode, count
falls below 10.
NARROW WINDOW; DIVIDER RATIO
BETWEEN 522 - 528 (60 HZ) OR 622 -
628 (50 HZ)
The divider switches to this mode
when the up/down counter has
reached its maximum value of 15
approved vertical sync pulses. When
the divider operates in this mode and
a vertical sync pulse is missing within
the window, the divider is reset at the
end of the window and the count
lowered by 1. At a counter value
below 10, the divider switches to the
large window mode.
An anti-top flutter pulse is also
generated by the divider system. This
inhibits the horizontal phase-1
detector during the vertical sync
pulse. The width of this pulse
depends upon the divider mode. For
the large window mode the start is
generated at the divider reset. In the
narrow window mode the anti-top
flutter pulse starts at the beginning of
the first equalizing pulse. The anti-top
flutter pulse ends at count 10 for 50
Hz and count 12 for 60 Hz.
When out-of-sync is detected by the
coincidence detector, the divider is
switched to count 625. This results in
a stable vertical amplitude when no
input signal is available.