SYNC AND FRAME STAGES

This page looks at Sync Separators and frame stages.

In order to create a stable picture on the TV set, the Line and Frame timebases must be locked, or synchronised, to the transmitted (or locally generated) picture signal. This is why the video waveform, as well as carrying video imformation, carries sync pulses.

Here’s the video waveform (one line of the picture) after demodulation. This is one line of the picture and is in fact the part of the VITS signal used by the BBC for measuring transmission characteristics.

A: Sync Pulse.
B: Colour Burst.
The rest of this is the picture information and in this case a black and white set would display a grey scale and a colour set would show either grey scale if the colour content on the steps were of the same phase or very desaturated colour bars if the phase varied on each step.

SYNC SEPARATORS.

The Sync Separator is biased to strip the sync pulses from the video waveform and then create two sets of sync pulses, for Line and Frame. Here is a typical example. (BRC1400 Chassis.) The 30FL1 pentode is biased to cut off on video and therefore strip the sync pulse train as shown.


The Line and Frame pulses are of course of different frequencies and are separated out by RC networks. Different time constants are used to separate the two out.

Larger voltage pulses are needed for valve timebases than for transistor ones. This is why for valve sets the pentode sync separator is used. Anode potential varies by about 100v and Sync Pulse Amplitude averages about 18v on 405 and 14v on 625 Line sets. This varies from set to set and scope traces are often provided in the manual for the set.

Transistors work better as switches and need a much smaller input to make them change state. Tends to be up to 1v video amplitude resulting in a 0.3v sync pulse train.

In the case of a transistor sync separator stage driving a valved timebase, the transistor itself switches the HT as shown in this example from the ITT CVC2 circuit. The sync separator transistor (BC116) is fed from the +15v LT rail and it drives the Sync Pulse Amp transistor BF163 which operates from the +290v HT rail. As the CVC2 is a colour set there is also a pulse feed to the burst gating on the Colour Decoder.


SYNC PROBLEMS.

These fall into three categories. Lack of Frame Sync, Lack of Line Sync and Lack of both Frame and Line Sync. Again, as a quick check I often plug in a replacement valve!

When both line and frame syncs fail, look carefully to see if the rolling and drifting picture information looks to be correct. If not look at the IF and Detector Stages. If you have a scope, check for the video input to the control grid of the valve itself. In the example above, a common suspect is Capacitor C1.Also worth a look is the valve’s bias conditions, R4,R5 and C2.

For lack of Line or Frame Sync, use your scope to follow the pulse amplitude and pulse trains from the separator stage to the requisite oscillator stage.

A transistor sync separator works in much the same way and is faulted similarly.

THE FRAME TIMEBASE.

 The frame timebase works at 50Hz and consists of 2 or 3 stages. Some valve sets use a sync pulse amplifier stage. Here is an example of a Grounded Grid Sync Amplifier.

The sync pulses are then used to control the frame oscillator stage. This generates a 50Hz waveform and an LC circuit is used to create a sawtooth to drive the frame output stage.

The Height control if on this stage acts to vary the amount of frame drive to the Frame output stage.

There are a number of capacitors here which affect the shape of the scan waveform via linearity controls. The scan must be uniform of course and the classic symptom of linerity problems is that the circle on Test Card F will be an irregular Oval (or Egg) shape. A scope check on the waveform will reveal the affect this has on the scan waveform, however my advice is to have a capacitor changing problem if linearity problems are experienced.

If these capacitors fail, the result is severe frame cramping at the bottom of the picture or in severe cases the scan comes up to the middle of the screen. Linearity problems are common on old sets as the capacitors in the frame oscillator stages are often in need of replacement. The sawtooth waveform becomes severely distorted as the capacitors, now low in value, reduce the time constant of the CR circuit or render the linearity controls ineffective.

Here is a typical circuit on which the points can be illustrated. This is the Bush TV109, circa 1962. Incidentally have you seen the Hammer Film ‘Quatermass and the Pit’? A very similar set is quite clearly seen showing a colour picture!!!!!!


Many timebases use a CR circuit to create a sawtooth to drive the frame scan coils. As the frequency is lower (50Hz rather than 15kHz) the inductance of the coils is not enough to create an effective sawtooth current waveform from a squarewave input.

FRAME FAULT FINDING.

Frame faults fall into three catergories. Complete Failiure, (frame collapse, horizontal white line), Partial failiure (lack of height) or distortion (linearity problems). In cases where the LT rail is derived from the Frame Output valve a complete failiure can be accompanied by loss of sound. This is the classic ‘Line Across Screen and No Sound’ Symptom associated with the PCL805. Its always worth trying a replacement valve first!

Fault finding on the frame stages is usually fairly straightfoward. For total frame collapse, check first for HT on the anodes of the Oscillator and Output Stages. It’s also worth checking for volts on either side of the height control as this can often be the problem as the control itself can go open circuit. Its always worth trying a new PCL805 as well as a quick check.

The Frame output stage, a pentode, drives the frame output transformer primary winding which is in series with the anode. It is biassed in a similar way to the Pentode Output Stage in a valve radio set. Remember my comment about the coupling capacitor or ‘That Capcitor’ in the power supply section? Same applies here!

A frame output transformer failiure is a rare occurrence. Open Circuit windings are the most common problem. Best check is with the ohm-meter and the DC resistances should be in the manual.

Lack of Height with decent linearity is often down to lack of HT on either the oscillator or frame output stage. Another cause of this is a resistor going high in value in series with the height control.

Linearity problems can result in people having flat or pointed heads. The circle on the test card will look egg shaped. In extreme cases, the scan will be extremely stretched at the top of the screen culminating in a bright line in the centre of the screen. The sawtooth waveform is created in many cases by a CR network in which a capacitor is charged up. This capacitor tends to go very low in value with age and this results in it charging up too soon and the waveform becomes distorted resulting in severe linearity problems.

To Summarise fault finding on Valve Frame Stages,given that the valves are OK, for Frame Collapse check first for the HT rails and anode voltages then the frame oscillator. For lack of height check resistor values. For Linearity problems look at capacitors.

As frame frequency is 50Hz, its also worth checking for power supply faults that could cause ripples on the HT rails.

TRANSISTOR FRAME STAGES.

These tend to be reliable and suffer the same sort of problems as their valved equivalents, but are more prone to damage from EHT flashovers. The transistors themselves do tend to be reliable but output stages in particular often need to be well heatsinked.

Some frame output stages employ a push pull output stage which works in a similar way to the much loved audio circuits! Crossover distortion here has been known to result in linearity distortion across the centre of the screen, often manifesting itself as a white line across the middle of the picture.

COLOUR FRAME CIRCUITS.

The main difference between Colour and Monochrome frame circuits is the higher requirements of the colour circuits. The frame output stages are stabilised to compensate for variations in supply voltage or valve ageing. This is because scan amplitude is critical to maintain convergence in early colour sets.

Another interesting aspect of the frame circuitry on early colour sets can be seen on this Decca Bradford diagram here.

CRITICAL HOLD CONTROLS.

Sometimes the Frame or Line Hold controls can be very sensitive and critical. This occurs when there is insufficient amplitude coming from the sync separator, there is poor HT smoothing causing a ripple which affects lock. If discriminator diodes are used to maintain the correct frequency then these can go open circuit or high in resistance. These sort of faults really need an oscilloscope to be traced effectively but the cowboy element will often tend to solve the problem the unscientific way and go on a capacitor changing binge!!

Another more simple cause of this problem is dirt or track wear on the control itself. Replacement is usually the best option but in some cases the original type of control may not be available and a replacement control from a similar scrap set may be just as bad. This seems to be a particular problem with the Bush TV141, and TV161 series of sets. A replecement control can be fitted and wired in place but this does affect the appearance of the back of the set. I have in some cases been known to leave the original control in place but open circuted to make it inoperative and fit a separate preset control inside the set!!!

Of course aesthetic restoration at the back of your set is entirely up to you to decide how far to go!!