G6 Electronics

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The Philips G6 chassis made use of hybrid technology.  The design employed 21 thermionic valves, 17 transistors and 44 other semiconductor devices.  The basic G6 block diagram is shown in Plate 1 below (clicking on the image will open a much larger detail).

Plate 1 Philips G6 Block Diagram

The complete G6 block diagram and the associated circuitry will be described on this page in due course.

THE G6 POWER SUPPLY – Described by Mike Phelan

The G6 power supply – see plate 2 below (click to open a larger detail) is quite a simple and conventional beast, as it predates the general use of using line-output-derived supplies; we are entirely in the 50Hz region here! 

There are six things that come straight from the mains:

  1. The valve heaters (excluding CRT and the GY501)
  2. HT supplies HT2, 3, 3a, 4, 5, 6.
  3. A negative supply HT7
  4. Degaussing to R1062
  5. System switch solenoids on D/Std chassis
  6. Mains transformer L1510-1514 for LT supplies HT8 and 9, and the CRT heaters.

1 – The valve heaters are in series and straight across the mains – no thermistors of other droppers present.   There is some filtering with capacitors and a couple of small chokes to prevent interaction between stages.

2 and 3 – The HT supplies come from a pair of half-wave rectifiers X1101 and X1102, originally BY100 but more usually later BY127s.   A couple of spring-off resistors protect against excess current on all the HT rails; if one springs off, there is usually a line timebase fault – stopped oscillator or S/C PY500.

The supply and some of the other rails have a somewhat rudimentary stabilization and spike removal by using VDRs.

HT3a is for the two valve stages in the chroma amplifiers.

4 – R1062 is a PTC thermistor, termed a “posistor”.  When cold it has a very low resistance, so the maximum current flows through the degaussing coils L1736, mounded on the CRT shield.  As it heats up, the current falls to a minimum, so the CRT is demagnetised; the VDR R1050 eventually going open-circuit when the voltage across it falls.  R1064 ensures that the posistor is kept hot with some current flowing.

5 – These are operated by a switch on the tuner; one is on the convergence panel and the other one above on the main chassis drives the line output panel and, using a bell crank, the IF and timebase switches.  The main solenoid has a pair of switches to give a snap action so the system switch terminals do not arc.

6 – The mains transformer mounted on the bottom of the chassis feeds the LT supplies with a full-wave rectifier and the 6.3v CRT heater.  

The G6 power supply does not cause many problems, thankfully.

Plate 2 Philips G6 PSU Circuit Diagram


Described by Mike Phelan

The G6 TUNER, IF, LUMINANCE DETECTOR, AGC, SYNC SEPARATOR STAGES are shown in Plate 3 below (click to open a larger detail).

The tuner and IF stages are fairly conventional, and follow the same ideas as most dual-standard monochrome and colour chassis.   A microswitch on the tuner operates the system switch solenoids as described in the power supply section.

The IF output from the tuner goes through three transistorised IF stages (T2142, 2143 and 2624) and a detector diode X2625.   There is some fairly complex filtering and switching to cope with the two standards – the vision IF is 34.65 on VHF, and 39.5 on UHF.

On VHF, the 38.15 sound IF is taken off using a series-tuned circuit C2561 and L2563 to feed the sound IF to the two stages T2140 and 2141, thence the AM detector X2549. 

On UHF, the sound is 6MHz FM, and is transmitted as a sideband of the luminance signal.   The sound IF strip is very similar to a radio AM/FM one that has both sets of IF transformers in series for each stage.  The  FM ratio detector diodes are X2530/31 – this or the AM detector go to the triode half of the ECL86 via a volume control, thence to the pentode part with feedback from a winding of the output transformer fed to the cathode of the triode.  On VHF, this cathode voltage is fed via R2070 and R2061 to the base of the first sound IF as AGC; on UHF the limiting of FM makes this unnecessary.   A rudimentary network on this feedback gives a switchable tone control. 

Back to the luminance –the output from the detector X2625 has to go to several places:

·       IF AGC

·       Luminance output

·       Sync

·       On UHF, the 6MHz input to the sound IF.

·       4.43 MHz to the chroma section 

The sense of the video is opposite on VHF vs UHF as well, so for all these reasons, a phase-splitter T2144 is employed.  X2153 is the AGC diode, and the video signal is “ironed out” by C2041.   T2145 amplifies the AGC and a voltage from the contrast control is applied to the AGC bias fed to the bases of the two vision IF stages.

Because of the fact that the chrominance channel has narrow bandwidth based on 4.43MHz, the signal is delayed slightly when it finally arrives at the CRT; the same has to be done to the luminance channel to avoid a visible mis-registration.   A delay line is used – this physically is a plastic tube about 12cm long and 1cm in diameter with an earthed foil (connected to HT in this case) and a fine linear winding along its length.   It acts as a string of inductors in series with capacitance to earth, and delays the luminance signal to match the chroma. 

X2154 is a 4.3 volt Zener and used as a reference for X2152 which is a black-level clamp.  Only used on UHF. 

Finally, the signal goes to the control grid of the PFL200 (the “L” section) whose screen voltage is varied with the brightness control.   From the anode, the amplified signal goes to the CRT and to the grid of the other pentode of the PFL200, acting as a leaky-grid sync separator.

From the anode, field sync if fed directly, and line sync to the triode-connected EF80.   These parts we will revisit when we cover the respective timebases, likewise the beam current limiting .and black-level clamping.

Plate 3 G6 Tuner, IF, Luminance Detector, AGC, Sync Separator Stages


The G6 chrominance section and associated circuitry are shown in Plate 4 and Plate 5 below (click to open a larger detail).  A circuit description will follow in due course.

Some Chroma Theory – kindly provided by Mike Phelan

Colour decoder fundamentals

To avoid making the circuit explanation of the chroma circuit unwieldy, I felt that it was better if the decoder principles were separated from this, allowing the circuit explanation to proceed without going off at too many tangents.

I have deliberately simplified this – there is no reference to Simple PAL and very few mathematics!  We will also forget U and V and refer to the R-Y and B-Y colour difference signals.


To understand how the decoder works, it is necessary to know a little of how the colour transmissions are sent.  A requirement was always that monochrome signals could be received by a colour set, and that the converse was true – colour transmissions had to be received by a monochrome set.   

So a normal monochrome signal, complete with sync pulses is sent (the Luminance or Y signal) and the colour information is additional.   Colour can be defined in terms of three primary colours – Red, Green and Blue.   If we send the Y signal as well, only two other colours are needed.

I had terrible problems trying to get my head around the fact that we only sent the red and blue, and somehow the green mysteriously appeared from nowhere!

Then the light dawned – if was not red and blue that were sent, it was R-Y and B-Y.   As Y is a mix of R,G and B in various proportions to make white,  R-Y and B-Y contain the green (actually G-Y) information, as can be proved mathematically, but we wont bother with that stuff here.

Look at it as a 2-dimensional map of every possible colour – we only need two cp-ordinates to find any point.

R-Y if you look at it on the screen, goes from green to red, B-Y from yellow to blue. 

Oh, all right, then.  All we need is:

Y = 0.3 Red + 0.59 Green + 0.11 Blue
R-Y = 0.7 Red – (0.59Green + 0.11 Blue)
B-Y = 0.89 Blue –( 0.59 Green + 0.3 Red)

OK – At the transmitter, the colour is modulated on to a 4.43MHz subcarrier, out of the way of the video sidebands and the 6MHz sound.   It is AM, but to modulate both the R-Y and B-Y each bit is put on at a particular point in the phase of the carrier.  R-Y is shifted 90 degrees from B-Y and reversed on alternate lines.  The carrier is suppressed, but so we can get it back at our end, as 10  or so cycles of burst are transmitted during the back porch of the line sync period.

Finally, the set needs to know which way round the R-Y signal is on each line; this is achieved by swinging the burst either side of the carrier by 45 degrees on each line.  Enough of that – on to the decoder.

The decoder.

Many of us have never met a discrete chroma section in the flesh, and never needed to know how any of it worked; it is all in a chip nowadays.   Even the G8 had a couple of these in it.

Treating the decoder as a “black box” we have a chroma signal (modulated subcarrier) going in, together with line pulses for various purposes, and R, G and B coming out.

There are really three main things to achieve; the chroma signal needs to be amplified and separated into its R-Y and B-Y components; there needs to be a subcarrier oscillator to replace the one suppressed at the transmitter and this oscillator needs to be phase-locked to the incoming burst; the R-Y and B-Y need to be matrixed to give G-Y and these three colour difference signals need to have the Y component added back in to give R. G and B.    Many hybrid sets, including the G6, do this by using the CRT itself.

The chroma amplifiers

These are two or three stages in number, and have tuned circuits like IF transformers.   There is provision for chroma AGC (ACC) and a means of turning the channel off altogether (colour killer) when a monochrome transmission is received – not often nowadays! 

Often it is necessary to override the killer when fault finding.

To separate the R-Y and B-Y components, we use a delay line that delays the signal by one complete line period.  Nothing like its luminance counterpart, it is a block of glass with a pair of piezo transducers – one for input has the signal reflected across the glass and picked up by the output transducer.

Later delay lines are much smaller and use multiple reflections.

So – we can get the direct and delayed chroma simultaneously.  If we add these together, because the R-Y is inverted on alternate lines, it cancels, leaving 2 x B-Y.   Inverting either the delayed or direct chroma and adding them together cancels the B-Y leaving 2 x R-Y.   We have separated the two colour difference signals, but need to put the subcarrier back in before we can use them.

The subcarrier oscillator

A crystal-controlled oscillator is used here, as the phase can be changed sufficiently to lock it to the burst.  

The oscillator demodulates the R-Y and, with a 90 degree phase shift, the B-Y.   These two signals are fed to a pair of synchronous demodulators, either a diode bridge or a par of diodes and a centre-tapped inductor.   Whatever, the idea is to switch on the demodulator to pass the signal though at the correct time or phase.

There is still something to do – the R-Y is inverted on every line, so either the subcarrier or R-Y signal fed to the demodulator needs to be switched on each line.  A bistable fed by line pulses does this.  It’s termed a PAL switch.

The burst channel

The chroma signal is taken from the amplifiers and passed through a burst gate, preceded or followed by a stage of amplification or two.   The burst gate is normally closed; a delayed line pulse timed with the burst on the back porch opens it.   The amplified burst stripped of its chroma goes to a phase detector – normally two diodes and two capacitors; this gives a DC output that varies with phase and goes to either a varicap diode or a “reactance valve” to control the subcarrier oscillator phase.

A free-running oscillator because of a fault, will show coloured horizontal stripes running up or down across the screen, usually it is necessary to override the killer (q.v.)

The burst signal can be rectified for ACC as well, as it is a constant part of the chroma signal.

Bistable, ident and colour killer

Wait a minute, though – the burst is swinging (it was in the sixties, remember?) through 45 degrees on each line.  Doesn’t matter as far as the phase detector is concerned because of time constants.

We mentioned the bistable earlier – this operates the PAL switch.  It will happily start in either phase; you will get reversed R-Y – green faces and pink grass – when it feels like it!

Because of the swinging burst, a half-line-frequency square wave (7.8KHz) termed the ident signal, appears at the burst phase detector.  This is amplified, usually by an LC circuit, and used to “steer” the bistable.

If we rectify it and iron out the bumps with an RC circuit it can be used to turn off the colour killer as well – see above

Colour difference   

The last piece of our jigsaw.  As the R-Y and B-Y signals emerge from the demodulators, a soupcon of mixing proportions of them together gives us G-Y.  

On sets such as the G6, the three colour difference signals are amplified and fed to the CRT grids.  As the luminance goes to the three CRT cathodes, the tube itself forms the addition of the Y component back into the colour difference signals, leaving us with R, G and B.     Later sets to this by adding the Y component to the CD signals and then just supply RGB to the CRT cathodes; the grid voltage is not fed with any signals, so can be used for beam limiting and the like.

Back to the colour difference – as we have completely lost the DC component in all the processing, each CD output needs to have some sort of black-level clamping.   Typically, this is done by a triode for each CD channel, it is normally biased off but connects the grids to a line pulse, turning the anode current on.

Chroma cct Description by Mike Phelan

OK – as we found, the chroma signal arrives from the vision detector through C2052, suitably filtered.  T2755 amplifies it, and the resultant output is applied to the tuning meter,  and to the grid of V7001.  The EF183 is a vari-mu valve chosen for the ACC bias via R7143. 

The following stage V7002 amplifies it further and drives the chroma delay line.  The direct and delayed signals are summed with R7161 and R7162 to give B-Y. and the same signals are subtracted by the transformer L7581/2/3 to give R-Y and inverted R-Y.  These are switched by the bistable T7013/7014 flipped by the line pulse from L5505.   This PAL switch reverses the R-Y signal on alternate lines, to restore the transmitted reversed R-Y signal. 

To maintain the correct phase, the 45 degree swinging burst via C7092 is fed to the base of T7015, the ident amplifier.  The 7.8 KHz tuned circuit consisting of L7634 and C7094 apply a signal to one side of the bistable with C7095, to “steer” it.  

From here,  we now have both B-Y and R-Y signals that need to be demodulated.  First they are amplified by T7011 and T7012 respectively.  R7173 is a preset to adjust  the relative gain of the two signals.  As you see, the G6 is a twizzer’s delight; many a good set has ended up with poor colour performance when field engineers were let loose with a trimming tool. 

We’ll leave the chroma signals at that point and look at the burst channel next.   The circuit the diodes X7316 and X7317 fed from L5505 line pulses is the burst gate,   The anode of X7317 normally sits at a negative voltage because the line pulses charge C7043. 

R7188, C7045 and L7574 delay the falling edge of the line pulse; the delayed pulse coincident with the back porch where the burst is,  earths X7317.   The anode of this is modulated with the composite chroma and burst by L7509, and arrives at the grid of V7008, the burst amplifier.   Except when the gate is open, the control grid is biased off by the negative voltage on X7317, so only the burst appears at the anode. 

The amplitude of the burst (and therefore the ACC bias) can be varied by the colour control using the negative feedback of C7086, the cathode bypass.     

The components in the can in the anode of V7008 are the burst phase detector, aka burst demodulator.   It works by comparing the phase of the burst with that from the subcarrier oscillator and providing a DC output to vary the phase of the oscillator.   Also, ACC bias and 7.8 KHz ident signals are available.   The diode X7628 rectifies the burst for ACC, the ident signal comes from the slider of R7618 which is use to adjust the subcarrier oscillator phase.  R7275 and C7102 rectify it further to give DC, which goes to the triode grid of V7009a (reactance valve) which behaves as a capacitor.  The anode controls the pentode half which is the actual oscillator.

X7655 and associated components regulate the bias on the pentode grid, and the output of the oscillator is fed back to the junction of X7629 and X7630 as a reference, and also to the chroma demodulators.  In most sets, there is a 90 degree shift for the B-Y reference signal and the PAL switch operates on the oscillator; here, the PAL switch is in the R-Y signal, and the B-Y phase shift is done in the chroma demodulators; it is Philips, though! 

We have now brought the two story lines (signal and burst) together and have two colour difference signals.   The final chapter is to follow them, picking up G-Y on the way, and delivering them to the CRT grids so the luminance can be added by the cathodes.

PCF200s are strange little valves.  The 10-pin base can fool you, but they give less trouble than their PCL84 friends

The demodulated R-Y and B-Y chroma signals are fed to the (pentode) grids of the respective PCF200 colour-difference amplifiers.  R7235, R7243 and R7245 allow the two signals to be mixed to give G-Y.   

Put simply, the R-Y (red) from its anode is mixed with a bit of B-Y (blue) from its anode to make a sort of magenta – negative B-Y !   This goes to the B-Y grid where the valve inverts and amplifies it to give G-Y (green).

By now, the Twizz Fairy will probably been at the presets, but if adjusted properly, colour rendition on these takes a bit of beating. 

Earlier chassis have a tint control that varies the cathode bias on the R-Y and B-Y stages differentially.

From the anodes of the three pentodes and through the coupling capacitors, the then amplified signals go to the CRT base panel.   The triodes of the three PCF200s are clamps; the anodes are connected to the CRT grids, the cathodes held positive, and the grids at chassis potential. so they are normally non-conductive.   A positive-going line pulse from the ubiquitous L5505 drives the grids positive, clamping the three signals and the anodes to a fixed voltage via the cathodes during the blanking period.   The 10 meg anode loads ensure that the time constant is very long, so the current through the coupling capacitors does not change appreciably during the forward scan; that would give poor LF response; colour bars would have tilted tops viewed on a ‘scope! 

Loose ends!   The output from the burst phase detector is fed the first grid of V7003 with a long time constant C7088 / R7614 which removes the vestiges of the 7.8KHz ident signal.   The cathode of this valve is connected to the slider of R7179, the ACC control, as we have already mentioned.   

When colour is present, the first triode is turned hard on by the voltage from R7614, and the cathode voltage is added to the ACC bias from X7628 to bias the first chroma amp.

With no burst, V7003 is turned off, and the colour killer operates by the slider of R7179 going to chassis.   When this happens, the other half of V7003 turns hard on, switching V7004a off.  Both of the anode/grid couplings are using VDRs instead of capacitors. 

V7003a drives the relay on the CRT base; when it is turned off by a monochrome transmission, the relay changes the relative greyscale components to the tube cathodes by increasing the blue and reducing the green – this gives a typical monochrome CRT tube display instead of the “warmer” Illuminant D of a colour set.   So it says.

There is also a link to the contrast control from the grid of V7003a to track the colour level with the contrast, and finally, the earlier sets have a switch to turn the colour off by earthing the grid of V7003a. 

Plate 4 G6 Chrominance Part A

Plate 5 G6 Chrominance Part B


The G6 line output and associated circuitry is shown in Plate 6 below (click to open a larger detail).  A circuit description will follow in due course.

But First – Some Valve Line Output Theory– Described by Mike Phelan

It is worthwhile giving this subject a bit of an airing, as there are probably a few misconceptions about its operation, and the basic principle is the same on both colour and monochrome sets from the post WW2 era to the demise of the valve. 

As tubes got bigger and flatter, more EHT and therefore scan current was needed.   Running at 10125 and eventually 15625Hz, the amount of energy wasted by turning the current off during flyback and that needed for the scan became unwieldy.

So, the ingenious reclaim or efficiency diode was devised; instead of the energy being wasted, most of it is put back, and as a bonus we have a  high-voltage supply for the line output stage, CRT first anode and field oscillator.  The unsung hero of the piece! 

A few assumptions and conventions first:

·       Voltages have all been rounded

·       Zero forward voltage on PY is assumed.

·       CStray is a combination of one or more actual capacitors and the capacitance of the transformer, scan coils and other things, and appears in series and parallel to the transformer – easier to call it one capacitor in parallel.

·       Tx is shown as a single winding; in practice it will be several, and includes the scan coils.

Ok – here we go.   We’ll say that it is already running, Cb is charged, the spot is about ¾ way to the right and PL is turned hard on by its grid drive.  The anode voltage is very low – say 50v – the valve is really a switch.   We are at t0

Tx current is increasing linearly (L and C are very large as C is really the entire HT line) as the scan progresses  and moves the spot to the right.  The current comes from Cb and the HT via PY.  As the scan progresses and Cb charge is used up, all the voltages on Tx fall slightly, so more comes from PY as its cathode approaches 300v. 

At t1, flyback is initiated by the PL grid being driven negative by the drive waveform.  Now the fun starts.   The valve is switched off; as Tx had a heavy current,  this current cannot just be stopped.  Tx and CStray are now a tuned circuit with (1) connected to Cb and sitting on 600v.

Tx is now a generator, not a source; the falling current gives the first part of the flyback and charges CStray to a very high voltage, because of the inductance x rate of change of current.  

We are at t2.

Aside: Without any intervention, Tx and CStray would just go into oscillation until losses made it decay to nothing.   

CStray immediately discharges into Tx, so the voltage falls and Tx current increases in the opposite direction, sending the spot to the left.   As the voltage falls, point 2 drops below HT, and PY conducts. 

This is t3 and the end of the flyback.

PY and Cb across the winding stop the oscillation by damping the tuned circuit, leaving the core magnetised and the current at maximum but in the opposite direction.

As PY is now conducting heavily points 1 and 2 cause current to flow into Cb, charging it to a  slightly higher voltage.

Tx is now a generator, not a source, so the current is in the opposite direction to t0 t2, but the voltage is in the same direction.

The current decays almost linearly towards t4, and the spot moves to the right.  Eventually the current would decay to zero exponentially, but just before t4, when the spot is just left of centre, the positive grid drive from the line oscillator slams the PL hard on – the current through Tx now starts to increase exponentially because of its inductance.  The time-constant is long and the voltage is about 700. The two overlaps on the current curves cancel each other out to give a fairly linear scan.

We have reclaimed enough of the flyback energy to provide about 40% of each scan, and as well, give us a 700v supply for other things.  Neat, huh?

Plate 6 G6 Line Output Circuits +


The G6 field output and associated circuitry is shown in Plate 7 below (click to open a larger detail). 

Frame Timebase Circuit Description – Mike Phelan

Unlike most of the G6, this at least is fairly conventional.  V4002a/b are used as an astable multivibrator,  with the grid of V4002a being fed with pulses from the sync separator.  R4102 and R4101 (height control) are fed from the boost rail and charge C4033 during scan;  using a high voltage supply gives a fairly linear ramp.  V4002b is cut off at that time; when it conducts on flyback, it discharges C4033.  The resultant sawtooth is fed to the grid of V4003a, a cathode follower.

The output from this goes to the grid of the PL508; there are feedback paths from the cathode and from the output transformer to earlier stages to allow linearity control.

R4120, vertical correction, allows some of the cathode waveform to be fed to the reactance valve on the line oscillator to straighten verticals.

Earlier sets have a protection circuit that prevents screen burn if there is no waveform on the anode of the PL508; R4109, a VDR, acts as a rectifier and biases off the grid of V7004b when the timebase is running by using the flyback pulses from the PL508 anode.  

R7209 is connected to the grid of the PL509 line output valve and during the line flyback, charges C7052 negatively without affecting the line output stage.

If the pulses from the PL508 anode are absent, V7004b has no bias and conducts heavily, stopping the line output stage and therefore the raster, so no horizontal line can appear.

The secondaries from the frame output transformer are to feed the scan coils, the raster-correction transductor and the convergence.

The transductor is a saturable reactor with windings from the frame line output waveform that interact with the line scan waveform to correct the width at frame rate – pincushion correction.

The static current on the frame scan coils can be adjusted with the shift control R1074, and its leads can be optionally reversed to give more control.   C1025 is an important component; if it dries up, there will be lack of height and R1074 will get very hot! 

There is little point in going into the rest of the convergence circuitry; it will not really help fault diagnosis at all; in a nutshell, various waveforms derived from line and field are obtained from RC and LC circuits, and most of them are adjustable.

The red and green convergence coils are at 4 o’clock and 8 o’clock, and many of the controls adjust these in a differential way.   The blue coil at the top only affects horizontal blue lines, and the blue lateral coil vertical ones.  All four coils have magnets for initial coarse adjustment. 

As it is a dual standard set, and the line timebase has switchable speed, there are two lots of convergence for this, but I don’t think many people will notice 405 being out! 

Plate 7 G6 Field Output Circuits

I have added a page on converging the G6 chassis (here).