LINE TIMEBASE & EHT PROBLEMS
This section deals with the single most troublesome area of any old TV set, the Line Output and EHT stages. Many sets have ‘no raster’ type problems and this is the area which is normally responsible. First of all I am going to tackle Mains Derived EHT supplies. This is because although they are now very rare and are confined to VERY old sets, I want this section to be seen for reasons relating to safety.
MAINS DERIVED EHT.
This method of creating EHT involves an extra winding on the Mains Transformer to step the mains suppy up to several Kilovolts to create EHT for the CRT. The usual arrangement is shown here.
In the event of a short circuit either in the valve or capacitor C1, the fuse F4 will fail. Note the chain of 4.7M resistors , these are bleed resistors and ensure that C1 discharges quickly as soon as the set is swiched off. When working on such a system, however, do not trust to luck and bleed resistors and ensure that there is no residual charge lying in wait for you.
The usual problem with this sort of EHT generation is that the EHT winding on the transformer has a flashover or develops shorted turns resulting in overheating and /or low EHT. The symptoms to look for are unfocussed white blobs on the picture and the brightness of the picture reduces as the picture size increases.
Should this situation arise (and it probably will) there are easier ways to create the EHT. First of all, a Diode Tripler (such as in the BRC1400/1500 chassis) will generate EHT when connected to the anode of the Line Output valve. Another possibility is to use a separate RF unit or one of those high voltage generators used in Neon Signs to give you EHT and create a raster.
LINE TIMEBASE AND LINE FLYBACK EHT SYSTEMS.
As the Line Timebase and EHT stages are linked so closely, it is impossible to cover one without the other. This is the system that is almost always used in post war sets, and all sets from 1950 onwards as far as I know.
It all starts with the line oscillator, which operates at 10.125 KHz on 405 lines or 15.625 kHz for 625.The system switch on a dual standard set operates on this stage to change frequency. A typical dual standard line oscillator circuit is shown here. This is from the Philips 170 Series sets, also seen as Cossor, Stella, Alba and under the Granada Rental flagship in about 1966.
Both valves here are double triodes, ECC82. V401 strips and clips the line sync pulses from the demodulated video signal and the line oscillator is locked to these. Note the action of the system switch on the variable resistor network to equalise line lock on both 405 and 625 line standards. If a 405 line signal is available, the setting up procedure I use is to set the line hold on 405 and then switch to 625 to adjust the 625 line line hold control for line lock on 625 lines. (This control is switched out on 405 line operation.) If there is no 405 line signal available, set the 405 line hold control to exactly midway and then set up the 625 line control.
The second ECC82., V402, is the line oscillator and this generates the line drive for the line ouptut stage. Drive is fed from the first anode of this valve to the line output stage via capacitor C413, highlighted. You’re looking for about -50 (405 Lines) or -57 (625 Lines) on the control grid (Pin 2) of the PL504 Line Output Valve. This is another example of a coupling capacitor that is always worth changing.. Make sure that the replacement is suitably rated for the conditions and frequency to which it must operate.
A LITTLE THEORETICAL INTERLUDE!
The waveform fed to the line output stage is not a sawtooth. Here’s why: look at the diagrams here.
When a square wave is fed into a resistive load, the resulting current flow will of course be a square wave. However the line output transformer has a very high L/R ratio and is almost totally reactive at line frequencies. The resulting current is therefore a sawtooth waveform.
THE LINE OUTPUT STAGE.
Line drive is fed from the Line oscillator to the Control Grid of the Line Output Valve. This is the first place to look if you hear no line whistle or have no raster. If there is no line drive then the Line Ouptut valve, usually PL something, will not be biased and will visibly overheat. The Efficiecny or Boost Diode (PY something!) may also visibly glow.
In this conditon avoid running the set for long periods as these valves may be damaged by the unfavourable conditions caused by the fault condition.
There is normally a good negative voltage on the control (first grid up from the cathode) of the line output valve. If this is absent check for operation of the line oscillator.
Line flyback generates a large amount of back EMF which is used to generate the EHT. The sudden cessation of current through the line output transformer induces a positive potential of about 1kVon the anode of the Line Output Valve. The transformer primary is extended and normal transformer action increases this potential to the create the EHT. This is rectified by the EHT rectifier valve (DY or EY). A small winding on the line transformer provides the voltage for the heater of this valve.
This transformer can be the single biggest cause of line timebase/width/EHT problems on a vintage TV set as a single shorted turn can render the LOPT inoperative. Some line transformers are more reliable than others however and I will make a few comments on what to look for later. Bush Restoreres especially are advised to read this section!!!
A PRACTICAL EXAMPLE.
This is the Line Output / EHT stage from the ALBA T990 series from about 1965. (I decided it was time we had a change from Bush and Philips sets and this example is easy to explain and spot the various parts of!)
Line drive is fed to the control grid of V10 (PL500). The anode of this is connected to the Line output transformer. Line deflection coils on the tube neck are L32,33,34. The boost diode V9 and C83 become active during flyback and the 1KV potential at the top cap of V10 is boosted to 15kV by normal transformer action. This is applied to the top cap of V11 (DY 86/87 or an 802 will do here!). Note the heater winding around the EHT overwind and the choke coil L35. Rectified EHT is supplied to the Anode Cap on the body of the CRT. (In this case the CRT is AW47-91) which operates at about 15.5.kV.
WHAT GOES WRONG??
As I have previously stated, this is the single most troublesome area of Old Telly Restoration and not only this, I MUST STRESS AGAIN THAT THE HIGH VOLTAGES AND RISKS CONTAINED HEREIN MUST BE TREATED WITH THE UTMOST RESPECT!!
It is often the first reaction to immediately suspect the Line Output Transformer, as their reliability varies from set to set. However jumping to a conclusion is never the right way to fault find any apparatus even though experience and ‘gut feeling’ often plays a large part!
Listen first for line whisle and look at the Line Output and Boost Diode valves. A visibly overheating Line output valve and no audible line whistle (rotate the line hold control) points to the loss of line drive and this can be comfirmed by the presence or lack of line drive. Look for a negative voltage on the control grid of the Line Output Valve. In the Alba example shown this is Pin 2 of the PL500. HT to this valve comes via R94 onto the Screen Grid, Pin 7. Check R94 and C78.
One of the happy habits of a neon screwdriver is that it will glow if there is line drive and the valves in the stage are operating. A handy and quick check that the stage is doing something but it dosen’t give an exact indication as to how much!
If the operating conditions are not correct and line drive is absent/low then this valve can be damaged and if this valve does not light then a replacement will fail in time as the biasing conditions kill the valve. It is worth trying replacement valves here if there is line drive and whisle but no raster. Another couple of favourites here are (looking at the ALBA circuit) C84 and C83.
Another thing to look for is whether the EHT rectifier valve is lit. If you’re lucky it COULD be the valve heater so try a replacement! If not, then for one reason or another there is nothing reaching the heater of this valve. Switch off the set, allow it to cool down and remove the top cap of the DY87. With this top cap safetly positioned, get a well insulated handled screwdriver and see if you can draw a spark from the top cap connector. This spark should be blue and should jump about an inch . If it is not entirely healthy or non existent then there is low or no EHT being generated.
Remove the top cap of the Boost Diode (PY88) and see if the EHT is partially restored. If it is, suspect the Valve, either of the two capacitors or the LOPT. (Shorted turns). If there is no EHT then there could well be an open circuit on the LOPT, probably the EHT overwind or maybe C84.
If there is a a good healthy spark then this narrows us down to the heater winding or the choke coil (and in some cases a small resistor about 0.16 Ohm) in the valve base. The base will come out of its holder and simple ohm-meter tests will show up theo problem. If its the heater overwind then you can remove it and put some EHT cable in the sleeving and wrap it round. If there is an open circuit choke coil then replace with a 0.1 ohm wirewound resistor!
If the DY87 is lit and there’s no EHT/Raster then there is a possiblity of this valve being faulty. Try a replacement valve. I had this once on a Philips 170 series chassis and the v was at fault. Fitted a DY802 (you can do this!) and the set lived again. Rarely is it this valve, though we always try a new one in hope!
There is of course the possiblity that the lack of a raster is not down to the EHT stages. In this case may I refer you to the video stages section of the website (when built! Sorry!).
FAILING EHT AND VARYING PICTURE SIZE.
When the EHT is too low, the picture size will increase, the brightness will decrease and the brightness control will probably affect picture size. The picture ‘gets closer and fades away’. A common problem (I call this Bush Lopt Disease but it does affect other makes as well, I have a Ferranti Dual Standard set like this as well!) is when the set comes up OK and then gradually the EHT falls off and the EHT rectifier valve gradually extinguishes. As this is most common on Bush sets then I will use Bush as a practical example.
The problem is most likely to appear sooner or be worse on 625 than on 405 lines due to the higher operating frequency. As the set fades away, let it run until the set has lost raster, and then switch off. The chances are that the EHT overwind section of the LOPT (especially if a black pitch covered one) will be warm and sticky to the touch. The insulation in between the tunrs on this winding will be breaking down as the set warms up. This tends to necessitate LOPT replacement but there are a few dodges that you can try, look under the experts tips but if you’re NOT SURE, stop and seek help.
Treat width adjustments with care. Remember the voltages involved!
For lack of width., check the line oscillator, line output and boost diode valves for low emission, check that the HT is at the correct level, width control setting, possiblity of a faulty width coil where fitted (Dual Standard Bush Sets again!) and I’m afraid the LOPT. If the width control is at the extent of its travel check the series and associated components for changes in value. Another common offender is a change in value of the output valve screed feed resistor. (R94) in the Alba Circuit.
Here is a Bush Line Output Stage, from the TV135R (close relative of the TV125) series of sets.
Here, 3V2 is a PL36 and 3V3 is a PY800. The EHT rectifier 3V4 is a DY87. The width coil is 3L4.
The system switch makes necessary changes by switching in or out windings on the LOPT. The switch is shown here in the 405 line position. Width problems associated with the LOPT (and they often are on Bush Sets) are again worse on 625 than 405 line standards.
Incidentally when the width creeps in and a black splodge appears in the middle of the picture, experience tells me to suspect the LOPT.
LINE LINEARITY PROBLEMS.
Cramping on one side of the picture can be caused by a number of things. The Line Linearity control or sleeve could be incorrectly set, but if it appears not to have been disturbed, there could be a low emissiom boost diode or capacitor. If the control can be adjusted to restore correct linearity, then you may get away with this but I would tend to be suspicious if the control is at the end of its travel.
Capacitor problems again can be the cause of this as a low value capacitor on the line output stage could result in the ‘clipping’ of the line drive waveform.
STRATIONS or RINGING ON THE LINE TIMEBASE.
This is another problem which is seen quite often. Vertical and stationary variations in brightness on the left hand side of the picture (or the start of the line scan) is caused by the line output stage being undamped. Again look for capacitor problems especially the boost diode and capacitor and possibly the smaller value capacitors in the line scan coils section of the stage.
The theory behind this aspect of Line Output operation and damping can be seen in greater detail here.The waveforms shown display the effects on the line scan waveform when the stage is damped or undamped.
I will be making comments on the sense of mass capacitor replacement at a later date, however it can be seen that a capacitor replacement session on the line output stage can work wonders!!
TRIPLER & MULTIPLIER CIRCUITS.
The EHT Overwind and rectifier valve system was superceded in the late 1960s by the solid state tripler arrangement. This is best shown in the BRC 1400 / BRC 1500 series sets from about 1968 onwards. An array of diodes and capacitors (usally hidden in a module) in the format of the Cockroft Walton multiplier as shown here. A 7kV pulse is effectively stepped up to 20kV. This is commonly known as a ‘stick’ or a tripler module. A few such modules can be remade (e.g. the Baird 700 Colour series) but many are simply throwaway modules. In the case of the BRC1400/1500 series sets, this module simply plugs onto the LOPT and the CRT anode.
This has several benefits. A lower ratio overwind gives better EHT regulation, Fewer corona and insulation problems and less problemx with Ringing and Vertical Strations.
The EHT requirements for the early colour sets were much higher than for monochrome, typically 25kV rather than 15kV. Line Output and Efficiency diodes became larger (i.e PL509 and PY500) and the DY86 was replaced with the larger GY500. An addition was the Shunt Stabiliser Triode valve. PD500, which regulated the EHT. In the Bush Dual Standard Colour sets such as the CTV25 series, focus voltage was also derived from the Line Ouptut Transformer and rectified with a DY86. Focus voltages themselves were in the region of 5kV.
This is a valved Colour TV Line Output Stage. The basic principles of operation here are the same as for monochrome albeit on a much bigger scale! Note also that this stage also provides the supply voltage for the frame stages and the A1 Voltages on the CRT.
For correct colour representation on bright and dark picture scenes, the EHT currentneeds to remain constant. Note how the Boosh HT, as well as providing the A1 voltages via D1 and the unrectified frame supply via R7, also biases the shunt stablilser triode valve V4. EHT current is adjusted (if necessary) with preset R9.
With all such stages, it is important to follow the setting up procedures descibed in the Manual and to avoid the temptation to judiciously twiddle the pots. Weak spots are the Valves themselves (a cautionary note: do avoid cheapo chinese replacement valves here if at all possible, there are still plenty decent types knocking around and be wary of second hand valves here apart from to prove a fault), the LOPT (I’m afraid) and capacitors. Another thing to look out for is a clean and sound connection on the Top Cap (Anode) of the EHT Rectifier valve V3 (GY500). A bad connection here can result in reduced EHT, poor regulation and can even be a fire risk. Check this carefully!! The Bush Dual Standard Colour Sets were known in the trade as the ‘Burning’ Bushes and with good reason so beware!
Many such stages are shielded and in the case of the G6, the stage is disabled when the covers are removed. The PD500 is also shielded. Not only does this valve dissipate 30 Watts but it also emits X-Rays. So do please treat these circuits with respect!! Ah yes, the G6...
PHILIPS G6 : IS THE FAULT IN THE LINE OUTPUT AND EHT STAGES?
May be a silly question at first, but consider the cost of a Colour CRT and the speed with which it could be damaged by Frame Collapse. (Horizontal White Line!). The G6 has a protection circuit to prevent this - Incidentally this was omitted in later versions of the G6 because of the confusion it could cause.
The Philips G6 protects the CRT in this situation and it is well worth a look at how this operates. V7004b (half of a double triode type PCC85) will conduct when the Frame stages fail, applying a strong negative bias to the Pin 1 of the Line Output Valve PL509. This will disable the line output and EHT stages and therefore protect the tube from a horizontal line of Ion Burn in the middle!
Pin 1 of this valve, the anode of this triode circuit, is connected to the Line Output Stage via resistors R7208 and R7209. A quick check as to whether the No Raster fault you have is line or frame related is to disconnect R7209. If the line output and EHT stages burst into life and you are met with frame collapse, breathe a sigh of relief and turn your attention to the Frame section of the set.
SOLID STATE LINE TIMEBASE AND EHT STAGES
The BRC2000 series has an interesting arrangement which I am reliably imformed is copied on some computer monitors today. I have re-labelled part of the circuit below.
The Line Timebase board contains the Line Oscillator Stage, Line Driver and Line Output Stages. There are two Line Output Transformers wired in parallel.
The Line Driver Transformer T2 also provides line oscillator feed to the EHT generator board. On this set the EHT is generated separately from the Line Output Transformers. The Line Oscillator Stage and driver are common to both line timebase and EHT.
The drive to the EHT board is taken from a winding on the Line Driver transformer. The Line Output transistors (which are prone to going short circuit!) drive the two parallel line transformers. Opinions as to the reliability of these varies - I have found them to be reliable on my sets but some engineers seem to lay in cloves of garlic when they are mentioned. As well as driving the Line Scan Coils (connected via the convergence board) they also generate line pulses for other parts of the set and also provide the A1 Voltage (about 1KV) via Diode W5 and filter capacitor C23. In practice these do tend to fail.
A confusing fault can occur with regard to the electronic trip circuit on the Regulator Board. This trip circuit will operate if current drain on the Line Timebase board is too high and the supply voltage to the Line Timebase board will cease.
This arragement is shown here.
The 55v Rail to the timebase board is adjusted by setting the Width Pots (one for 625 and one for 405) on the regulator board. The Regulator and driver are protected by the electronic trip circuit TR8 and TR9. If this trip operates, remove the Line Timebase board and measure the voltage off load. If it remains low then the first thing to look at is the Width Pots themselves as they can go high in value. Another potential source of confusion occurs when the line timebase board fails to draw any current and the voltage remains high. Check L6 on the timebase board for dry joints, check for volts on the line hold controls - also a swift knock on the system switch with the handle of a well insulated screwdriver can restore results. Set up the width control with the set up and running.
The EHT Generator and tripler unit is shown here.
This was seen in its day as a real ‘on the edge’ design and before the two transistors TR6 and TR7 were actually very prone to failiure, so much so that many engineers recall filling sweet jars with burned out examples at £17 a time back in 1967! The manufacturers of these devices changed the specifications of the transistors to solve this problem in the end.
Line Drive is fed to the EHT generator transformer via TR7. HT to this transistor is current regulated by TR5 and TR6 via the system switch and the EHT transformer. An extra winding on T1 is used on 405 Lines, This is switched on the System Switch on the Line Timabase board.
This HT rail, and hence the EHT voltage itself is regulated by a control voltage at the base of TR5. A sample of the EHT voltage is fed to the differential amplifier TR2 & TR3 and then the inverter amplifier stage TR4.
Feedback protection is provided by TR1 and Current overload by diode W2. (Bottom of circuit.)
AND FINALLY SOMETHING MORE CONVENTIONAL.
The more conventional arrangement for a solid state Line Output / EHT stage on an old colour TV set is still to have the EHT generated from the LOPT. The tripler will always be in module form and should it fail, in many cases a Power Supply fuse will fail (dead set) or (if you’re unlucky!) the Switch Mode Power Supply will trip.
In this case, the first thing to do is to disconnect the tripler from the LOPT and try again. If the set comes to life (obviously without a Raster) then replace the tripler. Incidentally the Late Great Les Lawry Johns (much loved and sadly missed regular contributor to ‘Television’ & before that ‘Practical Television’ ) warns us that the huffing and puffing of a Switch Mode PSU shutting down after three tries will ‘deal the Line Transformrt a Mortal Blow’ as well. I’ll leave that debate for another day.
Diode Split LOPTS were first seen in 1977. Blame B & O it was their set! In these transformers the EHT diodes are included in the LOPT and the capacitance in between windings is used with them to genereate EHT. As this is current practice I will stop here.