Technical Info and Restoration Tips On Televisions

 

    THE BUSH TV11A, TV11B, TV12A, TV12B

    The BUSH TV11 was well ahead of it's time, it was one of the first AC/DC sets and one of the first designs to use an efficiency diode. It throws up no major problems. It's wholesale use of tag boards as in common with most BUSH TVs' allows the repairs to be hidden behind the tag boards. My own TV11 is the first and only set where I have completely hidden my repairs, even underneath the chassis. Before you try out the TV11 you must disconnect the EHT smoothing capacitor. There is a section below on this webpage on EHT capacitor substitution.

    THE BUSH TV22, TV24, TUG 24

    The BUSH TV22 performs very well when it has been restored. Before you try it out on the mains you should replace the capacitors in the frame time base, other wise the oscillator transformer may burn out. You should replace or at least remove C29 (0.01uF 250V A.C.) which is connected across the mains. Now you can switch it on with out fear of causing any damage. Faults you may find and their cures are listed below:-

  • EHT present, but no raster: This may well be because the A1 voltage is low, you should measure it with a high impedance voltmeter e.g. a DMM. The A1 voltage should be 300V, if it is low it is due to a faulty Metrosil. See below in this webpage for information on Metrosil substitution.
  • No frame output: This is probably due to an open circuit oscillator transformer. The same transformer is used in the TV12 for the line oscillator, but any blocking oscillator transformer will work. The line oscillator transformer from a BRC 1500 chassis will work but you will have to make a mounting plate for it, on my TV22 I used one of these transformers and held it in place with cable ties.
  • Poor line lock: This fault is probably only experienced when using certain standards converters. Standards converters can produce minute spikes in the video waveform ,but the only sets affected by this are very early TV22's, it is actually due to the sync separator having to higher a frequency response making it very sensitive to interference spikes. Within the first year of production a modification had been introduced in the form of a 10pf mica capacitor (C32) between the grid of the sync separator and chassis, if you have to fit this modification I recommend that you use a 50pF capacitor instead.
  • Tube Faulty or Low Emission: MW22-16 tubes are getting hard to find, a MAZDA CRM192 can be used, unlike the early 2v heater MAZDA tubes the CRM192 has a more reliable 12.6V 300mA heater and the same neck diameter as a MULLARD tube. The MAZDA tube is aluminised and this makes it superior to the MULLARD tube. There are plenty of ECKO TMB272 sets in poor condition that can donate a MAZDA CRM192 tube. To use a CRM192 tube it must first be sprayed with Aquadag, see the section on this page on EHT Capacitor Substitution for details on how to coat a tube with Aquadag. The fact that this tube need 12 volts for the heater instead of 6 is not really a problem, you might have to change the mains voltage setting to 230 volts, all my sets are on the 230 volt setting because since the EU changed the rules on the mains voltage limits the voltage in my area has gone down below 235 volts anyway.
  • No EHT, line whistle quiet or non existent: This is probably due to L.O.P.T. failure. Failure of this in the TV22 is usually due to rusting of the laminations, this makes it overheat due to eddy currents preventing oscillation and causes breakdown. The EHT winding however is wound on a paxolin tube and so it survives, this is good news because the EHT winding is the only one which is wave wound, it is easy therefore to rewind the transformer. First you must get some new laminations cut, sets from 1951 use "I" shaped laminations which I can supply. Rewind the transformer using 0.2mm diameter wire. There are 60 turns between pins 1 & 2, 631 turns between pins 2 & 3, 1042 turns between pins 4 & 5 and 312 turns between pins 5 & 6. Make sure you wind the wire in the same direction as the original, otherwise the EHT will be wrong. Before you try your new L.O.P.T. you must replace the PL38 which is probably dead. If you have already pulled apart the EHT winding of your old L.O.P.T. and have lost count then do not despair, with little modification you can use a L.O.P.T. from an ECKO TMB272.

    THE BUSH TV56, TV53, TV62

    The BUSH TV56 is the second band 3 BUSH TV design, the set has mean level AGC and flywheel sync, the TV62 is one of the first post-war sets to use an electrostaticaly focused tube. The strangest effect on the TV56 is the disappearance of the picture after having changed the input signal going to your band 1 modulator. The effect is caused by the AGC circuit. When the video signal has been temporarily disconnected from the modulator the set is fed with an un-modulated carrier, this causes the AGC to be turned up until the set is driven into distortion by the blank carrier. When the signal is restored to the modulator it cannot pass through the IF stage and to the AGC and so the set is locked in a vicious circle. To restore the picture you just have to disconnect the aerial wire from the set for about 2 seconds. Another problem on the TV56 is failure of the combined concentric On-Off-Volume/Brightness control, see the section below on this page on On-Off-Volume control replacement.

    THE EKCO TMB272

    The EKCO TMB272 is one of the only valve TV sets that can run off a 12 volt battery. You must disconnect the EHT smoothing capacitor before you try the set out. You will probably find that the frame circuit is different to the one given in the NEWNES books, the main difference was the addition of a sliding trimmer to act as a Top Linearity control. If after replacing all the nasty nF capacitors in the frame timebase stage you find that the frame linearity is very poor it is probably due to MR1 misbehaving, you can replace MR1 with a 1N4007.

    THE FERGUSON 951T, 941T, 941TS

    Make sure you disconnect the EHT smoothing capacitor before you try the set out. Lack of raster may be because the A1 voltage is low due to failure of the Metrosil, see the section on this page on Metrosil substitution. This set can produce strong interference at UHF, this does not cause problems with the set itself, but it will affect your standards converter. It seems these sets always happily radiated lots of UHF rubbish, substitution of the line oscillator (V15, EF91) may cure the problem or try adding a capacitor of about 10pF between the oscillator's grid and chassis, you may need to change C45 to ensure that the correct line frequency is in range of the line hold control.

    THE INVICTOR T102

    The T102 is one of the strangest TV designs, it does however produce excellent pictures. It is the only thing I have seen that uses a transformer which is fed from a mains dropper. Some T102's have a fixed resistor in place of R67 and a variable capacitor for C53 which forms the line hold control, sets with this modification are 625 line capable. Before you try your T102 make sure that you disconnect the EHT smoothing capacitor (C55), see further down on this page for the section on EHT capacitor replacement, some T102's have an aquadag coated tube so C55 is not needed anyway. The HT power supply is a voltage doubler circuit using a PZ30 for the two diodes, this valve is highly stressed and the heater-cathode insulation can breakdown which sends the reservoir capacitors into orbit! Lack of EHT was due in my set to a broken ferrite core in the EHT transformer. The worst thing about this set is that it has two output transformers running at line rate (10.125KHz), one is for the EHT and the other is for the line output, my young ears cannot stand the set being switched on for long!

    SPENCER WEST 957

    The SPENCER WEST 957 is one of the worst sets ever made, it's poor reliability makes it a difficult set to find today. The load bearing transformers (LOPT, Frame output and Sound output) are all unreliable. In my set the LOPT had already been replaced with an old RS replacement type, the sound output transformer had also been replaced. I replaced the frame output transformer with one from a BRC 1500 chassis and then changed the feedback arrangements so that the frame output stage resembled that of the BRC1500, this produced a fairly linear picture. If you thought that multiple transformer failure was it's worst feature then you are wrong. The worst feature of the 957 is that it is transformer-less, but the tin casing is earthed. In order to prevent the casing of the turret tuner (which is connected to mains neutral) from shorting to the outer casing of the TV (which is connected to earth) the tuner is wrapped in rubber covered Pianola cloth, this makes for a good fire hazard. Another bad feature is that instead of using a cone shape to protect the end of the tube (the tube is longer than the rest of the set) the 957 has the speaker mounted back to front on the outside of the set to cover the end of the tube. This means that the speaker's terminals are exposed, so if the insulation of the sound output transformer breaks down (a common fault with this set) the speaker terminals will dish out nasty shocks! It comes as no surprise that SPENCER WEST originally described the set as being intended for occasional use.

    METROSIL Substitution

    A METROSIL (the proper name is a Voltage Dependant Resistor) is an early semiconductor device, it usually comes in the form of a green or red painted ceramic disc which is about the size of a 10p piece. METROSILs were used in early TVs to provide the A1 voltage for the tube. Replacements are not available, so you must use a diode plus some other bits to substitute for the METROSIL. The main thing to bear in mind is that a METROSIL is a high impedance device.

    In my BUSH TUG24 I used two 1N4007 diodes connected to a potential divider made of a 1.5Meg and a 1Meg resistor. Across the 1Meg resistor I placed the 0.1UF capacitor (C18), the A1 voltage was derived from the junction of the 1.5Meg and 1Meg resistors. Note that the resistors should be high voltage types. See diagram in fig 3.

    Alternatively you could try using the 250V stabilising VDR from a BRC 1500 chassis, it is a carbon coloured rod with a dash of green paint on one end, I have never tried one of these as a METROSIL.

    Fig 3

    VISCONNAL EHT Smoothing Capacitor Substitution

    VISCONAL EHT capacitors are used in early post-war sets that the EHT derived from some source other than a mains transformer (e.g. the LOPT). The casings of VISCONAL capacitors consist of a brown BAKELITE cylinder which is sealed at the bottom with a spun aluminium base. The BAKELITE cylinder is filled with a PCB based oil which is highly carcinogenic. If you try a set with one of these capacitors connected it an explode spraying carcinogenic oil everywhere (I speak from experience). You can replace the capacitor with a new one (which will not look the same) or you can coat the CRT with foil or AQUADAG which you must then connect to the chassis or you could open up the old capacitor and put the replacement one inside.

    To open up the original capacitor you will need to use a lathe. You must wear gloves whilst cutting the capacitor open. Use a lathe tool with a sharp angle on it and cut the bottom end of the capacitor between the screw thread and the outer edge. You must use the lathe on it's lowest speed to avoid spilling too much oil. Have a tray to catch the drips of oil from the lathe. When you have opened the capacitor you have to extract the internals. Now drain out all the oil and clean the inside of the capacitor with meths. How you fit a new capacitor inside is up to you, but you will see that when the capacitor is opened in this way the aluminium ring is left intact which means that if you sit the capacitor on a flat surface the opening cannot be seen. In order to dispose of the oil (and the gloves which you used) from the old capacitor you should get in touch with your local council.

    You can buy AQUADAG in the form of a graphite spray which is available form RAPID electronics of Colchester. Before you spray the coating on you must clean the tube thoroughly and mask it off, the finished result is identical to an original AQUADAG coating.



    Fig 5 Removing the masking tape from a re-coated MW22-16 tube.


    On-Off volume control Substitution

    Many old TV's and radios use switched 500K log pots for volume control and power switching. These large value logarithmic switched pots are no longer available, the answer is to make them up from bits of others. You will need to order a switched pot (of any value) from the 'Standard Type with Switch' range from Maplin Electronics (such as stock no. FW41U) and a 470K log pot from the Maplin 'Standard Range' (stock no. FW27E). It is then an easy task to dismantle the switched pot and swap the track with the one from the other pot. By using this method stereo switched pots can also be made up.

    If the pot that has to be replaced is part of a multi-control concentric assembly then replacement of the inner most control (usually the on-off volume control) is not too difficult because the replacement pots are smaller than the originals. The way to do it is to remove the tinplate cover from the old pot and then remove the old mains switch from the cover. Then drill a 10.5mm hole in the back of the cover so that the new pot can be bolted to it. By using an extra nut (Maplin stock no. FP06G) the new pot can be mounted as far back from the old cover as possible, this should allow enough room for a spindle coupler (Maplin stock no. RX29G) to be fitted within the old cover. A spindle coupler is required because an inner concentric pot usually has a smaller than standard shaft. Next you should remove the old shaft and wiper, cut the old wiper from the shaft, leaving the shaft intact. The shaft of the new pot must be cut very short, so that it fills half of the shaft coupler. The whole lot must be reassembled, you may have to shorten the original shaft and you may have to drill out the centre of the BAKELITE track assembly.


     
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