The following is based on an articlal written by F.P.Rozee that appeared in Practical Television magazine in December 1962.
It is highly dangerous to say the least, and I'd add that you'd be wise not to attempt this in a transistor set !
However the author claims some success using this method so perhaps it could be developed into something a lot safer and maybe a little more controlled.
Welding the ends of the open-circuit heater is obviously the only solution, so that what is required is a voltage of sufficient amplitude to bridge the space between the ends of the break in the heater. Also there must be sufficient current available to effect a weld, and the regulation of the supply must be rather poor in order to obviate the possibility of a further fusing on completion of the weld.
These requirements exist in the television set itself, the source of this supply being the cathode of the booster diode. There is a pulse of some 3,000 to 4,000V at this point. Tapping the supply is usually easy as modern booster diodes have a top cap cathode. With some older valves the cathode is brought out at one of the base pins and where this type of valve is used, connection will have to be made under the base at the appropriate pin.
There are other points in a set where it may be thought that a suitable supply exists, one being on the line deflection coils and another at the anode of the line output valve. These points are, howver, unsuitable as there is a risk of burning out the line output transformer on completion of the weld.
Check first to make sure that the open-circuit is not at the point of soldering the lead out wires from the tube to the base pins, nor caused by poor contact between the pins and the holder. When it has been definitely determined that the o/c is in the heater itself, proceed as follows.
Make certain that the set is switched off. Remove the holder from the base of the tube and short the heater contacts on the base connector together to give continuity of heaters through the set (this is, of course, not necessary with parallel heater sets). Clip a jumper lead between one of the heater pins on the tube base and the chassis making certain that the clip at the tube end is clear of other electrode pins.
Take a short piece of e.h.t. lead or other well insulated wire, strip the insulation back sufficiently to wind and twist up round the shank of a high voltage insulated screwdriver. This will serve as an insulated prod, the other end of the wire being stripped back and connected to the cathode of the booster diode, either by placing it under the top cap clip or soldering it to the cathode pin at the base, depending on the type of valve. Preparations are now completed and the weld can be made.
Making the Weld
Switch on the set and hold the screwdriver prod clear of the chassis, when the line output circuit is well and truly functioning, touch the prod against the free pin of the tube heaters, ensuring that no other electrode pins are touched when doing this.
An arcing will be seen inside the tube neck and this will continue for a second or so then cease. Remove the prod immediately. The weld should now be completed.
If the arcing continues and a weld is not made, it is possible that either the hearer wire is brittle and has shattered, or that its fusing was caused by a great excess of current and the gap in the wire is too wide to be bridged.
The writer has had some measure of success. One repaired heater has been working for well over a year and another worked for about three months before going heater to cathode s/c. A third refused to weld. A check was made with each of the above sets, as it should be, to see if there was a reason for the heater failure. With the first two sets everything was in order, but with the third it was found that an h.t. decoupling capacitor had been cut into by a valve heater tag. This had passed h.t. through the lower part of the hehter chain and had no doubt blown a sizeable piece out of the tube heater. Based on the above experiences, it would seem that there is probably a 50/50 chance of a successful repair, but as the tube is useless there is everything to be gained by attempting to save this very costly component.
The reader is advised to take more than ordinary care when handling the wire connected to the booster diode.
Owing to the high voltage present and the availability of current in this highly inductive circuit, a very severe shock will result from any contact.
When connecting the set to the mains, make absolutely certain that the chassis is connected to the neutral pole.
|Page copyright (c)
|CLICK HERE TO RETURN TO THE
TELLY TECH PAGE
17th September 2001