In the event of a LOPT failiure resulting in loss of EHT as the set warms up, this is often down to the EHT overwind on the LOPT having turns shorting as it heats up and the insulation between the turns breaks down.In this case, it is sometimes possible to replace the overwind with another one from a similar LOPT, or a tripler/stick can be fitted. The BRC 1400/1500 ones are good for this, often a 3 stick will suffice. The stage may need to be modified to lower the EHT however as the picture will be too small due to excessive EHT. This is very much a trial and error procedure: be careful. I have so far only tried this on Monochrome sets.
Another occasional source of troubles is the EHT rectifier valve's heater overwind. This usually consists of a turn or two round the main body of the LOPT connected to this valve's base heater pins. If this causes problems, check the series resistor, usually 0.2 ohms or less. and if the winding is at fault try some EHT cable in the sleeving.
Very Old sets employed Mains Derived EHT. If this fails and an RF unit is not to hand, try taking a tripler from the Anode of the Line Output Valve. This leaves a bit to be desired but its better than nothing!
In the case of the BUSH TV125 and some of its near relatives. a BRC1500 Lopt can be fitted. Keith Hamer and Garry Smith describe this method in the October 1981 (I think) issue of TELEVISION.
A word of Warning. If you decide to get a LOPT rewound, the firm that do so may want the insulation removing first. Black Pitch Bitumen is used on some LOPTs, especially the Rank Bush Murphy sets in the 1960s. This can be removed by melting in the oven, NOT THE MICROWAVE!, if you have a VERY accommodating wife or live on your own, or this black pitch stuff can be dissolved in Lighter Fuel. DO NOT get this black stuff on your hands or on any work surfaces you are house proud about. It is horrible and takes a very long time to remove, I once spent a whole evening washing my hands in Lighter Fuel, then cleaning the Kitchen, Bathroom and Door Handles.
Some EHT sticks and triplers, such as the one in the Baird 700 series dual standard colour receiver. have 'open', i.e. unsealed triplers. These can, if they fail, be rebuilt with the correct diodes and capacitors. Look in the Workshop Manual for the details.
As the largest and most expensive part of a TV set, not to mention in some cases the hardest to obtain, the CRT has been the focus of a number of 'dodges' over the years in both professional, private and complete cowboy workshops! Here are a few tricks that are worth a go if you've nothing else to lose.
Inter Electrode Shorts: The most common is heater to cathode, but grid to cathode shorts can occur. These are usually caused by debris or particles that lodge in between various electrodes and causing a short. Try increasing the heater volts, i.e off a separate power supply. Not so much as to open circuit the heaters of course, just warm things up. Give the neck of the tube a firm knock or two to dislodge the offending particle...being careful not to fracture the tube of course! Try this a couple of times, then refit the TV's tube base and try the set now!!
Incidentally, I once did this with an old International Octal Valve that I couldn't get a replacement for. Joy!
If this fails, go for the more technical approach. Get a good large smoothing block, some croc clip leads and a good DC supply. Charge the Capacitor up good and properly, then spark it across the offending electrodes. This will hopefully serve to 'blow' the short as if it were a fuse. Do please rememnber, though, that if the heater is involved in this, to tie both heater pins together to minimise the risk of it going open circuit! Incidentally if you use a 50uF cap charged to about 50v you can also blow those annoying little tin whiskers which cause AF117s and suchlike to fail!
I have heard tell of low emission CRTs ;'boosted' in this way, but I have never tried it myself so I can't comment. Also, one rather mad dealer once advised me, with reference to an interelectrode short on a CRT, to 'Crack it one with the EHT.' I did not do this!!!!!
If a set's tube is a little low, try shorting out or reducing the value of any resistors in series with the heater, or some other way of warming things up just a little by increasing the heater voltage. This is called putting the set on 'boost' and it can prolong the life of an ailing CRT, but there's no way of knowing how long for.
A tube reactivator, if you have one, can often clear shorts as well as boost a tube. These vary in complexity and various designs have appeared in 'Practical Television', sorry, now called 'Television'. I am not going to reccommend a 'Bulb Bopper' however, as I have never actually resorted to this sort of brutality. Yet.
Sometimes a reactivator can let you down. Mullard Tubes respond well to being 'bopped', 'boosted' or in some quarters 'given a kicking'. Mazda tubes generally don't. However, sometimes ecxessive heater volts, be careful of course, can sometimes be used briefly to give a weary gun its first 'belt'!
If all else fails, look under the Bits and Pieces section of this website...
Sometimes, very frustratingly, the top cap can come off an otherwise perfectly good valve. Murphy's law states that it will be a valve with good high emission and of course the very one you don't have a spare for in the kitchen cupboard!
This usually leaves a piece of wire sticking up out of the valve. If it's too short to solder to, file the glass back VERY carefully until there's enough. Leave the valve for a while and make sure it dosen't turn white. If it does, the valve has lost its vacuum so you may as well bin it. We tried...
Tin the wire on the valve. You can either solder to it directly (not that you would resort to such coarse measures of course?) or, fill the original or replacement cap with solder, connect it to the wire on the valve, solder carefully and then use a touch of glue to secure the top cap...make sure it won't melt or crack the glass when the valve heats up though!
There is often more than one of a particular type of valve in a set. For example, a PCF80 may be used in the IF, and another as the Line Oscillator. One will have led a much harder life than the other. Sometimes, swapping the two over can make a marked difference to a set!
Generally speaking, if and RF or IF transformer core is stuck, leave it alone. However, if you HAVE to adjust it, try the following:
A squirt of switch cleaner and a few nimutes wait, or a quick and careful warm up with the hairdryer or a soldering iron can loosen it at least temporarily. Failing that, try adding another core. If a ferrite one sends you the 'wrong way', try a brass one.
For cracked cores, carefully remove the transformer from the set and very carefully drill or break out the core. Be careful not to damage the threads on the coil former into which the replacement will go. Remove all debris and clean the inside of the former before you insert the replacement.
Unfortunately, these often have to be adjsuted. They don't often jam, however, as they are much larger with a wider thread. However, the former can fracture, leaving the body of the coil hanging from the base by its wires. Simply araldite it back down for a good strong coil before trying to tweak it. I heve never tried inserting extra cores on convergence coils so I will not advocate or comment on that here.
I once was presented with a problem: a low emission valve for which I did not have a replacement to hand. It was an IF valve in a BRC1400 series set. I wired this valve up to the reactivator and 'bopped' it in the same way that one would a CRT. After all, the CRT is basically a valve isn't it? It came up extremely well indeed and the gain was so much increased that I ended up realigning and modifying the set to minimise the intercarrier buzz for which they are famous...