BLACK & WHITE TUBES AND TROUBLES.
This page looks at what you can expect to find on the back of the Monochrome Tube Necks of old.There are details of what is on a Colour Tube neck in the Colour Science page, which also looks at fauilts relating to the Colour CRT.
First, a word of warning. Very early mono CRTs did not have an EHT connection, the body of the tube was the final anode. If there is no EHT anode connecction on the body of the tube, then ths body will be dangerously live when the CRT is powered. Also in some cases, such as a 1947 ALBA set that I have, this connection may not be insualted. So beware!
The Tube base connection, which must be secure, will have the CRT heaters, the cathode, grid, first anode and maybe a focus connection. (The focus may be mechanical or Electrostatic, we will look at focussing arrangements later.)
Video information is fed to the CRT Cathode, Picture Brightness is varied by varying the bias voltage on the First Grid. Next is the first Anode, which is fed from the ‘First Anode’ or A1 Supply. Here is a typical example and the voltages to expect. This is from the Mc.Carthy TSH312 from about 1955. CRT type is 12XP4.
TUBE PROBLEMS AND ‘SOLUTIONS’.
The obvious solution to a faulty tube is replacement, however as I mention under the Experts Only Tips page of this site, there are ways around this. Some of these were sanctioned in the trade and used quite shamelessly especially in Rental Sets and before CRT Regunning started. Now I’d like to stress that these things do have their roles, merits and place, but I will be telling a true tale later on to demonstrate that in some cases ‘Enough is Enough!’.
The first problem one can expect to come across is the Low Emission Tube. First steps to ‘cure’ this was to wind up the heater volts a bit. As a friend of mine in Liverpool often says, ‘Glow Me Beauties Glow!’.
When the CRT heater is part of the valve heater chain, it is not usually possible to increase heater volts and leave the CRT in the heater chain as all the other valves will be over-ran. When this happens, the solution remove the CRT heater from the heater chain, using a suitable resistor to re-ballast the valve heaters. This is resistor R in the diagram below.
The heater is supplied from the Boost Transformer which is supplied from the mains as shown.
Another way in which this principle was applied, albeit with rather less finesse, was to simply run the CRT heaters across the mains via a suitable resistor. As the CRT emission fell the resistor value was reduced to warm things up even more.
Here is a true story that I felt I had to include at this point to illustrate a time when things went a bit far!.
I was asked to go out to look at an old Philips Monochrome Set that had a ‘dim picture’. When I arrived at the house, the set, a Philips 170 Series, was displaying a viewable but not excellent picture with, I was told, the Brightness Control turned up full. There was a pleasant glow behind the set and at first I thought that there was some sort of decorative lantern behind the set.
‘Oh No, its been like that since the last time it was repaired’, said the customer.
I looked at the back of the set and found that the CRT heaters, rather than glowing, were close to being incandescant! The back of the set had been hacked away to let the heat escape. I yelled something rather coarse as my hand flew to the on-off switch.
When the set had cooled down enough for me to look closely, I found the tube (AW47-91) heater was running off the mains via a surprisingly low resistor...!!
In situations such as this, the CRT cathodes will have nothing left to give and the only answer is replacement. This is also a safer way to solve the problem.
Another option in these cases is to ‘Reactivate’ the tube. The most basic principle of this is what I refer to elsewhere as the ‘Bulb Bopper’. The idea behind reactivation is to blast the cathode clean to restore emission, but there is no way to guess after this how long the CRT will last. Some would last years but when a CRT has little left to reactivate, then the results will be short lived if any are obtained at all. Please, beg, steal, borrow or buy a decent CRT reactivator!
HEATER CATHODE SHORTS.
Heater Cathode Shorts caused a number of various problems. When the CRT heater is ran from a separate winding on the Mains Transformer, the effect of the fault is extra capacitor across the video input to the cathode. This results in lack of definition and smeary pictures, ‘pulling’ on whites and even sync problems.
Where the CRT heater is part of the valve heater chain, the video signal is short circuited and the positive potential at the CRT cathode is lost. The grid becomes positive with respect to the cathode. This results in Excessive Brightness and the screen is brightly lit at all settings of the brightness control. This is the typical Heater Cathode Short symptom.
Where a cathode follower stage is used in between the video-amplifier and the CRT cathode, the video ampliier valve may be overloaded and its load resistor overheating.
Many CRT reactivators will clear inter electrode shorts. However there was another method used to get around the problem of a Heater Cathode Short. Again, a similar principle of a low loss heater isolating transformer was used to keep the CRT in service. The isolating transformers varied in quality and also could offer varying degrees of ‘Boost’ for the CRT heater. If such a transformer is seen in an old TV set, now you know why it is there and also that the CRT is probably knackered!!
THE SOFT OR ‘GASSY TUBE’.
When the CRT vacuum is impaired, there could be a milky effect on the inside of the CRT, internal flashovers, a blue glow in the CRT neck, apparent loss of emission or poor focus. Focus may not be constant over different brightness levels. Replacement is the only option.
One characteristic of very old sets, that is sometimes joked about, is the dot on the screen after the set is turned off. Before the days of dot suppression, this problem could result in a burn mark in the centre of the CRT. This was often an expensive problem with projection sets.
Mention of Ion Burn very conveniently onto the tube neck assembly, as we can start with the Ion Trap Magnets.
ANATOMY OF A TUBE NECK.
Here is an example of an early CRT neck.
When a CRT neck assembly is being adjusted, remember that the various controls may well have been untouched for years and have siezed or jammed so gentle and careful treatment is necessary to avoid damage to the neck assembly or even the CRT itself. Remember it’ll be old glass housing a vacuum! Gentle and a careful touch of lubrication. If something is jammed and you can get away without adjusting it then LEAVE IT ALONE.
THE ION TRAP MAGNET.
Next down from the tube base is the ION TRAP magnet. In early CRTs, the electron gun was bent away from the front of CRT. This is because it emitted Electrons, which landed on the phosphors and gave us a picture, and also ions, which are larger and tended to burn the tube face away. What the Ion Trap magnet did was to deflect the electrons to where they needed to go (the tube face) while keeping the ions deflected safetly away. Incorrect adjustment of the Ion Trap resulted in loss of picture or severe picture distortion as the electron beam was deflected away from the screen.
There is in many cases a groove or a mark to show where the ION trap should go. It should also be secure on the tube neck. If it is loose or there is evidence of it having been moved (such as dislodged dust!) then the way I set it up is to gently move the magnets about and watch the screen.
When a set has mechanical focus, the focus lever often protudes from the back of the set in an arc. The focus lever is angled outward to facilitate precise focus adjustment without removing the back of the set. There is also a coarse focus control, which moves the focus unit along the CRT neck. My procedure for focus adjustment is to set the focus lever to the middle of its range and adjust the coarse focus for the best possible focus and fine adjust for the best results.
Another way of mechanically adjusting the focus is shown here. The Focus knob adjusts the spacing between the two ring magnets.
The use of Electrostatic focus removed the excess weight and mechanical complexity of the CRT neck assembly. An extra electrode, the focus electrode, in the CRT is used and the focus voltage on it is varied with a potential divider as shown here. It is a simple matter to adjust the control or ‘tap’ for the best focus.
The other purpose of the tube neck is to deflect the dot over the screen! The Line and Frame Timebases create the waveforms for line and frame scan and these are applied to the Line and Frame Scan Coils on the CRT neck to deflect the electron beam up and down and across the screen.
Here’s a more recent tube neck diagram (Bush). The shift plates position the picture on the tube face and plugs and sockets connect the scan coils to the timebases. The deflector clamp should be secure but not excessively tight as to risk imploding the CRT.
Another interesting addition is the deflection correction magnet. This is a permanent magnet that is has no electrical connection to the CRT neck and is mechanically positioned for the best picture shape. It is necessary because the CRT face is curved. The magnet corrects picture distortion as a result of the shape of the CRT and is simply positioned to suit the viewer. Rarely have I had to adjust one.
Scanning coil faults are mercifully rare. The most common problem is a severely distorted picture (trapezium shaped) as one set of coils goes open circuit. The break may be on the contact tagstrip in which case a careful repair can be effected. If not then the best bet is a new assembly. Easy to change, just seat it on the tube neck and position it for best scan. Getting one can be tricky but a lot of TV collectors and old time TV shops still have these things kickng about.
AND THAT’S IT.
Now we know what’s on the neck of a black and white CRT. Large amounts of this apply to colour sets as well, but these are discussed on the ‘Colour Science’ Page.