Basic Television Restoration

Before anything else

This is by no means a meant to be a definitive guide, just a quick start reference guide. Each stage will present you with many varied challenges to be overcome, it is assumed the reader has some prior understanding of vintage electronics & the safety required before attempting an old TV.  Likewise If you are at all unsure or are unaware of the inherant dangers these old sets present, I would suggest you stop reading now. 
 

The Basics

Get the service sheet and familiarise yourself with the circuit this is the road map which if studied will help you to understand how the set is designed to function, if you cannot read a schematic worry. Have a lot of caps on standby, TV's take a lot and I mean a lot. Have the best tools ready, your multimeter and common sense, think safety at all times never take anything for granted check, check and double check.

1. Cold checks
 
First, a quick visual check for missing bits or obviously damaged components, check heater continuity with meter, check and operate the on off switch. Ensure chassis is wired correctly if ac/dc chassis should be at neutral again use you meter to confirm this. Finally ensure the TV is set for the correct voltage, you will be surprised how often this is not.

2. If you are of a nervous disposition , avoid the pop

The set can be just plugged in but be ready to switch off, in most cases it will either warm up or nothing.
 
Alternatively for those who are concerned about easing the old girls in, then the next approach is as follows:-
 
Snip the capacitor that is across mains (check the schematic), this is the mains filter cap/ RFI suppressor it's called many things. Now with a variac, lamp limiter or both and connected via an RCD if your consumer unit is not fitted with one, give it a little taste of mains to see what happens. If you have a variac try starting it off at around 50v, increasing by 20v every 5 mins. A damp dusty smell of mains droppers and likely a bit of smoke as the dust burns off, don't worry this is quite normal from a set that has not been powered on for a long time. If the smoke is not eminating from the dropper try to quickly determine where it might becoming from, possible an electrolytic. DO NOT stick your face or head directly over one, if these suddenly let go they can cause great harm. Switch off and disconnect from the mains, feel the can, does it feel hot, if so then it will need replacing.
 

3. Eyes & Ears

Use all your senses to monitor what is happening. Are the valve heaters glowing correctly or are they too bright, you don't want to fry all your valves if the heater decouplers have failed or overrun individuals through other cap failures. Look for obvious signs of burning components or wiring or burning smells, listen for a hum from the speaker is it the deathly van Helsing rumble of main smoothing trouble or just normal/crackles, look for bubbling caps, etc. and be ready to switch off!!
 
If you're very fortunate you may start to hear ( if you still can) the 10,125Hz line whistle from the line output transformer acccompanied by something on screen. It helps to have the contrast and brigtness set at mid point but have a twiddle just to see if you get a glimmer. The line whistle may sound odd, sort of strangulated and not healthy.... Don't leave it like this too long as you may put the LOPT under stress. More often than not the set will appear to be dead with perhaps just the valve heaters glowing.
 

4. Here We Go

At this point we must start with the power supply  get this sorted out first. Get  the H.T & EHT near as to spec (consult service data), check the rectifier often a Metal affair and the main smoothing cans these may have dried out and exhibiting leakage ( electrical & physical). As I was once told by an engineer power supplies are little different from a radios, just a lot more beefy. Once the power supply is sorted and HT is on spec move on to get the line/frame time-bases operational.
 

Interpretation of Electrode Voltages

 
Electrode Indication Notes
Anode
  1. No HT
  2. Low HT reading
  3. High HT reading
  1. Short to Chassis within valve or capacitor. Anode load O/C
  2. Anode load high, leaky coupler to previous stage
  3. Valve not conducting, anode load low
Screen Grid
  1. No HT
  2. Low HT reading
  3. High HT readin
  1. O/C feed resistor., S/C decoupler
  2. Feed resistor high. O/C anode load
  3. Screen feed resistor low (PL81 favourite)
Control Grid
  1. Heavily negative
  2. positive reading
  1. Drive to stage satisfactory,  stage oscillating (if timebase) stage unstable (if i.f. stage)
  2. leaky coupler "soft" valve
Cathode
  1. High reading
  2. No reading
  1. Valve conductring too heavily
  2. Bias capacitor shorting. Valve not conducting at all

5. Now Here

Next the line timebase it's a little more complicated to start with you will soon get to know your way around them. Again schematics are essential and some reading on their function is necessary too. Very few major problems will be encountered and some sort of results are very often obtained with components that are faulty and the more you do the more familiar with them you will become. I won't go into them here but reading up on the time-bases is advised.
 

6. The Screen Will Now Spill the Beans

With the power supply and line output stage operational the set will start to tell you what faults it has, by showing you on screen. This is the TV talking to you about the issues it's having with components, look at the screen and take it in, I find this a lot easier than radio now. Typical results would be a very distorted raster from the vertical scan caused by leaking waxy capacitors, or the most often I encounter, frame collapse.
 
A modern tool to have is your digital camera, take detailed pictures of the tag boards/PCB/ hard wiring paying particular attention to the stage you are currently working on. Take notes too this is very important get disciplined in this matter, it will save you from yourself. I suggest and pass on the advice I was given, replace one component at a time, switching on after each and checking the results. It may seem boring but very worth while advice until you gain more experience and a great way to observe cause and effect.
 
The TV faults will begin to unfold as you go, this is where you begin to learn and see results from your work. Never replace more than two components without a check due to the reasons mentioned earlier, do so at your peril. I was often told until it sunk in "it is important to get something on the screen no matter how distorted and blurred" this was drummed into me over and over, it's sound advice and the best advice I was ever given even if it scared the pants off me at the time. Valve bases, switches and pre-set controls can now be cleaned too.
 

7. Need These too

Handy to have lengths of different coloured flex with insulated crocodile clips at the ends. These are used as temporary extensions when working on sub assemblies, tuners,transformers so you can remove them from their position but still in circuit. These are easily obtained from ebay or you can make your own.
 

8. Clean that Stage

When you have obtained some promising results you can then start to think about the clean up, dust and muck on the section you've been working on, then move on to the next.
 

9.Now you can do the Big one

Once the set is running fine the last clean of the chassis can take place and places you have not touched so far, use a small paintbrush, and the nozzle of a vacuum cleaner. Be very careful it's very easy to damage components and if the set does not work after you have cleaned it you will know that it's something you have just done and the fault should be easy to trace.
 

10.Swot

Read read read, get books I recommend:- Spreadbury Vol 1 ( time base circuits)& Vol 2 ( Receiver & Power supply circuits) Practical Television magazine, especially Les Lawry-Johns Correcting Television picture faults by John Curra, contains lots of examples of Test card faults, their cause and remedy It's very difficult to just wing it through and you will just cause more problems than you solve unless you're a gifted individual.
 

Got a Blank Screen? There are seven services from the receiver to the tube.

 
1. Heater power supply
 
2. video signal (modulation)
 
3. Grid bias (brightness)
 
4. Focus coil current
 
5. Line deflection
 
6. Frame deflection
 
7. E.H.T supply
 
Complete failure of of services 2,3,4,5,6 can occur without causing a blank screen. For example
 
1. Self explanatory
 
2. If modulation is removed that will leave a plain raster, which level will be decided by the brightness control.
 
3. If bias is removed the result will depend on how it is done, +ve maximum will make the screen go peak white. It can go down to chassis or possibly even more negative than that, when the screen will go blank; or it can be non existent when the grid will become free and anything can happen on the screen.
 
4. If focus current failed the spot would simply become defocused.
 
5&6 If either of these fail vertical or horizontal line will appear across the screen.
 
So really 1 3 & 7 can cause a blank screen*
 
If EHT is of the fly-back type then EHT is dependant on the line TMB circuit running correctly, with the exception of a very rare kind of fault, therefore failure of the line deflection service (5) is almost bound to result in a blank screen if line fly-back EHT is used, EHT can still fail independently of (5)
 
* Causes of blank screen
 
  • Faulty tube No mains supply (1&7)
  • Heater supply to tube low or absent (1)
  • EHT low or absent (7)
  • Grid/cathode bias too negative (3)
  • Beam deflected off screen (rare) (5&6)
  • Failure of HT volts to first anode (tetrode tubes only)
  • ION magnet misplaced (if one fitted)
  • line TMB failure (5 & 7)
  • line TMB oscillation weak (5 & 7)
  • line TMB Freq low (5 & 7)  

This following table should give some pointers

Symtoms Observations Check around Check Particularly
No picture, no sound
  1. No valves light
  2. except for eht rectifier, all valve light normally
  3. Some valves light brightly
  1. Heater chain
  2. HT Supply
  3. Heater Chain
  1. Mains lead, mains dropper,fuses, on-off switch
  2. HT Fuse, HT rectifier, main smoothing electrolytics. If still in trouble look for burned out resistors. Try disconnecting the various HT feeds systematically
  3. Heater-cathode leak in the last lit or first unlit valve in the chain
Picture satisfactory, no sound
  1. Screwdriver touched on centre tap of volume control produced no clicks or hum in the speaker
  2. Screwdriver touched on centre tap of volume control gives audible results in speaker, but no clicks are heard when channel is changed
  1. A.F. and sound ouput stages
  2. Sound i.f. and detector stages
  1. Sound output valve, o.c. lousspeaker, oc sound output transformer primary winding 
  2. Sound i.f valves, detector valve or diode, screen grid decoupling capacitor on i.f. valves
Picture satisfactory, distorted sound
  1. Distortion varies with volume
  2. Distortion does not vary with volume
  1. Sound output stage
  2. Interference-Limiter stage
  1. Sound output valve, loudspeaker, grid coupling capacitor, cathode bias bypass electrolytic
  2. Resistor from HT to upper end of limiter diode, Capacitor from lower end of diode to chassis
Sound satisfactory, no picture
  1. No raster, line whistle or EHT
  2. No raster or EHT, line whistle is audible
  3. No raster, line whistle and EHT are present
  4. Raster present, varied by brightness control
  5. Raster present, but not varied by brightness control
  1. Line oscillator and output stages
  2. Line output transformer
  3. C.R. tube
  4. Video amplifier i.f. and detector
  5. C.R. tube and brightness circuits
  1. line oscillator valve, line output valve, efficiency diode, HT fuse to timebase where fitted
  2. EHT rectifier, efficiency diode capacitor, line output transformer, scancoils, EHT smooting cap if fitted
  3. CR tube, ION trap magnet (maybe loose), boosted HT line,brightness control
  4. Video amplifier valve, vision detector, Vision i.f. valves. Check that HT is present at anode and screen grids of all vision i.f. valves
  5. CR tube for inter-electrode shorts. Open circuit video amplifier anode load or choke. Unstable vision i.f. stages due to inadequate decoupling
Screen lights but no picture or sound
  1. Rotating  channel selector produces clicks and flashes
  2. Rotating channel selector does not produce clicks or flashes
  3. Hiss in sound and "snow" on screen
  1. Tuner unit
  2. Tunder unit and i.f. strip
  3. R.F. stage of tuner, aerial
  1. Mixer valve, oscillator anode load resistor high,broken tuner contacts. Failty common i.f. stage (if fitted)
  2. Tuner valves,common i.f. valves HT feeds to tuner and i.f. strip
  3. R.F. amplifier valve, aerial coil in turret tuners, aerial downlead,plug and socket, components at rear of aerial socket
Sound satisfactory, picture negative
  1. Picture dim, controls have little effect
  2. Picture bright, buz on sound
  3. Brightness normal,no buzz onsound
  1. CR tube
  2. a.g.c circuits
  3. Limiter stages
  1. CR tube with disconnected cathode
  2. A.G.C. and sync valves. Valves controlled by a.g.c. Diodes on a.g.c. line
  3. Limiter valve or diode, limiter control open circuit

Picture distorted

 

 

 

 

 

 

 

  1. Bottom of screen cramped or folded
     
  2. Top of screen cramped or distorted
     
  3. Insufficient height
     
  4. Insufficient overall width
     
  5. Insufficient width at left
     
  6. Insufficient width at right
     
  7. Bright band of cramping at left
     
  8. Verical lines bend or curve
     
  9. picture tilted
     
  10. corner shadow, picture central
     
  11. Corner shadow, picture off centre
     
  12. Line and frame hold controls need adjustment
  1. Frame output stage
  2. Frame output stage
  3. Frame output stage
  4. HT supply
  5. Efficiency diode
  6. Line output valve
  7. Line output stage
  8. Sync and timbase stages
  9. Scanning assembly
  10. Scanning assembly
  11. Scanning assembly
  12. Sync stages
  1. Frame output valve and cathode bias capacitor and resistor
  2. Frame output valve, output transformer, grid coupling capacitor, low boost volts
  3. Frame output valve, low boost
  4. HT rectifier, line output valve, screen grid feed resistor on line output valve
  5. Efficiency diode valve and its associated capacitor
  6. Line output valve and cathode resistor if fitted
  7. Low line drive from oscillator. Efficiency diode capacitor "brushing" in eht circuits
  8. Heater-cathode leak on sync seperater or line oscillator valve. Faulty electrolytic smoothing
  9. Scancoils need truning slightly around neck of CR tube
  10. Scancoils not fully forward. Displaced ION trap magnet. Faulty CR tube gun assembly (take tube out, fit upside down)
  11. Positioning magnet maladjusted , focus or ION trap magnets displaced
  12. Sync separator valve, coupling capacitor from video amplifier
Picture will not hold still
  1. Picture rolls or judders
  2. Parts of the picture more left or right as bright objects near the edge of the screen
  1. Sync stages
  2. Sync, video and aerial stages
  1. Frame sync or interlace valve or diode, faulty screen grid decoupling capacitor on sync separator
  2. Sync seperator valve and coupler. Resistor from HT to video amplifier cathode, Vidion detector diode, aerial picking up multiple (ghost) images.
Dull picture
  1. Little control over brightness
  2. Brightness control causes picture to enlarge and go dim
  3. Brightness satisfactory, but control poor
  1. CR Tube
  2. EHT supply
  3. CRT video and R.F. stages
  1. Low emission CR tube
  2. EHT rectifier valve, faulty line output valve or transformer
  3. CR tube, video amplifier, vision detector diode, r.f. amplifier valve
Sound and picture take a long time to appear
  1. Picture comes up small
  2. Sound and picture arrive together suddenly 
  1. Power unit
  2. Tuner unit
  1. Low HT rectifier
  2. Local oscillator valve and anode load
Picture slow to appear
  1. Sound arrives as normal
  1. Line output stage
  1. Slow heating efficiency diode valve

Finally

 
A signal source is needed, an Aurora is the one to get, it provides Test Card C and also will convert 625 to 405 so you can watch freeview/sky dvds etc. If you don't want to get one yet then a good modulated signal from a sig gen will produce bars on the screen, but to set the TV up properly you will need the Test card the aurora produces or a test pattern generator.
 

Safety

Get a scope and ISO T/X if you intend to use it is a must, the final word is be careful EHT won't kill ( although mains derived EHT WILL!!) but will sting and hurt like hell, it's the bad jump away when you react that can cause more problems. A1 volts and HT are nasty so same care as in radios is required. Hope that all makes sense and is of some help, It's what what was drummed into me by a couple of very knowledgeable individuals and although I kicked and screamed and deviated from time to time their advice paid off and is now my mantra.
 
Finally you should not attempt any repairs as a complete novice, indeed if you are unaware of the dangers that lurk within old TV's and Radios and there are many,  you should leave it to someone who does. You have been warned these sets have a high potential to electrocute yourself.
 
Regards
Chris