16-03-2011, 07:26 AM
For the benefit of anyone who is entering vintage TV restoration, the following I have learnt from TV engineers.
Before anything else
The Basics
1. Cold checks
First, a quick visual check for missing bits or obviously damaged components, check heater continuity with meter via mains plug or pins on the back, 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. Avoid the pop
Snip the capacitor that is across mains, this is the mains filter cap/ RFI suppressor it's called many things then with a variac or lamp limiter or both which is what I do 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. A damp.dusty smell of mains droppers and maybe a bit of smoke as the dust burns off is quite normal from a set that has not been powered on for a long time.
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 or burning smell, 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!!
4. Here We Go
Start with the power supply and get the line/frame timebases operational. Power supplies are very little different from a radios, just a lot more beefy
5. Start Here
The line timebase is a little complicated to start with but with these well constructed designs on the sets, very few 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 you become familiar with them
6. The Screen Will 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 waxies, or the most often I encounter, frame collapse. Use your digital camera now, take detailed pictures of the tag boards/pcb/ hard wiring with particular reference to the stage you are working on take notes this is very important get disciplined at the point. I suggest as I was advised replace one component at a time switching on after each and checking the results, boring but worth while until you gain more experience and a great way to observe cause and effect. The TV fautls 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. it is important to get something on the screen no matter how distorted and blurred this was drummed into me and it's the best advice I was ever given even if it scared the pants off me. Valve bases, switches and preset controls can now be cleaned.
7. Need These too
You will need 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 easily obtained from ebay or 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
Last clean the chassis and places you have not touched so far with 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:-
Got a Blank Screen
There are seven services from the receiver to the tube.
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 flyback 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 flyback EHT is used, EHT can still fail independantly of (5)******
Causes of blank screen
If Flyback EHT used the
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 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.
Chris
Before anything else
- 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 60-70+ and one set I had 100+!! although the former is the norm.
- Have the best tool at your side your multimeter
- Think safety at all times never take anything for granted check, check and double check.
The Basics
1. Cold checks
First, a quick visual check for missing bits or obviously damaged components, check heater continuity with meter via mains plug or pins on the back, 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. Avoid the pop
Snip the capacitor that is across mains, this is the mains filter cap/ RFI suppressor it's called many things then with a variac or lamp limiter or both which is what I do 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. A damp.dusty smell of mains droppers and maybe a bit of smoke as the dust burns off is quite normal from a set that has not been powered on for a long time.
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 or burning smell, 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!!
4. Here We Go
Start with the power supply and get the line/frame timebases operational. Power supplies are very little different from a radios, just a lot more beefy
5. Start Here
The line timebase is a little complicated to start with but with these well constructed designs on the sets, very few 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 you become familiar with them
6. The Screen Will 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 waxies, or the most often I encounter, frame collapse. Use your digital camera now, take detailed pictures of the tag boards/pcb/ hard wiring with particular reference to the stage you are working on take notes this is very important get disciplined at the point. I suggest as I was advised replace one component at a time switching on after each and checking the results, boring but worth while until you gain more experience and a great way to observe cause and effect. The TV fautls 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. it is important to get something on the screen no matter how distorted and blurred this was drummed into me and it's the best advice I was ever given even if it scared the pants off me. Valve bases, switches and preset controls can now be cleaned.
7. Need These too
You will need 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 easily obtained from ebay or 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
Last clean the chassis and places you have not touched so far with 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 ( timebase 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
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 flyback 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 flyback EHT is used, EHT can still fail independantly 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
- ION magnet misplaced
If Flyback EHT used the
- line TMB failure (5 & 7)
- line TMB oscillation weak (5 & 7)
- line TMB Freq low (5 & 7)
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 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.
Chris
