Complete loss of picture - any suggestions?

[DoctorWho]

Well I had been getting on well with my Cossor 912 TV repairs.

Oh dear, now I have a worse problem!!

I had a good look at the circuit and I could not find C130 (0.01uf) within the RF strip (I've since come to the conclusion that it must be elsewhere within the main chassis). Due to the blue glow of the valve (previously discussed) I was investigating further.

Anyway I initially took some voltage readings which were as follows:

Pin 1: 222 V DC
Pin 2: 0 V DC
Pin 3: 0 V DC
Pin 4: 101 V DC
Pin 5: 0 V DC
Pin 6: 0 V DC
Pin 7: 0 V DC
Pin 8: 0 V DC
Top Cap: 224 V DC

Since I couldn't find C130 I soldered a 0.01uf capacitor between pin 4 and chassis (presuming that it was missing).

The voltages rose as follows:

Pin 1 rose to 350 V DC
Pin 4 rose to 108 V DC
Top Cap rose to 293 V DC

Unfortunately the increase in voltage caused something to fail, the sound is still perfect and the set is still displaying a perfect raster, but no picture information is reaching the CRT, it just displays a perfect uniform raster.

I promptly removed the capacitor that I had added and the voltages returned to those I have initially stated.

Please can anyone tell me what is likely to have failed to cause the complete loss of picture? Is it likely to have been a valve or another component which has failed? I was so disappointed as I had a perfect picture and now nothing I tried changing the valve for the other identical valve I have but this made no difference.

Looking at the service data for the set it seems that Pin 4 should be around 110V, Pin 1 should be 0V and the top cap should be 140V, so there is definitely a difference between the voltages quoted in the service data and those I am actually finding in the set.

Any suggestion as to what I need to do to restore the picture, and also what, if anything, I need to do to correct the voltages to the valve would be greatly appreciated. I'm really perplexed now.

Thanks for the help.

Peter.

[peter_scott]

Hi Peter,

It might be worth checking the voltages on the pins of V8 and see if anything looks particularly odd. Especially on pin 5.

HTH,

Peter.

[Heatercathodeshort]

Ok Peter,
I have had a few thoughts on this and can only offer you some basic help as I doubt if you have much test equipment. Don't get me wrong, not a lot is required but you need the basics. You need to test to see if the vision signal is arriving at pin 5 [The control grid nearest the cathode] of the 61SPT. Without test equipment the only way I know of doing this is to connect a pair of HIGH RESISTANCE headphones via a small capacitor say .01uf between the chassis and pin 5. [Cap to pin 5, other lead to chassis, if your a bit nervous about wearing the phones you should be able to hear the vision signal with the phones on the bench]With a signal supplied you should hear quite a loud 'buzz' that will vary in pitch with the picture content. This will prove that the signal is reaching the video output valve.
[2] The cap you mention is the screen decoupler [middle grid] and placing another cap across this one will not affect performance.
[3] My bet is that you have disturbed something, maybe the video peaking coils. L10 is a good bet and you may have' prodded it' open circuit.
[4] Take GREAT care when disturbing components. I have great experience but still make slip ups to this day.
Suggest you try this and let us know what the results are. These are not the sort of sets to work on if your fresh at picking up knowledge but the best of luck with it. Regards JOHN.
[3] I think the valve is opperating ok due to the fact that the tube is biased ok due to the plain raster and feel its something before the control grid.

[DoctorWho]

Thanks to you both for your help with this. Unfortunately the only test equipment I have is a multi-meter, which, although a good one, obviously isn't high tech like a workshop. I don't have an oscilloscope and, I have to be totally honest, that I wouldn't know how to use it unless someone showed me how to. I'm fine with the more basic work (such as replacing old capacitors and rotten wiring, checking transformers and chokes for open circuits and the like, but I struggle with the more in-depth fault-finding).

I don't actually have any high resistance headphones, but I will see what I can get hold of, is a telephone receiver useable for this?

I'll check V8 and everything else I can find around the RF sub-chassis, I'll let you know what I find and what readings I get. The plain raster is fine, you can see the line structure and it'sperfectly uniform, it's just simply not receiving any vision signal. Changing the 61SPT made no difference here.

I will do some investigation work tomorrow and let you know how thinsg progress,

Thanks again for the help, I really appreciate it.

Peter.

[Heatercathodeshort]

Peter,
You can use a low impedence headphone but you will have to connect it into circuit using an old sound output transformer. Any type will do. Connect the secondary to the phone and the primary to the test circuit as previously described.
Thanks for you honesty. Don't worry about that but do take great care when working around the R.F. stages. You need to work back from the video output stage treating it like a radio until the loss of signal is traced. It must be carried out in a methodical procedure to save frustration. Remember..Its only an old telly!
Using a scope to check the circuit is easy once you have been shown the procedure. You are very welcome to spend a day with me in West Sussex and I can run over it with you. There are many good scopes available for free if you check around. You only need the very basic type with a bandwidth of say 3mc/s to do all you need. It is then very easy to check timebase and video waveforms and to track down the missing link. I think you have done very well with the 912 considering you admit to having a limited knowledge.
Regards. JOHN.
The notes below are general and not directed at Peter alone!
I would suggest that all restorers of vintage equipment have the following:

Some form of test meter, preferably analogue but if you are familiar with digital types, well ok then give one a try..

Basic tool kit including B.A. box spanners and a good selection of pliers, screwdrivers and small hand tools of your choice.

A modulated signal generator that will cover the television band to around 60mc/s.

An old scope. Many are scrapped these days due to being overtaken by new standards. Very basic type required.

A can of switch cleaner.....!

Most important..A CIRCUIT OF THE EQUIPMENT you are working on.

Old PRACTICAL TELEVISION MAGAZINES from the 50's [Les Lawry Johns articles] TELEVISION RECEIVER SERVICING Volumes 1 and 2 [Spreadbury]

A range of test leads say 2ft long and terminated with crocodile clips.

A MAINS ISOLATION TRANSFORMER.

This is the minimum required and most if not all of these items will cost little if you keep your eyes open.

Remember that as soon as you get some form of raster on your receiver the sets crt shows what area the fault lies..Rather like a big scope.....

[DoctorWho]

Well I have done some more investigation work, I did check V8 and the readings were as follows:

Pin1: 1V, Pin 2: 0V, Pin 3: 404V, Pin 4: 404V, Pin 5: 73V, Pin 6: 1V, Pin 7: 0V, Pin 8: 101V.

I did find a pair of hish resistance headphones and checked as advised, using a 0.01 condenser etc, at V7, I connected to each of the grid terminals and there is just a very quiet steady buzz, similar to that which you get when trying to tune in a crystal set, nothing loud. The only loud noise heard was when touching the heater pin, then a loud buzz was heard.

I tred the same on all of the other valves from the entry of the co-ax cable right up to the 61SPT and I couldn't find any appropriate buzz. I even tried substituting each of the 63SPT valves in turn with an EF50 just to see if any were duff, but no change there either. Similarly I also tried swapping the two SD6 valves over just to eliminate that possibility.

I checked each of the windings between each valve and they all showed continuity.

There are a number of extremely low valve condensers which, presuming they were mica ones, I did not change them. They are C101, 105, 109, 111, 113, 115, 118, 121, 123 and 128. They are such values as 50pf, 500pf and 5pf. Is it likely to be one of these which is causing the problem? Should I perhaps change these as well?

I'm really quite stuck again now as I don't know what to try or where to look next. It does seem that the vision signal isn't reaching the 61SPT. I've checked that the coaxial cable from the aerial socket to the chassis is intact, and it is.

Any suggestions as to what to try next would be greatly appreciated. Do I change the low value condensers or perhaps look for something else?

Thanks for your kind offer John, if I were nearer to you I would love to take you up on that offer. I wish I had a scope, but obviously I'd need someone to show me how to use it before I could diagnose this problem with it. If you can suggest a next step I would be very grateful.

Thanks again for the help.

Peter.

[Aerodyne]

This may sound a little odd, but...
you could set about things the other way around. Instead of checking for presence of signals at the video amplifier grid, feed in an AC signal via a cap from the heater line. If the vid amp valve and circuit is working you should see hum bar(s) on the screen. At least it will prove or disprove the amplifier stage.
I do recall seeing similar problems with faulty CRTs. An electrode might be o/c even though it seems unlikely from your description. Can the brightness be altered? Again, you could feed in AC to the grid and see if the raster is modulated with hum bars.

In honesty, my knowledge of TV work is pretty rusty. It is many years since I fixed the things for a living, so others might quite rightly take issue on these suggestions. I advise you to heed them, but try these if you are stuck and nobody else objects too strongly. I have NOT examined the circuit of your receiver...
Tony

[peter_scott]

Hi Peter,

Tony's idea of ac coupling between heater and grid is attractive but didn't you say that you had a Domino standards converter. If so I think it has quite a high RF output level (150mV p-p). Well you could ac couple it into any of the grid 1s of the RF stages V1, V2, V3, V4. I don't think it can help you at the video amp but Tony's suggestion is perfect for that test.

The Domino probably won't do much by the time you get to V4 but you might see something.

HTH,

Peter.

[Heatercathodeshort]

Hi again Peter,
First class advice there from Tony and Peter. Please don't disturb the signal circuits if you can possibly help it. Everything was o.k. until you added that capacitor and I feel there is a simple answer. Its difficult without test equipment and even the suggestions offered need to be 'decoded' by someone with some experience. Don't unsolder componenets unless you suspect they are faulty. I don't want you to end up with half a dozen man made faults to fight each other...We have all been there. We await your results.

[DoctorWho]

Thanks for your help and advice here. Can I just clarify what to do as I'm not totally clear on this.

Just to clarify firstly, the set was fine even when the additional capacitor was in place, the picture just vanished a few minutes later whilst the set was on. The brightness can be adjusted, it can be turned right down or up very high, there is plenty of control.

I don't have a Domino converter, mine is much older with an individual modulator with a very high output.

Are you suggesting that I take the RF output directly from the modulator and connect it, via a 0.01 condenser to the G1 of each of the 4 RF valves, one at a time? If so, do I connect the screen of the co-ax cable to the chassis as well, or via a similar condenser?

One other thing I did remember, when the picture vanished the sound of the set changed, you can still hear the line whistle clearly, but when a set is running normally the sound of the set running changes with each scene change on the CRT, now the line whistle is just constant, there is no change in sound with scene changes. This does not have any effect on the TV sound which works perfectly.

Are those very low value condensers likely to be faulty, should I replace them?

If you could just clarify what exactly I should do with the signal from the modulator I would be grateful, sorry to sound dim, but I don't exactly understand what it means to "AC couple" it.

Thanks again for the help.

Peter.

[peter_scott]

Originally Posted by DoctorWho

Are you suggesting that I take the RF output directly from the modulator and connect it, via a 0.01 condenser to the G1 of each of the 4 RF valves, one at a time? If so, do I connect the screen of the co-ax cable to the chassis as well, or via a similar condenser?

Are those very low value condensers likely to be faulty, should I replace them?

If you could just clarify what exactly I should do with the signal from the modulator I would be grateful, sorry to sound dim, but I don't exactly understand what it means to "AC couple" it.

Thanks again for the help.

Peter.

Hi Peter,

You don't sound dim at all and you are understanding exactly what I intended
for testing with your standards converter. I don't think you should need a capacitor in the ground/screen lead as the 912 has a mains transformer and there shouldn't be any potential difference between the TV and converter grounds but put one in if you feel uncomfortable about direct connection.

I am only suggesting the AC coupling in the signal lead so that you don't disturb the dc conditions on the grid 1s on the valves, but there will probably be a capacitor in the output of your modulator anyway.

Sorry about the term AC coupling. AC just means alternating current as opposed to dc coupling (direct current) If you make the signal connection
without a series capacitor then any dc voltage on one side will try to pass direct current through any resistive elements on the other or will at least present a dc bias which may not be what you want. By using AC coupling (i.e. through a series capacitor) you are only connecting the (in this case RF) signal and not any dc voltages. (Transformers with separate pirmary and secondary windings also act as AC coupling elements.)

The low value capacitors that are flat and rectangular generally use mica to separate the plates and these tend to be much more reliable than those of the tubular types that use paper for plate separation. Personally I would think the there is only a very small chance that the micas are faulty.

I know that your fault only occured after you had tried the additional decoupling capacitor but I think John has already indicated that you may have inadvertantly disturbed something and this does sound very plausible. The act of connecting your additional capacitor is highly unlikely to have caused a fault.)

I hope you will find something with Tony's test and the standards converter.

Peter.

[peter_scott]

Hi Peter,

Have you checked the voltage on the Grid 1 of V1 and V2?

Peter.

[DoctorWho]

Hello Peter,

Thanks for the further help and advice, much appreciated. I'll have another go over the weekend and let you know what I am able to work out. Thanks also for clarifying about the AC coupling, I'll work along this process and hopefully may get some results. I didn't check the voltages on the 4 RF valves so I'll do that at the same time.

The thing that does seem odd about this fault is that when it actually occured the set was on and displaying a perfect picture, then after having been on for just a few minutes the picture simply vanished in an instant leaving a raster and sound. I even checked another set to ensure that a good picture was being transmitted, which is was, so when the fault developed the set was actually not being touched and was running. The way it happened so instantly was just as though something had failed.

Thanks again for this help.

Peter.

[oldeurope]

Are those very low value condensers likely to be faulty, should I replace them?

Hi, in my TS1105 these two micas failed after a few hours.
BTW I was able to rescue all the other caps electrolytics and waxies.

Kind regards
Darius

[DoctorWho]

Thanks for letting me know this Darius, the mica condensers in my set are just like the ones you have pictured.

I have just ordered replacements from RS and so, hopefully, they will arrive around Tuesday, so I'll fit them and let you know what happens.

Thanks again for the help.

Peter.

[DoctorWho]

I'm afraid replacing the mica condensers made no difference, I also tried a new 6V6G and similarly that made no difference. I'm afraid I shall have to seek help with this set as I do not have the know how to progress any further by myself unless anyone is able to suggest something else which I may have overlooked, perhaps the failure of a certain valve which is likely to have caused this problem?

Thanks again for the help.

Here is a picture of the set:

Peter.

[DoctorWho]

After another "last try" at this set the fault has been located. A resistor which sent the HT to the 2nd and 3rd of the EF50-type valves had gone open circuit, so both of these valves were not receiving any HT. Replacement of the resistor bought the set right back in to good working order.

The 61SPT video amp valve still has a slight blue glow, but the general opinion here was that it was normal and not a problem.

So now the set is fully up and running. Many thanks to everyone who helped with this set. The mica condensers were fine but at least now that they have been replaced the new condensers should last for many years to come, they are certainly working very well.

I'm amazed actually what superb picture and sound are obtained from this early set.

Here's a screen shot from the set taken this morning.