My First Television Set Stella ST1017U

[ChristianFletcher]

I managed to spend a few hour on the Stella television but unfortunately not made a progress towards diagnosing the vertical hold issue. Without a television signal applied I can see the multivibrator free running at just less than the 50hz which should be perfect. When I switch on the standard converter I can see the slightly out of phase frame output from the sync separator. These small sync pulse crawl along the multivibrator waveform but won’t make it lock. sometimes it locks briefly and the picture holds but then the multivibrator just start free running again. I’m not really sure what the size of the sync pulse need to be to make the multivibrator lock.

I have checked all the resistor values in the sync separator and the multivibrator a few had gone just slightly out of tolerance but not massively. I haven’t check the capacitors as it requires unsoldering them from circuit with the risk of damaging the tag strip.

My thinking is that the multivibrator part of the circuit is fine but the sync pulse are not big enough. I’m not sure how to diagnose this any further so not sure what I should do next

Ok some of this is hard to describe so please see link for 2 minutes video

https://youtu.be/2WSrjGiP5sU

Thanks Chris

[1100 man]

Hi Chris,
Interesting video! I would agree with your diagnosis that the sync pulses are not large enough to trigger the frame oscillator. As can be seen on the video, they almost do sometimes. I also agree that the resistor values that you mention were not wildly out of tolerance. Valve circuits are usually pretty stable with wildly out of spec values.

Could be a low emission valve - have you tried another one?

What does the video waveform look like going into the sync separator? It's possible that the sync pulses might be being crushed in which case the separator wont be able to work properly.

Sync separators obviously caused a lot of problems in the old days. There were whole books dedicated to faults and cures with just this small part of the circuit.

Do we have a circuit diagram for this part of the set? It would be interesting to study it.

Cheers
Nick

[ChristianFletcher]

Thanks Nick I was going try testing or replacing the valve in the sync separator I do have a basic emissions tester. Some of the voltage waveform measure wrong but I think this is because I’m using a modern DVM as I don’t own a VTVM

I have that book on sync separators but it’s a bit above my reading age right now as this is my first TV. I am just at work at the moment but will try and upload some parts of the circuit later. I will also see if I can make a video showing the input to the sync separator.

The picture looks very bright on this set with very poor contrast. Contrast, brightness and even volume adjustment appears to effect the vertical hold.

[ChristianFletcher]

I haven’t had the opportunity to take another look at the television but have been reading the repair books. In one of the books it mentions that poor contrast is caused by low video signal and then goes onto say that low signal could mean low sync pulses from the separator.

I would say that the contrast for my set is very poor. The picture is very bright and washed out. Adjust of any of the televisions controls but especially the contrast appears to have a massive effect on the vertical hold stability.

Is there a easy way to check the video level is correct. I assume this is another job for the scope but sometimes other method can be employed. Is this line of enquiry worth exploring

Thanks Regards Chris .

[1100 man]

Hi Chris,
The contrast control on most sets (but not all) adjusts the bias on the AGC line.
This in turn varies the gain of one (or more) IF amp stages. Thus the amplitude of the video signal at the detector, including the sync pulses, is directly affected by the contrast control.

The video signal is then passed to the 'video amplifier' which then drives the CRT. Usually the feed to the sync separator is taken from the output of the video amplifier to provide it with maximum amplitude signal.

With most 405 sets, if you turn the contrast down so the picture is pale and washed out, you will often loose frame lock and the picture will roll.

So you need to sort out why you have a weak picture. You should be able to get a very contrasty picture without having to have the contrast control up full.

The sync separator is probably blameless- it's just not getting enough video signal.

Check the AGC line & contrast control. Even slightly leaky caps will cause problems. There are also usually high value resistors associated with the AGC line.

Could be a low gain IF valve or video amp valve or the anode load resistor for the video amp. The video amp works hard and it's anode resistor is usually quite chunky and often goes high in value.

It would be handy to see the circuit but I'm not near my R&TV red books at the moment.

Cheers
Nick

[Heatercathodeshort]

Could you post a picture showing the actual symptoms on the screen?
Sync problems are rare on the TG100 series and it would be interesting to see the actual effect. Odd operation of the contrast control may be due to the fitting of an EF80 in the common I.F. stage V103 in place of the Vari u EF85. Worth a check.
Have you checked the insulation of the sync coupling capacitor C610 connected to the control grid of the sync separator pin 9 of V413B the ECL80 on the original Philips manual? This needs to be 100% for correct sync separator operation. The picture should lock perfectly with a very low setting of the contrast control. J.

[ChristianFletcher]

Thanks for the comments. I will try and look at these in turn and provide some feedback when I next look at the set. I wouldn’t say I can achieve acceptable contrast by adjustment of either the brightness or the contrast controls.

The screen looks too bright and washed out I think. If I manually adjust the frame hold control I can occasionally manually hold the frame but this would normally be split. As I showed on the previous link to the video the sync pulse will occasionally act to pull the frame oscillator into sync with the frame but it quickly looses the lock especially if the video content changes or if I touch the volume, brightness or contrast controls

Video of the screen look 19 minutes in https://youtu.be/kQdlDwxMbGo

Multivibrator showing application on sync pulses https://youtu.be/2WSrjGiP5sU

[1100 man]

Hi Chris,
I rather enjoyed the video - I wasn't planning to watch the whole thing but got engrossed! As you discovered, it's pointless trying to run a TV from a lamp limiter - the variac is much more useful!

You can't really tell anything useful from either pattern generator as they are both unknown quantities. The test card from the Aurora is much more sensible, although a greyscale 'stairstep' pattern would be better for fault finding using the 'scope.

Looking at your test card pictures at the end of the video clip, there looks to be plenty of contrast to me. That should be more than enough to drive the sync separator. Hook the scope up to the anode of the video amp valve and set it up so that you can see one line of the video waveform. You should clearly be able to see a well defined line sync pulse. It should have a nice square bottom to it and should look like the pictures in your red 'TV faultfinding' book.

It's possible that the sync pulses are getting squashed. If possible, post a picture of the 'scope trace and also the video amp/ sync separator part of the circuit.

Adjusting the contrast and brightness is a balancing act. Ideally, you would adjust the brightness so that the black areas are actually black and the contrast to give adequate highlights. That's fine on the test card, but it's a whole different ball game with live video.

That's because almost no TV's had black level clamping. This means that the illumination in the black areas of the picture changes with scene content. On a bright outdoor scene, the grey areas turn black and there is way too much contrast. Conversely, in a dark scene, the areas that should be black turn light grey, so the settings of brightness & contrast are a compromise. The effect is quite annoying and seems much more noticeable on some sets than others. By all accounts though, customers didn't seem to mind!

Cheers
Nick

[ChristianFletcher]

Thanks Nick again lots of really good information I will do as you suggest and start to make some recording tonight. Depends how I get on I will post a link and some pictures tonight or the weekend

Thanks again for everyone support.

Regards Chris

[ChristianFletcher]

I just had time to look at the output from the Anode of the sync seperator. Im only measuring 35 volts when the waveform information shows 62V Im wondering if this is significant. I did check R414 and C403. The capacitor was correct at 150pf. I swapped out the 720K for 680K.

I will have longer to take a better look tomorrow. The input was test patern C.

Video showing link to output from VAGC Sync Seperator Anode
https://youtu.be/Q39ZGj-PjKA

Thanks for all the advice and comments. I have too say I’m feeling a bit frustrated with myself not having ever worked on a television it’s quite daunting. I’m definitely learning I didn’t even know what a sync separator was a month ago. The picture actually looks worse than when started it looks like it’s missing lines now. More time tomorrow to follow your suggestions

[1100 man]

Hi Chris,
Don't worry - you're doing fine! Actually learning about what's going on is far more valuable and rewarding than just randomly changing components.

A DVM (multimeter) is fine for measuring almost anything in a TV. Originally it would have been an analogue Avo type meter, but the difference is not significant. The little rectangular box shows 63V on the anode of the sync separator. You should be able to measure that with a meter. Have you checked the resistor in the anode?

The VAGC bit refers to the other half of the ECL80 which will be dealing with the 'vision AGC'.

As a rather irrelevant aside, from your first video clip, you mentioned the control called the 'limiter'. The nature of the 405 system meant that any impulse interference, such as vehicle ignitions, received by the aerial, would come out as big white spots on the picture.

The limiter, sometimes called the spot limiter, could be adjusted to 'clip' off those white spots from the signal. Turn it too far and it will cut all the white areas from the picture. It must have been a real problem back in the day as even the cheapest sets had a limiter. They use an extra valve so would have added to the cost of the set.

With our locally generated signals, white spots are not a problem, so we don't need the limiter. It can just be turned to one end and forgotten.

Tomorrow, I should have access to my red R&TV books, so should be able to look at the circuit which will give me a much better idea what's going on.
Can you give me the model number of your set?

Cancel that: I've just seen it in the second picture of your first post. Stella ST1017 U.

Cheers
Nick

[Cobaltblue]

Model number added to title

Cheers

Mike T

[1955APREN]

Hi Chris
Just a few thoughts on your frame sync problems . Low voltage on pin 6 of
the ECL80 would seem to indicate the the valve is not biases correctly . what are the voltage's on the other pins of the valve most important G1 . What is the voltage on the centre of the contrast control is it near what the manual say.s.
Have you tested R407 R401.
Hope you find this fault, as HCS says I do not remember faults on Phillips set
like this. Keep up the good work you are doing fine.
Kind regards
derrick

[FERNSEH]

Bad frame sync can be caused by the electrolytic capacitors in the video amplifier going low capacitance.
This set doesn't employ the usual frame sync integrator to create the frame sync pulse. Instead a simple differentiator circuit is used.
Simple it might be but the interlace is excellent in these receivers.
The attachment shows the Stella ST1007 video amplifier. Later 110* models have the same video amplifier circuit.
Note worthy is the clever vision AGC system.

DFWB.

[FERNSEH]

Reference to the circuit diagram in the previous post.
In the event of C42 going low capacitance the low frequency component of the video signal will be fed back to the cathode of the final vision amplifier and will cause attenuation of the frame sync pulse train and other LF elements in the video waveform.
The 50Hz frame sync must be considered to be a serrated 400 microsecond broad pulse.

DFWB.

[ChristianFletcher]

Just for clarification on Anode voltage measurement. I was using a micronta analog meter that has a FET front end so it’s a high impedance meter. I would assume the measurement may have been made originally using a VTVM. My micronta measured 3.5v my Fluke 27 measured nothing and the oscilloscope shows a waveform of around 35vpk-pk


I did check the anode load resistor and it is correct and the supply to that resistor measures 198 for stated 200v rail

Going to spend some time making the detailed measurement today as has been suggested I will check the valves and types installed and report back with a marked up diagram and voltages

I may try a rope Derrick into a live Zoom chat later as I have cameras on the bench but offers open if anyone has time to or interested to have a look with me.

Thanks for the encouragement. We are very lucky living in modern times having forums like this and the internet to do research.

[ChristianFletcher]

https://youtu.be/c-CAko-BgAc
https://youtu.be/Z2x44sxAIpg

I will be posting a full video of this part of the repait. But I have traced my hold problem to a simple fault. The Anode voltage of the Sync seperator was found to be very low on the the DVM and about half what it should be on the scope. The 180K anode resistor had gone very high to over 1M ohm. I really dont know how I did spot that early as I had been through the resistors. I missed it anyway but the voltage checks revealed the problem. The picture still isnt great with some retrace lines but the vertical hold is rock steady now. Thanks everyone for the help and advice.

Reagrds Chris

[1100 man]

Originally Posted by FERNSEH

Note worthy is the clever vision AGC system.

DFWB.

So come on David, tell us how it works! I've found the circuit in the 1960-61 red R&TV book and true to form, Philips have included some unusual features!

It would seem that the sync separator, V413b (ECL80), is also used to derive the vision AGC. See the circuit in post#70. It would be interesting to know how it does this?

As TV circuits go, there are quite a few departures from the more standard methods used by other manufacturers.

1) Separate IF's for sound & vision rather than a combined 1st IF stage.
2) The use of an EBF89 as 2nd sound IF & detector. I've never even seen one!
3) EF80 as a video amp - unusual choice!
4) ECL80's - I don't usually associate these with TV's!
5) Sync separator also used for vision AGC.
6) Separate AVC used for the sound rather then combined AGC for both.

The final thing that strikes me is that the video is DC coupled to the CRT. That's pretty unusual, so the black level performance should be really good, although the mean level AGC might mess it up slightly. Unless, of course, the AGC is more high tech than that?

So overall an interesting TV!

Cheers
Nick

[FERNSEH]

1100 Man wrote:
"The final thing that strikes me is that the video is DC coupled to the CRT. That's pretty unusual, so the black level performance should be really good, although the mean level AGC might mess it up slightly. Unless, of course, the AGC is more high tech than that?
So overall an interesting TV!"

Hi Nick,
My interpretation of the purpose of the DC feedback between the cathodes of the video amplifier and final vision IF amplifier is to correct the short comings of the mean level AGC system. The DC voltage at the cathode of the video amplifier valve is rendered free of any video signals by the 100uf bypass capacitor.
We know the AGC voltage from the grid of the sync separator valve will vary with signal strength but also with the levels of the constantly varying picture content. On dark scenes the negative going AGC will be less than the voltage from a very bright picture. So it follows on a dark scene the cathode voltage of the video amplifier valve will less than it would be on a bright scene.
Because the cathode voltage of the IF amplifier is controlled to a certain extent by the video amplifier, on dark pictures the gain of the IF amplifier will be greater than it will be on bright pictures when the gain of the IF amplifier valve will be reduced because the cathode voltage is greater because of the increased DC component.
I hope this makes sense.

DFWB.

[beery]

Hi Chris,
That's great. I'm glad you got there in the end.
The big question is, did you enjoy the journey?

Cheers
Andy