ITT-KB VC52 Ballooning picture

[Colourstar]

I don't know how long this 1968 set has been dormant, but it's awake now- albeit with an issue or two...

Low EHT seems to be the problem, with the picture ballooning until it vanishes if the brightness is turned up. Reducing the brightness allows the image to be held, although its a bit dark at this point.

The LOPT section comprises a PL36, PY800 and DY87. A spanking new PL36 (thanks Brian!) cured the slight lack of width, whilst the other two valves have been swapped with used examples to no improvement. Technically of course these could be no better than the ones originally in the set.

No components below chassis have been touched yet. Any guidance very welcome!

Steve J

[Steve_P]

If the power supply is up to scratch, then try changing the boost capacitor.

The line output transformer on these is always dodgy, but you can help by drying out the LOPT. Then you could try some coax inner as the DY87 heater, or the Tripler trick.

Cheers,

Steve P.

[Danny]

Hi if you can see the dy heater glowing with the brightness turned down then whatch if it goes out with the fault condition if it remains lit then it will eather be the valve or the overwind i did once have a tube causing this but i dont think you would be so unlucky.
If the heater goes out then things get more complicated and you will mean checking resistors around the lopt and if the set has its origanal caps then replacing these will probrably cure the fault. you could try an eht stick rectifier to see if this improves the fault. Danny

Ps a nice clean example

[Peter.N.]

I would suspect that its moisture in the LOPT, very difficult to get rid of, it will need a number of days in a warm place, an airing cupboard perhaps. If it is damp it will normally get worse as the transformer warms up.

Peter

[Welsh Anorak]

Hi
Agreed, but it's worth trying the airing cupboard dodge. Have a look at the boost rail voltage as the brightness increases to see if the fault is in the primary circuit - if not the overwind's suspect.
What a clean chassis you've got there, by the way!
Glyn

[Focus Diode]

The boost voltage should be 750V. The LOPT is not the original one, evident by the white overwind and evidence of non original solder on its tags.

I wonder if the DY87 is being heated correctly? I don't recall seeing any illumination from this valve despite the heater supply (2.5ohm) resistor seemingly being within tolerance. Could it increase in value when warm?

This is indeed a beautifully preserved example!

Cheers
Brian

[Steve_P]

The heater does not take much to light it, but the heater voltage is obtained from the overwind, so if the overwind has a moisture problem or is faulty, the DY87 would not be heated properly.

Cheers,

Steve P.

[Heatercathodeshort]

The LOPT is an ITT component. It may have been replaced but I never replaced a single one in the hundreds that went through my hands. Brilliant receiver and probably the very best of all.
First make sure you actually have a DY86/87 or 802 fitted and not a 6.3v EY86. I've known it happen more than once. Turn all the room lights off and reduce the contrast setting to a very low value. Increase the brilliance [on a test card] until the screen is just illuminated and observe the picture width. Turn the brightness up and see if the width decreases before the picture balloons. If it does I suspect dampness in the transformer. Drying out may well return it to normal but it will take around a month in the airing cupboard before any improvement will be noticed.
Hope this helps. John.

[Colourstar]

Wow, thanks for all the replies in such a short space of time. Plenty of food for thought there.

I can confirm that the DY87 is indeed a DY87. I've tried a couple now with the same result. I've usually found them pretty reliable in the past. Good point though, as it would be all too easy to pop an EY in by accident.

Testing as per John's suggestion, there is no decrease in width before the picture balloons to oblivion.

What is significant is that there is no detectable orange glow from the DY87 with the image held on screen. It's actually glowing blue - more so when the brightness is on max and the picture has blown up and vanished. There is also a small amount of blue visible within the PY800 (which has been changed).

I believe the set has been 'dry' stored, but that's not to say some damp has infiltrated the overwind over a period of disuse.

I don't have central heating, which won't help matters. Maybe it's time to fire up the Rayburn stove and park the KB in front of that.

Steve

[Refugee]

I have fixed that fault with a new DY87 before or whatever it was in that set (1980ish).
The odd glowing indicates this too.
Do try a third one.

[Hybrid tellies]

You have got a nice example of a great set. For this fault I would check the HT line, the voltages around the line output valves. If when measuring the line drive on the control grid of the PL36 the fault cleares check the high value width resistors. But before checking these voltages run the set for aprox 20 mins with the top cap to the DY802 disconnected then switch the set off and feel the temperature of the EHT overwind it should be cold to the touch. If its hot the EHT overwind has shorted turns if it's just warm it could be dampness and a few weeks drying the set out should sort it.

[Welsh Anorak]

Hi
The blueish glow in the DY suggests that it's just possible the CRT is drawing too much current - unusual but possible. Try inserting a high value resistor (say over 10 meg) in series with the EHT lead (well insulated, obviously). If the fault worsens then it's the CRT.
Otherwise Simon's suggestions are good - especially the one about turning the set off first!
Glyn

[Colourstar]

Well folks the moral of this story is never to trust a used DY87!

I ended up pinching the one from my Dux TV/Radiogram which is known to be good 'un. Result: a nice orange glow and an end to the ballooning raster. Simple.

Interestingly a 1968 article by George Wilding in Television mentions identical symptoms to mine, but in that particular case the resistor in the base of the DY87 was at fault, having risen from 2.7ohms to between 4 and 5 - enough to strangle the heater. I checked the resistor in mine and it was slightly high, so I replaced it anyway. I'm also told that this resistor can read OK, but break down under load. All worth bearing in mind should this fault be encountered again.

The working DY87 has shown up a lack of width (the PL36 is new out of the box) as well as the usual linearity problems typical of a set that's been dormant some time. Just noticed tha the height is also having some minor spasms. I'm working blind at the moment- does anyone have the VC52 circuit?

Steve

[Focus Diode]

I had doubts about that valveholder resistor (as mentioned in post 6)!

It looks as if the boost rail voltage is down which would affect both height and width. The boost volts should be around 750V at C134 (0.1uF) to chassis. Note there's seperate width controls for 405 and 625-lines so will need checking on both systems.

Cheers
Brian

[Colourstar]

Nearly there! There's a good guide to servicing the earlier VC2 and VC3 chassis in the Oct/Nov 1966 issues of Practical Television, a good deal of which seems to apply to this later VC52.

The lack of width is on 405 only, which narrows down the suspects. A 220k from one side of the width control to chassis had risen to 275k, although replacing it made no difference. That leaves R151 a prime suspect, as it's a 1.5 meg, which could be well out of tolerance.

Whilst running the set briefly at 625 for the first time there was a loud whipcrack from somewhere, which was a little alarming although things continued as normal after this.

These days I'm in the habit of switching dual standard sets off before flicking from 405 to 625. A friend of mine killed the LOPT in his KB with the VC2 chassis when the switch jammed halfway.


Steve

[Colourstar]

Well it wasn't R151, despite having risen from 1.5meg to 1.8.

[Heatercathodeshort]

If the scan coils have been removed or disturbed check the linearity sleeve between the neck and the coils. It may have been pushed in too far. I can probably find you a VC52 circuit if you need it. The sheet is large. John.

[Colourstar]

Thanks for the kind offer John. I do now have a schematic to hand and am working through the components that affect 405 width. It could be the control itself that's to blame. Brian R (aka Focus Diode) has very generously copied me LLJ's guide to common faults on the chassis.

The linearity sleeve looks untouched and is well towards the rear of the tube neck.

Hopefully my next post on this will have a full width picture...

Steve

[Hybrid tellies]

I have taken a look at the cct diagram. Don't think there was a lot of difference between the VC1 and later VC52 chassis. If boost voltage is 750v on both systems check the linearity sleeve but not before checking the s correction cap which is C131 0.3uf 200vw also check C146. If the boost line is low on 405 check the value of the width control R154 it should be 1M. They can go low or high.

[Colourstar]

Thanks for your help Simon. Can I get away with a 0.22 for C131? I don't have a 0.33 to hand.
Incidentally, what is the function of the S correction capacitor? Is it a part of the timebase circuit that corrects raster distortion?

Steve