Itt kb vc200 linearity question
 

Just a quick rundown first.
This one has been sat in my stash for about 4 years. I thought it time to try and get this one going with my limited knowledge. On first power up unhealthy noise appears to come out the line output transformer. Not good so I put it in the summer house to dry out. After 2 weeks . No noise and a raster but that was it. Fault traced to a faulty lock fit Transistor .i changed 4 Plessey caps as I couldn’t get to frame to lock .i believe John (h c s) in a previous post said they test ok but often are faulty, that was the case here. Also a new PCL805 did wonders.
The vertical hold pot is a little iffy I may replace it later.

It’s almost perfect but both the linearity pots are at their maximum. I have checked resistors in the area and they all test ok. I am more than happy to leave the set as it is . Does anyone have advice on this or should I leave alone now it’s working.
Is there a reason there’s not much information on these sets . They can’t be rare as I have 3 sets in different disguises .
Andy.

Re: Itt kb vc200 linearity question
 

Did you check the cathode bypass capacitor on the PCL805 pentode, I think it’s 100 ufd?

Re: Itt kb vc200 linearity question
 

From the picture it looks good. The line linearity can probably be improved by adjustment of the linearity sleeve between the neck of the tube and the scan coils. Don't push it in too far and keep it straight. John.

Re: Itt kb vc200 linearity question
 

In your second photo, there is a large brown ERIE capacitor, vertically mounted. ERIE took over HUNTS and it’s highly likely this capacitor will be faulty, as it is a metallised paper dielectric type. It’s probably in the field timebase circuitry, so I would definitely replace it (but I don’t have a circuit diagram to hand).

Now I have looked at the circuit, it is in the field timebase: C77, 3.3nF, 750V.

Re: Itt kb vc200 linearity question
 

Hi Andy, there are a number of other mixed dilectric capacitors that really should be replaced. As Dazzlevision said, that brown capacitor (C77) is a must. Another important one will be the boost HT capacitor, C137 a 0.068uf. This is located just above the horizontal linearity control, L63. I would also replace C93, a 0.1uf mains RF bypass capacitor mounted near the on-off switch. This could go bang and give you a nasty surprise.
In short, if you spot any capacitors that look like the same style (grey Plessey) as the one's you have replaced, then suspect them too.

Lack of width could be due to a 10meg resistor going high value, R159. C138 is 0.3uf and could be another source of reduced width.

Here is a link to my adventures, hope you may find it of some use....
https://www.radios-tv.co.uk/communit...tvs/itt-vc200/

PS, there should be no linearity sleeve on the CRT neck, they are not used on this chassis.

Re: Itt kb vc200 linearity question
 

Yes, as PYE 405 says, any either yellow or grey coloured cylindrical capacitors with axial leads and branded “PLESSEY” or “ PLESSEY - TCC” should be replaced.

Re: Itt kb vc200 linearity question
 

As always, the mustards and the liquorice allsorts (unless obviously in distress) will probably be fine. That's the capacitors, not lunch!

Re: Itt kb vc200 linearity question
 

Thank you everyone for your advice.

Yes I did change the cathode bypass cap
I have changed the mains R.F. cap as well.
Andrew thanks for the link. I believe I seen it in the past but forgot about it.
I will check c138 later or just replace it. I also tested r159 .it tested out ok but I did find the r153 1M next to it has gone up in value by about 50% so might be worth changing that.
I will change the brown Cap as Daz and others have said. And also the other 2 plessys.
I don’t think I got a c137 .068 high voltage or a c77 at 750volt

Would I get away with a 650 volt for c77
Also how critical is the capacitance for c137

I obviously want to do it correctly .
Thanks again everyone. Andy

Re: Itt kb vc200 linearity question
 

I would say yes for C77, but I used a 0.068uf 1600v ERO capacitor for C137 that I got from ebay in my set. I wouldn't like to change the value much as it would probably result in incorrect boost HT volts.

Re: Itt kb vc200 linearity question
 

I agree. ITT wouldn’t have specified the less common “preferred value” of 68nF without good reason. Fit the correct capacitance value and the next dc voltage rating above that of the original part. I’d also use a Polypropylene dielectric type.

Re: Itt kb vc200 linearity question
 

Thanks both of you . I looked on eBay and I can only see the ero ones as new old stock.
Any chance you got a link.
I suppose I can get them fro Radio Spares but I try and make up a reasonable order.
Thanks again Andy

Re: Itt kb vc200 linearity question
 

The ones on Ebay are exactly what you need. Correct capacitance, voltage, axial leads and Polypropylene dielectric! The ones with the blue colour should be as good as new.

RS Components are unlikely to stock what you require.

Re: Itt kb vc200 linearity question
 

Daz .Parts on order. Thanks for taking your time and everyone else Andy

Re: Itt kb vc200 linearity question
 

like someone already said you should use a ac pulse rated cap for the boost voltage otherwise it may fail early, i use mkp caps from philips or similar (same dielectric). if not availbale then choose a very high dc rating for the cap like 1600v or 2000v.
if it shorts the flyback can go bye bye, its a critical part and fails often
the value is critical on most sets, too much difference and sometimes the high voltage doesnt even work or it may not be very efficient
coupling caps and boost cap are the first to change in any set

Re: Itt kb vc200 linearity question
 

A few months back I did a write up on a ITT/KB VC205 which is very similar to your VC200. I had issues with the frame linearity and changed out about four capacitors around the vertical output stage. My findings might be of some use to you.
Ref Line linearity, this chassis does not use a horizontal linearity sleeve but uses a horizontal linearity coil L63 damped by a 1.5K resistor R162.

https://www.vintage-radio.net/forum/...85#post1219085

Re: Itt kb vc200 linearity question
 

Thanks Portugal TV and Simon.
I did follow your write up on your set a few months back and the video. It had slipped my mind but I have just been reading it again
Andrew

Re: Itt kb vc200 linearity question
 

My mistake. I thought all 110 degree tubes employed them. Just checked mine. No lin sleeve. Thanks for pointing this out. John.

Re: Itt kb vc200 linearity question
 

Well I disided that I can’t improve on the original question on the linearity. I’m not going to possibly mess anything up not being a professional. I’m more than happy how it is.
Thanks everyone who gave me help and advised me to change the other caps. I also changed the vertical hold pot that was dodgy. Infact on removal it fell to bits.
I have treated it to some genuine ITT /KB legs.( I will take the old feet off and put them inside). i think it looks great.
Here is some parts I changed but not all.
Andy

Re: Itt kb vc200 linearity question
 

It certainly does, and a good job done sorting it out.:thumbsup:

Re: Itt kb vc200 linearity question
 

Glad you have sorted it out. Very nice tellies and look good as well.