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ITT CVC2, No colour on left side, colour on right...

 
Alastair
(@alastair)
Reputable V-Ratter Registered

Finally got hold of an early gen. Colour set, thanks to Neil29--Cheers, Neil!

Set is in very nice external and internal order. Cabinet I would say is in class A condition--No Scratches-- hugely rare on a 40 year old wood-veneered set, just a very slight scuff on one bottom corner.

Internally the set has had just a couple of minor repairs carried out pretty well in the past, that are actually documented on the rear of the service-manual that came with the set, including the dates done! Three repairs done, all resistors in the time-bases.

Set sorta works.--Well reasonably well, maybe a bit under-contrasted on monochrome, but has issues with the colour. --There Isnt any on the left hand side, and that on the right appears noisy.

Along the lines of the old Pye hybrid 691/7 series I suspected the 12K supplying the screen-feeds of the PCL84, decoupled by a 4.7uF cap, and confidence was high as I chopped out a Callins cap and a rather stressed looking 2W 12K.
I replaced 'em both, but made no difference at all. I seem to remember something like this--not on one of these--Never seen one before-- but as I'm suffering the lurgi today, cant think straight. Anyone shed any light....?

Supplies seem reasonable, 280V +B, and 150V screen feed supply.
,

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Topic starter Posted : 16/01/2015 10:45 pm
sideband
(@sideband)
Famed V-Ratter Moderator

Probably thinking along the right lines though...

Suspect supply decoupling capacitors....especially if they are Callins types! Don't know the set at all but check decouplers along the supply rail of the decoder. You may get away with just trying a good one across any suspects.

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Posted : 16/01/2015 11:12 pm
Anonymous
(@anonymous)
Illustrious V-Ratter Deactivated Account

Only ever worked on a CVC5 and that was 35 or more years ago.

The fact that the colour only appears at a certain part of the line scan makes me think that there is a line output derived supply with a smoothing issue.

Al

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Posted : 16/01/2015 11:35 pm
Refugee
(@refugee)
Famed V-Ratter Deactivated Account

I would cast my vote for a bad capacitor in the decoder for that fault.
Callins is the best clue I can give though.

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Posted : 17/01/2015 1:11 am
sideband
(@sideband)
Famed V-Ratter Moderator

Only ever worked on a CVC5 and that was 35 or more years ago.

The fact that the colour only appears at a certain part of the line scan makes me think that there is a line output derived supply with a smoothing issue.

Al

Yes indeed. It's typical of the sort of problem where line pulses are getting in to somewhere they shouldn't....! So not necessarily a supply on the decoder itself but a line derived supply to the decoder.

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Posted : 17/01/2015 9:17 am
Cathovisor
(@cathovisor)
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I'm inclined to agree with Trevor - it has the feel of something mis-timed and I'm sure Chris encountered it in one of his sets... can you post a picture?

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Posted : 17/01/2015 10:46 am
crustytv
(@crustytv)
Vrat Founder Admin

This exact fault was experienced by two members ( Tas and abctelevision) on two BRC 3000 video boards, I had a similar issue but in my case it was more poor colour on one half.

The fault for the no colour on one side, was a lock-fit transistor see here might not be the cause of this CVC2 fault but perhaps worthy of an investigation to eliminate it.

I've got the McCourts manual which covers the CVC2, many pages of known stock faults for this set. None of the listed mention this condition, shading yes but not the split no colour one side.

You will also note in the thread Neil states he has the same fault on his CVC2, the very same set we are discussing here. Either Neill didn't get around to checking to eliminate it or he did and it was not the fault. :aab

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Posted : 17/01/2015 11:28 am
neil1974
(@neil1974)
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You're right chris I never did get around to fixing the fault :cch .

Cheers
Neil.

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Posted : 17/01/2015 1:54 pm
Alastair
(@alastair)
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Ah--Thanks for the ideas guys, something for me to think about. I'm still suffering from a bad cold--which is odd as I haven't had one for about 5 years.
--This one however--Makes up for it completely, Its a real horror! Its odd, in that while doing something I cant concentrate at all, can't think straight! (Nothing new there really,--just much worse than usual!)

For those not familiar with the CVC2 its a metal-chassis type set. There's only two tiny PCB's in the set around 2" in size, one is a sorta IF pre-amp between the tuner and the main chassis, the other is in the line-timebase and has the Transductor on.

The entire decoder of the set is steel-chassis, point-to-point tag-strip as is most of the rest of the set including the IF Strip. There's only one Lockfit as far as Ive seen --on that IF pre-amp board,--I'll prolly replace it anyway, Another for the ceramic pot--got me well over a hundred lockfits in there now...
Ive seen there's another Callins on the decoder, and also one of those red Plessey type elect. caps. Most electrolytics are the Philips clear/blue tape wrapped type, with beige rubber ends and look OK.
--What is the concensus on these Philips caps....? Any good at this age?

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Topic starter Posted : 17/01/2015 5:51 pm
Refugee
(@refugee)
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Those Philips capacitors are usually reliable. I found a couple of the larger clip mounted ones with slightly perished bungs. They are still working fine and have had a generous dollop of spirit based varnish added to the bungs. They are from the early 1970s.

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Posted : 17/01/2015 6:51 pm
Anonymous
(@anonymous)
Illustrious V-Ratter Deactivated Account

I have found a few used as coupling caps used in record player amps to have dropped in value. normally I don't suspect them
Rob T

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Posted : 17/01/2015 7:01 pm
Alastair
(@alastair)
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Cd45 and Cd44 in the decoder replaced, One a 50uF Callins the other a 2.2uF 'TCC' (That I thought was a Plessey initially, This TCC cap was dated, 1969)

Full colour to the whole screen was the result. I still have lowish contrast, that I suspect is actually the CRT. It does look 'tubey' when the brightness is advanced, there is a loss of all definition to everything on the right of something bright on screen, above a certain point.

Shall have to see what I can do about that!

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Topic starter Posted : 17/01/2015 9:46 pm
Cathovisor
(@cathovisor)
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Cd45 and Cd44 in the decoder replaced, One a 50uF Callins the other a 2.2uF 'TCC' (That I thought was a Plessey initially, This TCC cap was dated, 1969)

What's the function of these caps?

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Posted : 17/01/2015 9:48 pm
Terry
(@terrykc)
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... there is a loss of all definition to everything on the right of something bright on screen, above a certain point ...

That might imply that the EHT sags when the demand peaks, producing poor focus until it recovers. Does this set use a valve EHT rectifier, that might be losing its emission? Try monitoring the focus volts with a scope to see if that is affected (easier than monitoring the EHT!).

When all else fails, read the instructions

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Posted : 17/01/2015 9:59 pm
Alastair
(@alastair)
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Cd45 and Cd44 in the decoder replaced, One a 50uF Callins the other a 2.2uF 'TCC' (That I thought was a Plessey initially, This TCC cap was dated, 1969)

What's the function of these caps?

The 2.2uF Cd45 cap decouples a supply derived from the 290V rail via a 47K to the Burst Amp.

The 50uF Cd51 cap decouples the 820 ohm resistor in the emitter of the same burst-amp transistor--Made mistake earlier--called it Cd44, its actually Cd51.

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Topic starter Posted : 17/01/2015 10:51 pm
Alastair
(@alastair)
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... there is a loss of all definition to everything on the right of something bright on screen, above a certain point ...

That might imply that the EHT sags when the demand peaks, producing poor focus until it recovers. Does this set use a valve EHT rectifier, that might be losing its emission? Try monitoring the focus volts with a scope to see if that is affected (easier than monitoring the EHT!).

Set has a EHT Tripler, and the focus is derived via a Metrosyl/VDR like a CVC9 but supplied from the final EHT supply, instead of a focus-tap in the tripler, via 6.8M and a 2.2M on its earthy-end, and a 100K to the CRT. Ive replaced the 100K so will do the 6.8M and the 2,2M next.

I was surprised that it had a tripler--I remember seeing a couple of early CVC5 type sets with EHT Recs and Overwinds years ago, with the odd name/make, 'ANTUR'.....

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Topic starter Posted : 17/01/2015 11:00 pm
Alastair
(@alastair)
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Well,--I checked out those resistors on the focus Metrosyl and they were OK.

As the colour--now full-screen wasnt very good, was lacking red although grey-scale wasnt too bad, I turned attention to the CDA stages. These (Like Pye 691/7) used PCL84. Two were new Mullard valves,-Neil, its previous owner having replaced them, and one was a well used looking 'Granada' one. I replaced this one with a Hungarian Tungstram valve and tried again.
--No apparent change in picture or colour quality.

A quick check of the Schematic showed that the Triode portion of each valve is used as a sorta 'loose clamp' holding the grid of the CRT at some pre-determined level, and allowing modulation via a 1 nan cap to the triode anode from the pentode amp section.

The Anodes of the Triode sections were connected to +B of 290V via a 10 Meg resistor, making the 'clamp effect' rather loose and hence modulatable with the CDA signals from Pentode half.
Each of these 10 meg resistors had gone high, the worst was on the Red stage and was 16 meg-ohm. --I changed all three, the others were 14M and 12M
--I then checked the various other resistors in the CDA, most had gone out of spec, one 220K in one of the G1 of a pentode had gone to 340 odd K. I replaced all these as well.
The couple of Matrix resistors for the Green source had also changed in value and were also replaced. (39K and 180K)

All in all--I replaced in the CDA stages-

3 x 10M
4 x 100K
3 x 220K
1 x 180K
1 x 39K

All above were 2W Metal-Film types (TE Connectivity, pretty Blue coloured ones), so should be good for the foreseeable.

This has improved the colour considerably, but the CRT still shows poor brightness, streaking and lowish contrast I tried the 'non destructive tube rejuvination' which improved brightness and grey-scaleing but not dramatically....

--I May have to borrow the 15W Bulb from the fridge, and introduce it to the Cathodes via the mains....... :aak

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Topic starter Posted : 19/01/2015 3:01 pm
Tazman1966
(@tazman1966)
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Subject: ITT CVC2, No colour on left side, colour on right...

...A quick check of the Schematic showed that the Triode portion of each valve is used as a sorta 'loose clamp' holding the grid of the CRT at some pre-determined level, and allowing modulation via a 1 nan cap to the triode anode from the pentode amp section.

The Anodes of the Triode sections were connected to +B of 290V via a 10 Meg resistor, making the 'clamp effect' rather loose and hence modulatable with the CDA signals from Pentode half.
Each of these 10 meg resistors had gone high, the worst was on the Red stage and was 16 meg-ohm. --I changed all three, the others were 14M and 12M...

A very common fault on the Pye hybrids causing odd grey scale drift depending on the colour content of the picture. I guess the circuit is very similar.

How about some pictures of the beastie?

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Posted : 19/01/2015 9:50 pm