ITT CVC7 - poor contrast and brightness

[neil29]

Hi again
I've got one of these sets, the trouble being there is not much in the way of contrast or brightness.
Any ideas?

Thanks in advance,

Neil.

[vince]

The ITT CVC7 i am sure this was the solid state chassis and all of the series i dealt with had excellent pictures and long lasting crts, however i dont recall any stock faults for lack of brightness/contrast, the cct diagram should help to narrow down your diagnosis.

Cheers Vince.

[dazzlevision]

Hi,

Is this set all solid state? Is the model number FT110 with a 110 degree delta gun CRT? The FT110 was an import from the German arm of ITT (Schaub-Lorenz, in Bochum). It sold around the same time as the CVC5 or CVC8 - early 1970s.

I've never heard of the CVC7. I thought ITT/KB colour set chassis went CVC1, 2, 5, 8, 9, 20, 25, 30, 32, etc.....?

Regards,

Dazzlevision

[Heatercathodeshort]

Hello,
If my memory battery still has enough power in it I think I am right in saying that the CVC7 is the later version of the CVC5 chassis. I believe it was used in the 'FEATHERTOUCH 100' and if I recall this was the first 'finger touch' channel selector set released in the UK. I sold all of this range from 1969 and was without doubt the most reliable colour television series of all time beaten only by the early sets from the far east. It is of course a hybrid design from the LORENZ factory in Germany. It was built at the K.B./ITT factory at Footscray. A very early version using an EHT overwind and valve rectifier was imported under a different name. Regards. JOHN.

[Stewart]

The CVC7 is a later version of the CVC5 hybrid chassis featuring the troublesome "feathertouch 100" station selector hence the model no FT100. It wasn't around that long - soon being replaced by the CVC8 with a new colour decoder circuit.

As to the flat picture, well, without actually seeing it it's hard to comment: It would be unreasonable to expect these sets with their delta tube to give the brightness and "punch" of later sets even when new. These sets where noted for the life of their tubes - but all good things must come to an end eventually - my own CVC9 has a very tired tube in it!

If the picture is very dark then a check on the tube electrode voltages would be a good starting point. If these seem fine it could well be the tube - but
it is worth making a few other checks though: Check all the solder connections in the tube heater circuit just in case a poor joint is causing a voltage drop.

R423 in the cathode circuit of the line output valve is worth a quick check too and was a stock fault: This resistor is soldered into two stand-off tubes on the PCB - it's the contact between the resistor leads and the inside of the tubes that used to be the problem. The feed for the beam limiter is taken from this point hence how it influences the brighness.

The beam limiter preset R144 could be out of adjustment - or have a poor contact too. I have the instructions on how to set this if needed.

D45 can also go high causing the contrast to shift...

Good luck!

[neil29]

Hi again,
Firstly the set is as said a hybrid chassis and it dates from 1973.

It's a 26 inch tube with the roll door so it is quite a heavy lump.

I have resoldered r423 and a few other connections but without any improvement in the picture.
I've also got a CVC9/1,the one with the simple remote(missing)and the ideal colour set up, it also has a roll front door on it.

Back to the problem; I'll try my best to describe it:
There's very little contrast and practically no brightness although, when the colour is turned up, it does obviously brighten the picture a bit.

As you can gather I'm a bit lost so any more ideas are welcome.

Neil.

[Danny]

Hi this has caused some old memories and I have had this problem and thought it should be the above resistor?

I also seem to remember the luminance driver transistor used to go o/c, also check that the wiper hasn't dropped off any of the preset pots like the good old Philips pots.
Can't remember the symptom but there was a resistor in the convergence box that used to go high. Has the yellow mains filter cap been changed with a gray one?
Oh the memories of that and what a pong, I will hunt my manual out and see if it causes any brain cells to come alive.

Danny

[Stewart]

Neil, I think your next step should be some voltage measurements on the tube base. Maybe if you can post your results here it will provide us with a few more clues.

What happens if you throw the service switch to its central position? It should provide a plain raster....

Stewart

[neil29]

Stewart, I did as you suggested and did get a blank raster.

Should I still do the voltage checks? If so, where do I connect the neutral lead from the meter?

Neil.

[Stewart]

Hello Neil,

Can you clarify something? When you say you get a blank raster in the centre service position - do you mean a blank screen (ie completely dark)- or can you actually see an actual raster with lines being scanned?

I can't recall now if the brighness control is in circuit in this position (left my manual at work from where the other posts were done). I think (and feel free to correct me someone) that this position was for setting the white tone - so you should have a moderately bright raster on screen?

[neil29]

Stewart, yes, I can see a raster when the brightness/contrast are turned up fully.
I don't know if this helps but I get the occasional flashes of blue on the screen.
Any ideas, because I don't have any?
Neil.

[Stewart]

Hi Neil,

The raster should be a nice, moderately bright white (with the front controls wound up) when the service switch is in the centre position (it's used to set up the purity of the tube). Are you getting that? If so, that suggests your problem is in the video output stages. If not, then we need to confirm those tube base voltages...Do you have access to a voltmeter?

Hopefully the flashes of blue are caused by poor connections at the blue output transistor or blue drive preset - both were common problems way back when....

Stewart

[neil29]

hi stewart.im getting a similar result as before.even with the brightness/contrast fully up the only thing visible is a dark grey/black raster.do you think the fault is in the video stages,or should i check around the line output valve etc as before.thanks,neil.

[Stewart]

The video stages are out of circuit with the switch in the centre position - hence they can be taken out of the equation if the picture is still dark.

Of the line output valves I would have thought the PY500A boost diode more likely of the two to cause problems - the A1 volts for the tube come from the boost line. But - as I've said before - voltage checks are the only way to pin this one down in my view. Given you have a second example it wouldn't be too hard to rule out the valves by swapping them over.....but I think it will all come back to making a few key measurements......are you in a position to make them?

[Focus Diode]

Any joy with the CVC7 Neil 29? When the service posistion is set to give a blank raster it should be bright white (when purity is correctly set). When set to the next posistion one should get a collapsed frame to facilitate grey scale setting up. If coloured lines at the centre of the screen arn't visible try adjusting the A1 controls on the convergence box until they can be seen. Each of the lines should be of equal brightness of course to allow for correct grey scale.

My own CVC5, a 20" RGD CR20 model, doesn't have its original CRT. The one fitted, which was not the original one was low emission. It was a Hitachi type. Ironically I had an identical Hitachi type ex GEC 2110 series which was known to be in first class order to hand which is as crisp as ever nine years later with first class grey scale on setting up. Should you replace the CRT in your CVC7 if it turns out to be faulty don't be surprised if this causes the IF transistors to fail, giving no raster, (unless service switch is set) and SW Radio on sound! This happened to me! They're on a small sub-panel and is far easier to remove this from the PCB using plenty of desolder mop to replace the transistors. I used 2x BF167s and a BF199 successfully with no further trouble.

Cheers from Brian

[peter_sol]

Hi
Unless you are familiar with the look of a flat(low emission) colour tube it can be hard to be sure but if the picture is milky looking with the contrast and brightness up full (colour to min) then you can be reasonably sure that the tube is duff.
Regards
Peter