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ITT CVC8 set 2

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freya
(@freya)
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This set appears to have seen much less use with hardly any dust or discoloured panels. Initial inspection shows it has had a few parts removed, the biggest being the front control panel, the 220k anode resistor in the PCL86 audio output, and 1 Meg grid resistor on the PCL805 Horizontal oscillator and finally the HT diode D54
With the missing HT diode replaced with a BA133 and source of the missing heaters found which turned out to be the plug not quite in on the board. :ccb
However it would not provide any line drive, replacing several capacitors on the horizontal oscillator board has restored the line drive which is a bit excessive at -97 volts.
The snowy raster looks good compared to set 1 which has a decidedly red tinge.
A problem has now shown up of which I have had no experience in for quite a few years, that being the tuner will not tune.
checking the voltages against the data I have found suggests I should have 33 volts ? at the tuner, which I do.
I may be in the wrong area which is why I need help to progress further please. The button bank is tested ok on set 1.

Any guidance gratefully received. :aab

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Topic starter Posted : 06/11/2015 8:31 pm
freya
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Been doing some more checks and the varicap voltage on orange/white is staying at 32 volts no matter which button is selected (variable positions)
attached part schematic

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Topic starter Posted : 06/11/2015 10:58 pm
Jayceebee
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Hi Stephen,

It's connector B2 where you should be looking for a voltage swing. If it also stays at 32v then check B3 is at earth potential, if so then it could be the internal resistor on the tuning head that connects the bottom end of all the tuning pots (top end in your cct) to B3 that is o/c.

I've added the path the tuning voltage takes to get to the tuner and ringed the suspect resistor. I've repaired the odd CVC8 and ISTR that the resistor is not a physical component but printed on the PCB the same as the tuning pot tracks. If it is o/c you try shorting it out with conductive paint.

John.

John.

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Posted : 06/11/2015 11:24 pm
Jayceebee
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Hi Stephen,

It's connector B2 where you should be looking for a voltage swing.

EDIT
That should have been "You should also be looking for a voltage swing at B2"

John.

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Posted : 07/11/2015 12:13 am
freya
(@freya)
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Hi John,
Thanks for the help, in error previously uploaded the wrong schematic, the one below is correct.
When I measure at the button bank (which works in another cvc8) it shows the voltage swing at its output at B2 (red arrow), however if I check at the tuner varicap voltage point A2, its solid at 32 volts.

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Topic starter Posted : 07/11/2015 1:05 am
Rebel Rafter
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Hi, Freya, RR here. If you're stuck with the full 32 volts at the tuner but the button bank is working fine it looks to me like transistor T6d on your diagram could be shorted collector to base, this would cause the tuning voltage from the stabiliser to bypass the button bank and hold the tuning voltage at the full potential at the tuner, preventing any tuning. So try checking this transistor next with your meter. RR.

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Posted : 07/11/2015 11:50 am
Niall
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If the voltages are different at A2 and B2 there must be a bad connection or broken wire, surely? From the diagram they should be connected?

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Posted : 07/11/2015 12:44 pm
Rebel Rafter
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Hi, folks, RR here. Because of the orange highlighting it's not clear what's happening where the orange lines cross, is there a connection here or just a crossover? It's a very long time since I last worked on an ITT hybrid so I can't remember. But surely however the circuit goes if there's a bad connection between points A2 and B2 there wouldn't be the full 32 volts at the tuner, although there is another connection going from the tuning voltage supply to "R373d in the power supply" on the diagram so maybe that's having an influence, I can't tell without seeing the rest of the diagram. Otherwise the only route I can see for the tuning supply would be via T6d but only if it's shorted, otherwise surely a positive supply can't get from an NPN collector to the base. There could also of course be a bad earth connection for the button bank. Lovely clean chassis, I've got a TX10 like that. RR.

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Posted : 07/11/2015 3:12 pm
Niall
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See what you mean, must be a crossover.

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Posted : 07/11/2015 3:51 pm
freya
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I worked out the problem in the end, turns out the main board has a slight variation which means two of the wires from the button bank come in different places :ccb
So changing these wires over has restored the tuning, however the convergence is terrible which means i will have to get a pattern generator for sorting it.

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Topic starter Posted : 07/11/2015 7:00 pm
Nuvistor
(@nuvistor)
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looks like the purity is out or the degaussing is not working, fix that before attempting convergence.

Frank

Frank

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Posted : 07/11/2015 7:42 pm
Jayceebee
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Hi Stephen,

Glad you've sorted that problem, I had a suspicion it was something like that as things just didn't add up. Wasn't the two orange and white wires was it? After my initial post I spotted that there were two but I ran out of time during editing. The two circits you attached are very similar just component numbering changes really.

Before you start on convergence check the purity as it looks a bit off in the top RH corner, make sure the autodegaus is working. This set uses a posistor, the blue component near the two large smoothing caps. Take it out and give it a shake. If makes a sound like a pepper pot it's faulty.

Sorry if I caused any confusion with the diagram Niall, I just assumed that because I didn't add a blob where the orange lines crossed that everyone would think it was a crossover, must learn to do loops in "Paint". :ccg

Looks as though it's going to be a good set.

John.

EDIT Post crossed with Nuvistor

John.

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Posted : 07/11/2015 7:44 pm
freya
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It rattles :aai and is very hot !
Where do you get these from nowadays ?

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Topic starter Posted : 07/11/2015 8:22 pm
Jayceebee
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A PTH451C could be relied upon as a sort of general purpose replacement except in some mainly Philips models which used one half of the device as a surge limiter. I will look in the garage tomorrow, I might still be able to find one or something suitable in the old coffee jars I have with various screws and other junk.

John.

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Posted : 07/11/2015 9:40 pm
Nuvistor
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I didn't think theses would be still available but beware this will be a 110volt one.

http://www.amazon.com/Waldom-Degaussing ... uctDetails

Frank

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Posted : 07/11/2015 9:57 pm
Jayceebee
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Hi Stephen,

I'm afraid a trawl through the various bit and bobs in the garage found a couple of unsuitable Philips 96203s and a Murata C360N but no PTH451C. I'm sure a post in the wanted section will bear fruit, failing that they are available on ebay at silly prices. Don't order from the States as it's also available as a 110v version.

John.

John.

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Posted : 08/11/2015 12:46 pm
Marc
 Marc
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Hi Stephen,

Would this Posistor on eBay do the job ?
http://www.ebay.co.uk/itm/PTH451C-POSIS ... xyOalTbg9w

Marc.

Marc
BVWS member
RSGB call sign 2E0VTN

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Posted : 08/11/2015 12:56 pm
freya
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Im in the hands of the experts Marc, colour is new to me :bba

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Topic starter Posted : 08/11/2015 1:00 pm
Marc
 Marc
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Im in the hands of the experts Marc, colour is new to me :bba

And me too !
I was going by Johns post giving part number PTH451C. :aab

Marc.

Marc
BVWS member
RSGB call sign 2E0VTN

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Posted : 08/11/2015 1:10 pm
Anonymous
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is it black and square? or white and round? i have both.
the white ones are ph9313
black one pt37p
i can send you one of each if you let me have your address.
Rob T

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Posted : 08/11/2015 4:52 pm
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