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ITT CVC9 no colour

 
BRC 3000
(@brc_3000)
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Morning All, A friend has a very nice CVC9 which has no colour and i'm trying to help him solve the problem all being remotely via a facebook group. First stop was to override the colour killer as per the manual. Still no colour information, just a slight green tint if the colour saturation control is advanced. So suspect the 4.43mhz sub carrier osc is stalled. Quick scope check confirms that this is the case with no oscillation at all at the emitter of t38d. So replaced the xtal from a scrap pcb but no change so tried another from another pcb but still no better. I cant imagine all the xtals are duff. Cold checks on all the components around T38 and the sub carrier amp show no obvious issues. the voltages around t38 and t39 as follows

Voltage readings for T38  sub carrier osc
B- 1. 8v  should be 6.7v
E- 1.2V should be 0.4v
C- 1.2V

Voltage readings for T39
B- 0. 9v   correct 
E- 0.33v  correct
C-14.6v  correct

I then wondered if the phase detector could be biasing the sub carrier incorrectly causing the osc to stall. A voltage check at the junction of R285 shows 4.30v rather than 0.45v so looks like that may be a clue? However cold checks all around the phase detector dont show any obvious issues. Any ideas anyone what I have overlooked or am missing. Any assistance much appreciated.

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Topic starter Posted : 10/05/2018 8:41 am
crustytv
(@crustytv)
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Posts: 10839

Have you checked out the service section up top? Among many other manufacturers, there's a CVC9 stock fault section there

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Posted : 10/05/2018 8:46 am
Nuvistor
(@nuvistor)
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I would break the control loop and apply 0.45v or thereabouts to the osc, if it still doesn’t oscillate carry on fault finding in that section.

 

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Posted : 10/05/2018 8:53 am
Terry
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Posted by: BRC 3000

 

I then wondered if the phase detector could be biasing the sub carrier incorrectly causing the osc to stall. A voltage check at the junction of R285 shows 4.30v rather than 0.45v so looks like that may be a clue? However cold checks all around the phase detector dont show any obvious issues.

Your cold checks around the phase detector won't show any problems if it is functioning correctly - and I think it is!

The function of the phase detector is to maintain the xtal oscillator at the correct frequency and phase. Obviously it has to get the frequency right first.

The oscillator isn't working so the phase detector says the frequency is low and is sending a correction signal to the oscillator telling it to speed up! Obviously, if it isn't working, it can't do that!

 

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Posted : 10/05/2018 7:26 pm
Terry
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Posted by: BRC 3000

Cold checks on all the components around T38 and the sub carrier amp show no obvious issues. 

What cold checks, exactly? How have you checked the capacitors and the transistor?

I would break the control loop as Frank has suggested. I probably wouldn't bother with the 0.45V unless it is vital to the operation of the oscillator. You obviously have the manual, so should be able to determine that for yourself.

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Posted : 10/05/2018 7:34 pm
BRC 3000
(@brc_3000)
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Thanks Chris, I have that stock fault list here already and I have shared it with the owner of the cvc9 who is working on the set. he tells me that he has checked the parts in the stock fault list and they test out fine.

Hi Frank and Terry  So I suggested that he lift one end of R289 to isolate the phase detector which he did whilst monitoring the reference osc on the emitter of  T38 with a scope. This brought about no change with no oscillation. He experimented with different Bias voltages for  T38 but still no oscillation. The xtal has now been swapped in to a working TV to prove it is functional. So it would seem that the fault can only be with one of relatively few components in the ref osc circuit! The parts in the circuit have been previously tested with a multimeter, cap meter and transistor tester and are apparently all ok. Unless there are any other suggestions its probably time to substitute T38 with a new, one along with the capacitors and see if that gets it oscillating again.

ref osc
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Topic starter Posted : 10/05/2018 9:00 pm
Nuvistor
(@nuvistor)
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Did you see the Servicing the CVC9 etc by E Trundle chassis  in the Data library ? There is a section about faults in the reference chain.

 

 

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Posted : 10/05/2018 9:15 pm
crustytv
(@crustytv)
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Along with the stock faults section up top under "service" there is also the "servicing the sets" section.

The CVC9 among many other popular sets of the period, have their servicing articles available online, especially useful for guests and those not yet with data library access. 

The "service" section up top is always a good first port of call for all manner of service related information.

https://www.radios-tv.co.uk/itt-cvc56789/

 

ss1
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Posted : 10/05/2018 9:20 pm
Norman-Raeburn
(@norman-raeburn)
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Hi Folks, We had a lot of these out on rental, I recall a tant in the colour circuit which was fitted the wrong way around in the pcb. The pcb was marked incorrectly, this may have just been a batch problem but is worth a look. The tant used to go leaky sometimes after many years of use. Norman

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Posted : 11/05/2018 9:12 am
crustytv
(@crustytv)
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Posted by: BRC 3000

[..] The parts in the circuit have been previously tested with a multimeter, cap meter and transistor tester and are apparently all ok. Unless there are any other suggestions its probably time to substitute T38 with a new, one along with the capacitors and see if that gets it oscillating again.

Agreed, I think we've all found testing and substitution are often two very different beasts, the times I've been caught out due to load condition not being reflected in test condition. When you're in this position of seemingly nothing at fault, I would replace regardless of testing results, that's the only true way to be certain.

Reading the service article,  it clearly suggests the oscillator capacitors C228 & C231, the amplifier tuning cap C232 and finally C200, are notorious for intermittent or complete shutdown of the oscillator. Also an imbalance in diodes D34 & D35 can be detrimental. Rightly as you stated above, replace T38 ensuring its a BC172C or BC109. 

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Posted : 11/05/2018 10:13 am
Cathovisor
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Posted by: Norman-Raeburn

Hi Folks, We had a lot of these out on rental, I recall a tant in the colour circuit which was fitted the wrong way around in the pcb. The pcb was marked incorrectly, this may have just been a batch problem but is worth a look. The tant used to go leaky sometimes after many years of use. Norman

Curiously, that precise fault also happened at the other end of the broadcast chain: in the Link 125 camera. In those cameras there was remote control of red and blue line amplitude, and one day we discovered during line-up that it was impossible to correct the red line amplitude of one camera remotely. That too turned out to be a shorted tantalum bead cap, fitted 'correctly' to a PCB that had been incorrectly marked.

Unfortunately, that PCB was the motherboard and it took a lot of dismantling of the camera to get to the offending component and change it.

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Posted : 11/05/2018 1:42 pm
BRC 3000
(@brc_3000)
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Posts: 48

 

Evening all,

 Well still no success with the decoder fault on this CVC9. we seem to have hit a brick wall with it now!

To recap, there is no colour due to the ref osc not running.  

components replaced c232, 231, 228, T38, Also d34, d35,  4.43 xtal and varicap diodes d42/43 using ba102 diodes. also tried to isolate the ref osc from the phase detector by upending r289 and giving t38 around 0.4v of bias but still no osc.  Voltage reading for t38 Voltage readings for T38
B- 1. 8v
E- 1.2V
C- 1.2V

Good burst signal going in to the phase detector and plenty of chroma at the chroma amps.
anyone any ideas what we are missing ?
What I would like to have done would be to isolate the ref osc completely from both the phase det and subcarrier amp so no other part of the decoder can be having an influence over it, idealy by using another working cvc 5/8/9 as a reference.  By disconnecting parts of the phase det and subcarrier amp  and see if the osc runs on the good set then compare how the bad set behaves but i dont have another one....... anyone offering to use theirs to compare. I dont want to give up on this set but running out of ideas. decoder diagram attached for reference. Thanks

cvc8 full decoder

 

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Topic starter Posted : 05/06/2018 8:27 pm