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Old 4th Oct 2020, 6:14 am   #1
Mikey405
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Default Dicey Decca CTV25

Good morning everyone.

Here's a fault on an old Decca CTV25 which I've not come across before. The cogging is an easy one but it's the shape of the raster which is the strange one.

I've had a quick poke around in my own clumsy way but I thought I'd post a couple of pictures up here and see if anyone has any ideas before I crack on in earnest.

The two pictures are: 1. The raster with the blue A1 turned down and - 2. The red and green A1 turned down.

Thanks everyone.

Kind regards.

From Mike.
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Old 4th Oct 2020, 7:54 am   #2
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Default Re: Dicey Decca CTV25

Ps. The same fault occurs on both standards.
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Old 4th Oct 2020, 8:52 am   #3
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Default Re: Dicey Decca CTV25

Hi Mikey
Could it be a partly shorted Line scan coils?
or pin-cushion fault ? try shorting out C500 on the convergence board.
or poor de-coupling on the time base board?
Keith
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Old 4th Oct 2020, 9:07 am   #4
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Default Re: Dicey Decca CTV25

Hi Keith.

Your first guess was also my first guess and I replaced the scan coils but alas it made no difference.

It could certainly be to do with the pincushion circuit but it's the wildly different raster shapes and sizes for each colour that is the interesting bit. Certainly way too much interaction between line and frame.

I shall investigate further.

Thanks Keith.

Kind regards.

From Mike.
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Old 4th Oct 2020, 9:29 am   #5
Peter.N.
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Default Re: Dicey Decca CTV25

Never seen that, you appear to have two pictures, one on top of the other. I at first thought a sync fault, but probably not.

Peter
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Old 4th Oct 2020, 9:38 am   #6
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Default Re: Dicey Decca CTV25

Hi Mikey
What about those ready to fall off connections to the convergence yoke / scan coils all on and in the right place?
Keith
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Old 4th Oct 2020, 10:01 am   #7
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Default Re: Dicey Decca CTV25

Hi Keith.

Oh yes, those dodgy connectors. I've been through those a dozen times making sure everything is wired correctly as per the manual...
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Old 4th Oct 2020, 10:02 am   #8
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Default Re: Dicey Decca CTV25

Could it be a buckled shadow mask?
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Old 4th Oct 2020, 10:02 am   #9
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Default Re: Dicey Decca CTV25

Quote:
Originally Posted by Peter.N. View Post
Never seen that, you appear to have two pictures, one on top of the other. I at first thought a sync fault, but probably not.
It is a strange one Peter.
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Old 4th Oct 2020, 10:03 am   #10
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Default Re: Dicey Decca CTV25

Quote:
Originally Posted by cheerfulcharlie View Post
Could it be a buckled shadow mask?
I think it would have to be outrageously buckled for that Charlie but it's certainly an interesting thought.
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Old 4th Oct 2020, 10:21 am   #11
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Default Re: Dicey Decca CTV25

That looks like an evil child has removed he back and had a twiddle!
Maybe try to break the faults down, sort the cogging first then the purity. If you cannot set the purity the tube may have an odd fault but I wonder if the degaussing is operating at a low level continuously? Certainly a strange one. Regards, John.
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Old 4th Oct 2020, 10:28 am   #12
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Default Re: Dicey Decca CTV25

Hi Mike,
how about one of the clamping diodes in the convergence circuits having gone faulty? Usually diode connected AC128 transistors.

DFWB.
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Old 4th Oct 2020, 10:34 am   #13
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Default Re: Dicey Decca CTV25

Hi Mike,
Good to see you back posting on the forum again after a bit of a 'rest'! I tend to find as the darker evenings approach, I get a desire to start playing with TV's again- seems more of a winter thing!

I think with this fault, I would sort out the dodgy video/ sync issue first. Then you can see exactly what the raster distortion looks like. Your picture looks like it has been crumpled up into a ball and then unscrewed! It just hasn't been unscrewed properly!

I guess there is some modulation of the frame scan at line rate causing it to reduce in height at one end? Presumably a pincushion/ convergence board issue? Assuming there is no line rate 'hum' on the supply to the frame stage?

Interesting fault on an interesting set!

Good luck
All the best
Nick
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Old 4th Oct 2020, 11:17 am   #14
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Default Re: Dicey Decca CTV25

Circuit diagram of the convergence board fitted in dual-standard sets.
The clamp diodes are in the line convergence circuits. These could be replaced with schottky diodes.

DFWB.
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Old 4th Oct 2020, 5:07 pm   #15
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Default Re: Dicey Decca CTV25

Turn it the right way round and it'll be fine!
Seriously, it looks like some line radiation is getting where it shouldn't get. Decoupling perhaps? Any history on the TV - was it working then this happened? Perhaps disconnect the pincushion circuit, though I'm not that familiar with the CTV25, possibly fearing for my sanity.
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Old 5th Oct 2020, 7:19 am   #16
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Default Re: Dicey Decca CTV25

Morning Mike!

That looks like a combination of more than one fault to me, mostly associated with the line oscillator and the flywheel discriminator/reactance valve!

I'd be looking for:–

a) Yakky polystyrenes in the PCF802 oscillator stage (pentode section);
b) H.T. decoupling to same;
c) Heater–cathode leakage in the PCF802 valve;
d) Grotty or unbalanced flywheel discriminator diodes;
e) Poor h.f. response in the video e.f. and preamplifier prior to the sync. take off–point – series electrolytic coupling and/or l.t. decoupling capacitors associated with same!
f) Yakky resistors feeding back the line t.b. reference sawtooth from the LOPT to the flywheel discriminator.
g) Wiring and earthing connections to convergence panel and rest of receiver, also earthing connections associated with line osc. PCF802 stage!

This sort of fault is one where you really need the full S.M. for the set rather than just the circuit–diagrams as the s.m. will show all the wiring harnesses, interconnections, etc., etc., in more detail than the basic circuits do!

Unfortunately the CD of early CTV manuals I had has got damaged so I can't look up the reference nos., but you'll be able to pick them out from your circuits easily enough!

Obviously it's impossible to adjust the static or dynamic convergence on that mess, the reason it's gone so bad is that the fault affects all the timing between the R/G and the blue sections of the line convergence circuits!

Incidentally this type of snag is really best checked on a pattern generator – do you have one?

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Old 5th Oct 2020, 8:33 am   #17
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Default Re: Dicey Decca CTV25

Morning John, David, Nick, Glyn and Chris.

Many thanks for all the input.

Re the "cogging" problem and purity, those will be quite straightforward I'm sure (famous last words?) - It's just the weird raster shape that's the interesting one.

Unfortunately I was rather tied up over the weekend and didn't get a chance to have a look at the set but I'll have a go this week and see what we can find.

Thanks all.

Kind regards.

From Mike.
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Old 5th Oct 2020, 12:37 pm   #18
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Default Re: Dicey Decca CTV25

The more i think of mit the more I'm inclined to the pincshion transductor. Since that's my opinion you can probably cross it off the list...
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Old 6th Oct 2020, 9:56 am   #19
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Default Re: Dicey Decca CTV25

This has got me going. I suspect you have more than one fault here.

For the strange shaped picture I agree with Glyn and would certainly try changing out the pincshion transductor. I have seen a few of these fail in the past and some that actually burn up.

When you tried changing out the line scan coils did that include the convergence coils as these can be separate from the scanning coils especially on older delta gun scan coil units.

For the purity if the degause circuit is working ok then hopefully once you have cleared the raster shape fault it will hopefully clear with a good set up.

If all the above is proved ok then I am afraid you could be looking at a strange CRT fault or man made faults courtesy of a previous owner.

As for the line tearing or cogging effect if there is no sign of EHT leakage then I would suspect a problem around the line oscillator or sync circuits.

Best of luck

Simon.
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Old 6th Oct 2020, 11:08 am   #20
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Default Re: Dicey Decca CTV25

Hi Mike.
That's an odd issue. A couple of things did come to mind. A problem with the scan coils or convergence yoke or possibly hum ingress to either. Either way an odd issue.
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