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Old 23rd Jan 2021, 12:59 pm   #1
Niechcial,Steve
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Default Decca first single standard colour chassis

This is a variation of the Decca CTV 25 Mk 2 dual standard chassis (the one with the tripler instead of the GY/PD valves). Apart from the IF strip, it is virtually identical.
I have a hum problem I have run out of ideas on. A faint 50 hz hum bar makes its way up the screen kinking the verticals as it goes and flipping the frame sync when it gets to the top. Using a 'scope, the hum is clearly visible on the demodulated signal where it comes into the main board from the IF, It is also clearly visible on the separated sync pulses. It looks very much like it's being generated on the IF panel.
I have scoped every supply line for hum (all good). I have thoroughly checked for poor earths on the IF , screens on leads etc. I replaced the electrolytic de-couplers on the IF and all the valves just in case there was a heater/cathode leak somewhere.
In short, I have no idea wher to try next. Help!
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Old 23rd Jan 2021, 1:48 pm   #2
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Default Re: Decca first single standard colour chassis

A vague memory on these is that the wiring loom's location is very critical. Might be worth a go?
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Old 23rd Jan 2021, 2:37 pm   #3
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Default Re: Decca first single standard colour chassis

Thanks Glyn. I'll give it a go.
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Old 23rd Jan 2021, 5:19 pm   #4
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Default Re: Decca first single standard colour chassis

Hi Steve and Glynn.

I hope all is well.

I had this exact fault on my pal Lucy's Decca CTV25 and turned out to be a nearly open-circuit resistor in the sync separator. (Hmm - can you have nearly open-circuit?)

Have a look at paragraph 10 in post 33 on this thread:

https://vintage-radio.net/forum/show...t=decca&page=2

I hope it helps.

Thanks Steve.

Kind regards.

From Mike.
PS. Thanks to my pal Tone who remembered the sync problem in the writeup. I'd completely forgotten.
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Old 23rd Jan 2021, 5:23 pm   #5
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Default Re: Decca first single standard colour chassis

I checked the cable forms which are all bound and rooted as per original. That did give me the idea though of whether there might be some residual field from the de=guassing coils. It was a long shot as there was no purity disturbance, and sure enough, disconnecting the supply to them made no difference at all.
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Old 23rd Jan 2021, 5:24 pm   #6
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Default Re: Decca first single standard colour chassis

Thanks Mike. That sounds hopeful. I will give it a go.
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Old 23rd Jan 2021, 6:24 pm   #7
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Default Re: Decca first single standard colour chassis

Mike and Tone, I am eternally grateful to you. Hum gone! The 82K had gone up to 128K, the 47K it formed a potential divider with had gone up to 67K, and the 4M2 base bias resistor was o/c. It was amazing it worked as well as it did! I might have got there myself in the end, but like the prodigal son I was 'still afar off'! Now I can go looking to find the colour.
I only bought this set as a consolation prize for not having a smokestack CTV 25 but having read that CTV 25 post, I'm feeling less envious. The only trouble with this LOPT so far was the paxolin panel on top which had carbonised and split. Someone had bodged it with araldite and sparks were flashing and fizzing everywhere. The transformer is plug in; no doubt because of the failure rate on earlier models. It was a fairly easy matter to dismantle the core and fashion a new panel with rivetted tags to go back. Thanks again.
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Old 23rd Jan 2021, 6:50 pm   #8
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Default Re: Decca first single standard colour chassis

PS on LOPTy issues, this set came with a large replacement metal 0.25 mfd Hunts cap boost rated at 1KV (subsequently discovered it should be the more usual 0.47mfd). As I was working on it to my horror I saw the picture shrink in symmetrically from all sides with no loss of line lock. Fearing the worst and switching off quickly, I switched on again whilst monitoring the boost. Up came the full picture again for about 30 seconds only to repeat the performance. When I felt the boost cap it was very hot. It measured about 500K. A replacement 0.47 sorted it out. I presume it might have been an oil filled cap? I've never seen one do that before.
The CTV 25 saga takes me back to the days of 1974 when I was but a callow youth and working for a dealer. He gave me a CTV 25 to sort and I was foolish enough to run it without the main earthing strap between chassis and control panel in place, Whether or no there should have been an alternate neutral return route in place I know not, but there certainly wasn't and a large dose of juice found an alternate route through many transistors destroying all in its path. I think it ranks as my most spectacular cock-up to date. Boss wouldn't talk to me for a week.
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Old 29th Jan 2021, 7:54 pm   #9
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Default Re: Decca first single standard colour chassis

This set is just about sorted now. Having fixed the hum thanks to Mikey and Tone, the only other electronic faults were the first Chroma Amp transistor, BF196, o/c, and both resistors in the boost feed to the frame osc. gone high. I am noticing a lot of these plastic lock-fit transistors fail o/c, many more than their metal equivalents. I changed the 10M clamp resistors on the 3 PCL84s for good measure, converged it and job done. The 4 push button tuner is not as complicated as the vhf/uhf one but it needed a bit of mechanical work. What looks like a fifth channel button is in fact a knob which tunes the other buttons through a series of plastic cogs. It was all thoroughly seized up. They originally fitted a UHF tuner and IF with AFC to these single standard sets but had abandoned it by this later variant. The tuner resets well and doesn't drift without it.
The tube is the original Mullard, and not bad. The set is ex-rental Granada badged, and has had a lot of wear, but looking around, seems to have been fairly reliable. Apart from a few valves, the only things I can see replaced are the horiz shift pot and 4 diodes of the R-Y synchrous detector.
Service is easier than the D/S because there is not a great smoke stack to get in the way of pulling the chassis back and flipping it on its side to access the print. There are no component numbers on either side of the board though, and the manual does not give you print side drawings so finding and de-soldering components is a nightmare. I attach some pics. The strange moire effect is on the camera, not the set. If you look at the chassis pic you will see the three coloured spindles for the A1 pots. A first for me on a UK set is that these go through the back and are customer adjustable. Does this say something about Decca's view on the longevity of British tubes? Having those for customer twiddles must have led to a lot of service calls.
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Old 29th Jan 2021, 9:13 pm   #10
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Default Re: Decca first single standard colour chassis

That's a damn fine pic given it's age, convergence looks spot on, never saw one of these, the Bradfords, hundreds of them. I was about when this was a current model and saw lots of other models, G6, Thorn 2000 etc but never one of these, that looks a fine set though, good repair.
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Old 29th Jan 2021, 10:50 pm   #11
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Default Re: Decca first single standard colour chassis

Likewise, never saw one, but hundreds of Bradfords, very good oicture.
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Old 29th Jan 2021, 11:07 pm   #12
Niechcial,Steve
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Default Re: Decca first single standard colour chassis

Thanks Tony
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