Dicey Decca CTV25

[Peter.N.]

The double picture looks like a convergence problem as there is only one picture on the blue image. Still could possibly be a line sync problem although I'm used to seeing cogging with square edges. I wonder what the power supply to the sync circuit looks like?

Peter

[beery]

Hi Mikey,
Years ago I had a CTV19, but it never did this.
I might be barking up the wrong tree, but could it be a fault with the degaussing circuit causing a bit of current to remain flowing in the degaussing coil?

Cheers
Andy

[Mikey405]

Hi Glyn, Simon, Trevor, Peter and Andy.

Thank you for the interesting ideas. I've not really done anything with this fault over the last week. Today (probably against Boris's express wishes), Tas visited and between us we sorted out the terrible EHT arcing problems (the set had had a fair old burn up in the stack) by re-insulating the cable as it came out of the GY501 base and replacing the EHT cap and its integrated 2K2 resistor. The odd trapezium shaped raster will be for tomorrow though.

Andy - I will double-check but I suspect the degaussing is okay because there is no "movement" in the pictures as there would be if AC was still flowing through the coils. Your old Decca CTV19 is down in Devon - never having had its back cover removed (not by me anyway).

Peter - I think you're right about the line sync problem. The "cogging" should be fairly straightforward to sort out though (picture content in the sync channel) and I'll get that fixed once I've got the frame and convergence all sorted out.

Trevor - Nice to see you back on the original-and-best forum. I also thought it may have been the scan coils originally but alas not. I've not ruled out some kind of short in the convergence yoke yet.

Simon - Re your comment regarding a previous owner - I certainly wouldn't rule out a man-made problem. The EHT stack was all loose and it'd obviously been opened up after the burn-up so it could be something mis-wired after they threw the set back together.

Glyn - That certainly hasn't been ruled off my list of suspects although shorting out the frame-half of the transductor didn't make any difference whatsoever.

Regarding the purity and convergence - I'm not concerned about that at all at the moment - Once everything is sorted out then I'm sure that will be relatively easy to set up. If not by me and my clumsy ways, then by Tas and his expert fingers and keen eye.

Thanks everyone. I shall report back any further work and findings.

Kind regards.

From Mike.

[Mikey405]

Hi all.

Well, this Decca is leading me a merry dance. The cogging is sorted and we now have colour too, but what about this weird convergence...

Replaced so far: scan coils, convergence yoke, pincushion corrector (actually the latter was removed and the frame circuit shorted but it made no difference).

From the pictures it looks like the R/G line-derived convergence is fine and those controls do what they say they should (more or less) but the R/G frame-derived convergence is all over the place. As for the blue, well that's just bizarre. Ignore the purity error - that was the result of changing the yoke (which includes the purity magnets). I'll sort that once the convergence is fixed.

The output from the frame transformer to the scan coils also has about a 20 percent line-rate waveform superimposed too so that is likely to be the reason for the overall slope on the picture.

Anyway, I'm tired now and am going to go and watch some television (one that works) from the front of the screen rather than sat round the back.

Thanks all.

Kind regards.

From Mike.

[Mikey405]

Some bits and pieces for anyone who may be interested.

The output from the frame transformer and a few pages from the manual.

Thanks all.

Kind regards.

From Mike.

[beery]

Originally Posted by Mikey405

The output from the frame transformer to the scan coils also has about a 20 percent line-rate waveform superimposed too so that is likely to be the reason for the overall slope on the picture.

Hi Mikey,
I can't say that I know much about convergence waveforms as I've not had much experience with delta gun sets.
Anyway, I'm clutching at straws. Is there any chance that line ripple is getting in via the feed to the height control?

Cheers
Andy

[Mikey405]

Hi Andy.

The supply rail from the line stage had a little line ripple on it but replacing C403 has put that straight and it's now clean. I think the ripple is getting in at the output stage. There are a number of connections to the convergence from the frame (two in the cathode and one from the frame transformer) so a good sniff around those points will be useful.

Thanks Andy.

Kind regards.

From Mike.

[Welsh Anorak]

My brain hurts so yours must!
Can you disconnect bits of the convergence circuit? Every time I look at the pictures I think it's the scan coils, which it obviously isn't. It's almost as if a DC component at line rate is getting into the frame derived convergence, but how? Could there be a fault in the convergence yoke? It's so bizarre I'd be looking for the person who's been here before and swapped two wires over.

[FERNSEH]

Try shorting the black and brown wires together at pins 8 and 9 on the convergence panel.
Wires connect to VR500 the frame scanning coils symmetry adjustment.

DFWB.

[frsimen]

Trapezium distortion is usually caused by an open circuit in one of the scan coils. In this case, it would be one of the frame coils. You've already replaced the scan unit, so it can't be that. However, the frame coils are fed individually via R500 the R/G H.2. potentiometer on the convergence board. That control is the likely cause of the trouble.

For the line frequency waveform on the field drive, perhaps the field shift circuit is worthy of investigation. C327 going low in capacitance could cause that fault.

Paula

[Mikey405]

Hi Glyn, David and Paula.

Well, a degree of success at last. After having already replaced the scan coils, convergence yoke and checked the pincushion transductor to start off with no success, and having shorted the R/G balance pot etc. as per David's suggestion above with NO noticeable difference at all, I decided to make sure all was well with the wiring again (the Decca CTV25 manual having more than its fair share of errors in this regard). Everything looked fine so I thought I'd see why shorting out the two halves of the frame scan coil connections had absolutely no effect at all. I took a measurement between the two "joined" ends and lo-and-behold it measured 50 ohms (it obviously should have been a complete short). On examination, the wire that connected two halves of the "half frame windings" (there are 4 actual frame coils in total) to the pin had virtually no solder on it and meant, as Paula surmised above, that effectively one half of the new scan coils was open circuit. Re-soldering the connection brought full frame scan. Hooray.

Who would have thought that I'd replace a faulty set of scan coils with another set having exactly the same issue (although I imagine caused by slightly different reasons - although I will now check the original set).

Anyway, I can now move onto other things like the focus, convergence, purity, lack of B-Y etc.

Thanks for everyone's words of help and encouragement so far. I'll carry on and report back soonest.

Thanks all.

Kind regards.

From Mike.

[Mikey405]

Picture: One quarter-section of the original frame scan coils is open circuit (although I can see no visible evidence of any issues apart from the multimeter reading). I really should have checked this but I didn't really expect the new ones to have the same problem. Oh well, enough of the excuses.

Thanks all again.

Kind regards.

From Mike.

[Mikey405]

Well, a replacement 4.7M resistor in the focus (although the focus control is still up at one end) and a cursory twiddle with the purity, convergence etc. and the old girl is looking a lot nicer. There are a couple of dodgy pots in the convergence and some other jobs to do but hopefully not too much work before I lose interest in the set as always seems to happen when I'm nearly finished with a set.

Thanks all.

Kind regards.

From Mike.

[Heatercathodeshort]

That was cruelty to TV engineers .Well done! John.

[Techman]

I remember working on one of these in the 70s. Full metal chassis and too big and heavy to get on the bench at that time, so had to stay on the floor to be worked on.

[Peter.N.]

That's looking a lot better now John, well done. CRT looks good.

Peter

[Tazman1966]

Incredible coincidence that both sets of scan coils were defective in the same way. Excellent detective skills Mike.

Don't lose interest on it yet. You've still got the cabinet to tart up!

[Mikey405]

Hi John, Techman, Peter and Tas.

Many thanks for the replies and comments. A similarly-faulty set of coils is not what I expected at all but I'll certainly remember it for the next time.

The set is now looking a lot nicer with convergence, whilst not up to Tas's standards, isn't too bad for my galootish efforts.

I was just about to adjust the 405 line convergence however when the system switch jammed and a wisp of smoke came from the timebase solenoid. Something else to sort out but hopefully it's only the small sacrificial resistor under the solenoid windings.

The focus is still not quite right, there is also a slight jitter on the line, the not-very-good-idea B9A plugs on the back of the chassis are playing up and the decoder needs minor attention (Hanover blinds) but they should hopefully be relatively straightforward.

Thanks all.

Kind regards.

From Mike.

[murphyv310]

Good stuff Mikey.
Something in the back of my mind tells me that dry or poor soldering on scan coils were not unknown and I bet you that some were replaced for that very reason.
Glad you've got a result.

[System A]

Well done Mike , it looks a good set .
Regards Gary.