CTV [Sticky] 1974 HMV 2726 Coloumaster; Thorn 4000

@crustytv What's caught my eye is that the "fold-over" appears to be in the opposite phase to the colour adjacent. I think it would be instructive - looking at the bars at the top - to feed in colour bars and look at the individual colours with both eyeball and scope. I think there is very definitely something amiss with the decoder.

@crustytv Could it be line phase (R315) needs adjusting?

Hi Jim, tried that too, still have the same condition.

I'll get colour bars up, then sniff the decoder to see if there's anything that jumps out.

Just so we know where we're at with colour bars.

Besides the odd effect, there are indeed "Hanover bars" in there too.

If L113 didn’t improve things then it does look like a decoder issue. The general look of the colour of the test card doesn’t appear quite right together with the reflections with phase shift on the left. First of all which type of decoder do you have, is it the one with the SN76228N IC? Whatever it is looks like B-Y is the most badly affected.

Posted by: @jayceebee

First of all which type of decoder do you have, is it the one with the SN76228N IC?

Yes

I tried to scope the B-Y signal at pin 8 of the IC, but it removes the colour from the colour bar

Ooops wrong IC, I was on IC156, I should be on IC157, I'll try again!

Pin8 of which, IC156 or IC157?

Answered. Pin8 of IC156 is the 4.43Mhz osc so I would expect that may happen.

Pin 8 of IC157, B-Y trace not good

@jayceebee The place I first worked at were very narrow minded about Thorn sets. the excuse was that they didn't carry the spares. When they bought out a local firm's rental accounts they disposed of all the Thorn sets as soon as a service call or upgrade was needed. I bought loads of lovely Alba and Marconi varicap 3500 sets in lovely condition for peanuts. I couldn't understand that they would dispose of these when at the same time we would refurb Pye Hybrid sets almost no matter what. Later on when I was self employed I would buy ex rental sets from a local firm on a weekly basis, they operated a "TV surgery" which offered set owners a repair while you wait service. Apart from Thorn sets (by then most of the 3/3500s and 8k s had gone the sets were 88/9K onwards) they saved the Thorns for my visit. I would do them but because the service manager couldn't pay me I would be be paid in ex rental or customer write off sets ! 

Happy days! Long summers tatty old escort van that never broke down and down the pub every night! 

Good luck with the remote control fault Chris! The only ultra sonic ones I can remember fixing were the G11 and the ITT CVC9. The ITT sets occasionally needed a transducer the G11... Well what can I say ? The remote panel was just about the only one without common faults or bad soldering! Usually the fault was the handset (mostly because it had been 'drinking'!)

@crustytv It's not great, but what do pins 2, 1 and 4 of IC157 look like?

I wonder if it's a problem with the delay line, I also wonder why it's taped up. This is the only evidence of some intervention by someone on this board, it otherwise looks as original.

Couple of possible suspects around there, C186 and C188. Have you gone through the adjustments of the ACC and also the delay line pots/coils? The delay line itself is not above suspicious. I’ve had those multi reflection types cause issues in the past, not sure if something happens to the properties of the glass or transducers. In one case the glass was physically cracked when I took it apart, I was amazed it worked at all.

Before I do anything with set up etc, I'm going to look at the Delay Line. It's certainly been out by the looks of the flux residue, and someone has strapped a cap on the bottom. This is not on the circuit. I may just replace it with a new one.

I'm not saying this is the cause, but a couple of pads just floated away with minimal application of the iron. The delay line has been opened before and glued, why? I would replace but it, but it's a UDL14, the closest I've got is a UDL11. I cont checked the fine wires to the glass and they are OK.

The extra 47pF capacitor is a mod designated C251 when a UDL14 is fitted but it says the cap should be on the input of the delay line which I take to be the L160 end. Looks like yours has been tacked on the output to me?

Try the UDL11 and remove the cap.

Fitted the UDL11, added wire links to compensate for lost pads. It's got rid of the odd shading, but now my Red bar is no longer red!

What's the difference between a UDL14 & UDL11, could this be the cause of my Red going a darker magenta? I'm tempted to put back the UDL14, now I've fixed the pads and repaired its case.

@crustytv Just looks like it needs a phase adjustment now - do you get that odd antiphase reflection on TCF?

Remember, you've changed one of the most critical parts in a PAL decoder. If that antiphase reflection has gone on TCF, I'd consider this a win... but you will be looking at a decoder lineup. 

Yes, reflection has gone, TCF looks much better.

I'm very tempted to put back the UDL14 and that cap on the correct end, with the fixed pads, I have a sneaking suspicion it would be OK now, might save having to do a full decoder alignment.