Workshop 1976 Labgear 7026 - Teletext Adaptor

Well... you have an explanation of the Tifax module, does that say on what pins the video input is fed to? That's a good place to start 🙂

Edit; PL2, pin 13 ground, pin 16 video in.

Sorry, yes I'd already scoped that, as that is the lead straight out the back of the ELC1043, that arrives on PL2/13/16. I thought you were looking for before entering the tuner.

No, I'd be looking at the output of the receiver board where the video goes into the decoder. Sorry I wasn't more clear.

But that seems odd, what you're saying?

Posted by: @cathovisor

↑

But that seems odd, what you're saying?

That'll be coz i'm a twonk, two white wires crossed, and I followed the wrong one PL2/13/16 is fed from here, and I had scoped that earlier too, but it looked like a mess.

@crustytv "Looks like a mess" is good - shows the issue is with the receiver board. I sort of thought a guess of the block diagram would be this.

@crustytv What voltage do you have at the AGC pin of the ELC tuner and any chance of a look inside that the larger IF module? Will be interesting to see the IC types. There is a pot next to the IF selectivity module and large IF can, this looks to me as though it may be for AGC crossover and is well worth a check.

@jayceebee That's a thought that has crossed my mind too, John - and I can see the dreaded Lokfits lurking as well. I wonder if that IF can was used elsewhere?

Not done much at all to with this today due to an idiot driver! They parked their car up the road on the opposite side, neglecting to apply the handbrake. I was sitting at my workbench, which is in an extension way down in the garden, there was an almighty bang!

When I went to investigate, the car had damaged the front garden wall, deflecting the car partially into my drive, totally demolishing a substantial pillar and about 5ft of my other wall. Missed my wife's newly acquired car by inches. To be honest it could have been so much worse, checking security camera footage, moments before people had been passing on the pavement, it's a fairly busy main road, so unbelievable it did not meet oncoming traffic. More importantly we are surrounded by two junior schools and two senior schools, thankfully 10:52 they were all in class, but it could have been a fatality in so many ways.

I did call the police as after an hour with the car across our drive and no show of driver. Eventually the cause was ascertained, the owner, was in a rush as they had to go to a meeting where they were being considered for ejection from the allotment association. She was obviously stressed and not paying attention, and paid the price, literally. As I say, could have been so much worse. 

Anyway, major digression....... back on topic!

Posted by: @cathovisor

↑

I wonder if that IF can was used elsewhere?

Spooky you should mention this, you may recall yesterday I installed another ELC1043/06. This was removed from one of my many PCB I have for sets in stock. I think the board is from a Rediffusion MK4, but I might be mistaken. Anyway, I noticed that the board resembled my Labgear receiver PCB ELC1043/06, large can, smalle can even the ic look in the same place, just a few more if cans.

I decided to open up the large and small cans on the board I stole the ELC1043 from. I then did likewise on the labgear PCB. I was amazed to see they were identical, except the smaller module has one extra coil than the labgear has. But other than that, they are the same, I even counted the number of turns of the coils on the small module, they matched. The larger module has printed coils, and they too match.

Side by Side comparison.

Posted by: @jayceebee

↑

What voltage do you have at the AGC pin of the ELC tuner

Will try and take a reading later.

Posted by: @jayceebee

↑

and any chance of a look inside that the larger IF module?

The above should satisfy

Now, from your very own forum at https://www.radios-tv.co.uk/vintage-electronics-blog-forum/early-ctv-ic-faults/

TCA270: Widely used vision synchronous demodulator i.c. Can be responsible for loss of or weak sync (check output at pin 10), a blank raster, or a weak picture.

I'm not for a moment saying this is your fault, you will need to get scientific on it. But a possibility, no?

Posted by: @cathovisor

↑

I'm not for a moment saying this is your fault, you will need to get scientific on it. But a possibility, no?

Indeed, and it's pin4 that controls the RF AGC. 

@jayceebee Ah, now that's interesting... I'd love to think we've just stumbled across the fault but as always - science, not guesswork.

I don't know if removing the cans, or having clip leads attached to both TCA270B pin 4 & pin 10, but I'm able to tune in a lot better. Still very grainy, with background buzz. Tune off signal, you can get test card tune, but lose colour.

Here are the results of Pin 4 and Pin 10

Pin 10- Video 9+V

Pin 4 AGC control 2.3V

Out of interest I did try teletext but nothing, although I've no idea how you operate this. I assumed you press "On" on the remote and the select a page. Not entirely sure if it's weak signal related, as when I press "on" I would expect the test card to blank to black screen. Perhaps there are remote faults too, still one thing at a time, signal first.

The workshop signal the box is receiving is OK and carrying "Live" teletext feed from my Pi server. This is it working from test set earlier this morning.

Interesting.....

With this marginally increased signal, I switched the box at the back to "text", I then get the blank screen and could make out teletext lines. However, the picture is very dark when switched into text mode compared to when on test card. As an experiment I entered in page 100, I could just about make out the characters, plenty of teletext corruption, which is expected with this level of signal. Whilst trying various things like tweaking the labgear tuner or the TV tuner, over some considerable time, suddenly the little text I did  see vanished, something died, got hot, who knows.

Some steps forward, some steps back, laced with clues.

New TCA270, and you'd be really hard-pressed to notice any improvement. If anything, off signal it's allowed me to tune in a better B&W picture. I even hooked in a new tuning head, to eliminate that from the equation.

Following on from experiments, switching the box into text mode. What I've discovered is the blank screen was not in fact blank at all. Raising brightness and contrast, you can make out the test card, so when switching to Text from picture on the rear, reveals a massive disparity on those settings, another fault, perhaps on the decoder card? I wish the 4 chips on my PAL board were not either deliberately scrubbed off in one case, or totally missing on the other three, for I could read datasheets and take voltage readings. One chip, the one above the bottom two, feels cooler than the rest, and oddly it has 5.3V on seven of its pins.

As in the previous post when I had everything on a knife edge tuning-wise and in text mode I could see the teletext lines, so it wants to work, but I think it has more than one fault. After a week of this I'm getting weary, so I think I will box this up and maybe come back to it, who knows data may turn up!

Thanks for your thoughts and input.

It's a long time ago but I think the voltage swing on the AGC pin of the tuner was between 2V to 5V and the lower the voltage the higher the gain. You measured pin4 of the IC but I wonder what it was at the tuner itself as there may be a buffer stage?

The IF panel with the printed coils reminds me of the PYE 725/731 series and they used to suffered terrible problems purportedly from the capacitor legs not being clean. If this was used in Rediffusion models perhaps @malcscott can tell us whether it suffered from the same issue.

Rediffusion never used these. The MK4 tuner was a U341 ? The MK1 used an ELC1043/05.

Thanks Malc. The more I look at it the more I suspect this is a PYE board especially with that slot cut in the centre. I did some copies of the LEDCo replacement with a SAWF design for @the_teleman and looks a similar size.

@jayceebee Looks like a Pye pcb to me as well.

The Tifax board from the Rank, which I have been using in this labgear, has a fault, I sort of knew this, but this afternoon's results confirms what I originally thought in the Rank thread.

Despite it being fairly ropey, by placing the original labgear Tifax board back in the unit, with some knife-edge tweaking of the tuner head and TV, it appears less faulty than the Rank one.  I managed to get a little further in text mode, as can be seen in this video. The remote can turn text on and off, but there's just a single line of garbage.

It could be the chicken and the egg, enough signal for a B&W test card, but not enough for the text decoders to resolve, though I don't think so, as I'd expect more than a single garbled line. Too many unknown faults, like why was the diode D2 a 1N4001 blown in half? The receiver PCB has faults, as do both Tifax boards, and I'm also suspecting the PAL coder board too.

Anyway, before consigning this back to the store room, a little light at the end of the tunnel. There is a chance a Vrat member may, and I stress may, have a spare one I can acquire and in much better condition. Watch this space, but for now, after one week, here endeth the torment of this one.

p.s.

I've been provided with info, so I know the four unknown ic's on the PAL PCB and the one on the receiver.

The If Gain module is indeed PYE CT200 or 731/725 as suggested. I can't get to my Pye 725 panels at the moment as they are in storage in the loft until I get the cabinet sorted. But a look at the CT200 manual confirms the panel is the same.

P.S sorry the pics are upside down I got a garbled page when I tried to upload the pictures...