Workshop 1976 Labgear 7026 - Teletext Adaptor

Posted by: @cathovisor

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Before we go delving into the 5V supply and why the fuse keeps opening,

Just for clarity, the fuse blew just the once, it does not keep blowing.

Posted by: @cathovisor

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t might be instructive to see if you get a variable voltage to the ELC1043 when you alter the relevant pot from the channel selector

Agreed, I need to find a pin-out cct for the ELC1043/06

Posted by: @cathovisor

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is that a sort of stand-alone Philips IF unit on the board in the big screening can?

It certainly appears to be

Posted by: @crustytv

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I need to find a pin-out cct for the ELC1043/06

I would imagine any TV that uses an ELC1043 will do for the pinout - I doubt the /06 bit is significant (unless it's something to do with AGC polarity, for example). I'm sure some of your older sets use them.

Edit: https://www.vintage-radio.net/forum/attachment.php?attachmentid=43143&d=1290632928

Also https://www.radios-tv.co.uk/community/black-white-tvs/pye-741/

I thought the difference between the  /05 and the /06 was the gain or more correctly the level of output to the IF?  

@slidertogrid You're quite right, Rich - the leaflet I placed a link to says the IF output of the /06 version is 'different' to the usual /05 version.

The third pin observed from the left is the tuning voltage, here are the test results:-

• 1st channel, selected, the voltage was at 27V, tuning across the band from one extent to the other did not vary the voltage.
• 2nd channel, selected, the voltage was at 2V, tuning across the band from one extent did vary the voltage. However, it was highly erratic, when you stopped tuning it would jump from 2V to 9V, 15V, to 3V, to 27V. If you waggled the selector, you could get it to stop, but not always. Currently, it has stopped working altogether.
• 3rd channel, selected, the voltage was at 3V, tuning across the band from one extent to the other did not vary the voltage.
• 4th channel, selected, the voltage was at 1.2V, tuning across the band from one extent to the other did not vary the voltage.

Conclusion; the TAA550 is OK, I can vary the tuning voltage, albeit not in a good way. Based on the above, it would appear the varicap tuner in the labgear is looking like the problem. Whether it will respond to some servisol & cleaning I will have to see.

Firstly, I'd suspect the switch unit - if possible, wire in a 100k pot between the TAA550 and ground, take it to the tuner and see if you can tune anything in then. A note of warning though - quite often the various pot wipers are fed through diodes to isolate them from each other. But what you've seen does explain why you couldn't tune anything in.

@crustytv Neither the 1043 tuner or the those types of switch units were that reliable although the 1043 usually failed intermittently and a tap on the side would make it work for a few minutes.

Has suggested by Cathovisor definitely worth trying an external pot.

Posted by: @cathovisor

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I'd suspect the switch uni

And you'd be right 👍 All contact tangs have broken away on both ends.

I will just clip link one channel in and see if I get anything, tuned in.

That worked, I can now tune the second channel across the entire .3V -28V range.

I don't think I have any 4 channel varicap tuners in stock. Long term I will hard-wire it to one channel, until such a time a 4-channel varicap tuner of the right size emerges.

That looks very similar to the unit fitted to the later Decca CS1830 and also the "Gypsy" portable.

It is similar to the models Mike suggests and one of those units is probably electrically the same, but the button size is different those units don't have the thinner buttons. The holes in the front may be large enough though. I think this unit is the same as used in the Pye CT200 maybe? I'm not sure if that had the tuning indicators though.... It's a shame Sendz are not still around they would have had them in stock for peanuts! 

It's fixed and now tunes across the voltage, on one fixed channel, not pretty but needs must.

Next..... another attempt at set up, although I'm not confident as when the TV is tuned to the text signal board, all I get is a blank screen, I should see 100 in the top line. This suggests that although I'm tuned to the labgear signal teletext signal, nothing is there. I'm more concerned as I tried another Tifax board. This suggests a possible fault on the text signals board, that has 4 ic's.

Have you tried to see if you can get a picture from your receiver section? That board looks like a PAL coder board, are there any signals from the Tifax board on the red/green/blue leads when in text mode?

The deeper I dive, the more confusing it gets

Edit:

At the end I actually mean when I switch colour off, picture goes off, if i switch to colour on it comes back.

I'm suspecting the ELC1043 now. I find if I tune in for good sound, no picture. Tune for picture I get this and lots of hiss, I can just about make out test card music buried in the hiss.

Well, it certainly looks like a tuner fault... however - does "colour off" operate on the PAL coder card or the off-air receiver?

Posted by: @cathovisor

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does "colour off" operate on the PAL coder card or the off-air receiver?

PAL coder PCB, the switch shorts the black wire to chassis

I'm no longer certain it's the tuner. I've tried two replacements, an ELC1043/05 and another ELC1043/06, exactly the same symptoms. Three tuners cannot all have the same fault, surely.

@crustytv Could it be the modulator, in that case - or indeed that IF strip? I'm assuming there's no aerial isolator to fail here (been there), but is it possible to look at the video output from the receiver board on a 'scope?

Sorry Mike. 😞 Without a circuit diagram, I'm afraid my experience level is not sufficient to fathom where to start on this board, I need diagrams and circuit descriptions, I've no years of experience to call upon to reverse engineer by sight.  Made worse by the fact all the ic's on the two video boards have no markings or have been deliberately scratched off. I scoped the one ic on the main IF board and found three waveforms, the final two were on the orange and blue wires.