Workshop 1976 Labgear 7026 - Teletext Adaptor

The ultrasonic tank works its wonders once again.

I need to replace this D2 diode, no markings left where it split, and it's not mentioned on the sparse diagram in the RRI book. On the Rank Arena board, I can make out IN4, but the other numbers must be underneath. I'm loathed to go pulling that one apart, but will probably have to. I could stick in a IN4001 as I seriously doubt it's higher, and it's certainly not a 1N4148.

It's definitely not a 4148, that's for sure.

Well, after some research this afternoon, I finally have the answer to the question of Teletext standards compatibility. Firstly, both TIFAX modules in my Rank Arena and Labgear have "D22N700 ISSUE 4" installed.

Here are the decoded numbers of all the XM-11 TIFAX modules, and their abilities are as follows:-

• XM-11 Standard TIFAX module meeting January'76 Teletext Specification. Code N700
• XM-11/S TIFAX module with Swedish character R.O.M. Code N705
• XM-11 DH/E Tifax module with double height character facility and standard English. Code N706
• XM-11 DH/S TIFAX module with double height DH/S character facility and Swedish characters. Code N703

The often mentioned modification to the Teletext standard, surrounded the new control characters added for displaying double height characters. This recently mentioned by other VRAT'ers, when I was working the Rank Arena AC6333

Further reading suggests in most applications this addition should not stop the older N700 boards working, as it is not essential. The change in the specification was made in such a way as to ensure compatibility with decoders not reacting to the double height control. Again, I seem to remember @jayceebee stating he didn't remember the older boards not working after the standards change.

But....

Where the Teletext system is used primarily for subtitling, the use of double height characters became more important. For that type of application, the XM-11 DH module, which does have the ability to decode double height control character, would be required.

Based on all that, I would expect to at least see something on screen from both my N700 modules. We shall see in due course.

@crustytv Do you have any videotapes from the last century, If so why not try playing them back and see if you can get some vestiges of a page as @WayneD did? It's much harder with a tape not played back on the machine on which it was recorded but worth a go. I've VHS stuff going back to 1978 which you are welcome to borrow.

Broken diode D2 is out.

Posted by: @jayceebee

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Do you have any videotapes from the last century, If so why not try playing them back and see if you can get some vestiges of a page

I do

Posted by: @crustytv

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Well, after some research this afternoon, I finally have the answer to the question of Teletext standards compatibility. Firstly, both TIFAX modules in my Rank Arena and Labgear have "D22N700 ISSUE 4" installed.

Here are the decoded numbers of all the XM-11 TIFAX modules, and their abilities are as follows:-

 

• XM-11 Standard TIFAX module meeting January'76 Teletext Specification. Code N700
• XM-11/S TIFAX module with Swedish character R.O.M. Code N705
• XM-11 DH/E Tifax module with double height character facility and standard English. Code N706
• XM-11 DH/S TIFAX module with double height DH/S character facility and Swedish characters. Code N703

That's very informative, Chris. I seem to recall - albeit vaguely - that it was suggested the TIFAX decoders wouldn't cope when the number of lines in the field interval used for carrying text data was increased in later (late 80s/early 90s?) years. Do you know if that's the case or are the pages generated by your various devices to the original Teletext standard? Pretty sure I read that in "Television".

I should mention that one or two companies marketed Teletext decoders in a 19" rack case with RGB outputs: we had a few at work back in the 80s but to get them to work with the studio's PAL coders necessitated a data bridge to re-time the text data from Network to the local pulse timings to make the output synchronous. In later years, when surplus BSB receivers flooded the market it wasn't unknown for them to have the section of PCB with the MC1377 PAL coder chip cut out and used to encode their output. I have a couple of data bridges that were ex-BBC Parliament doing nothing here...

When Sendz components acquired a load of Mullard VM6101 decoders I bought one and made a set-top decoder at work that fed the TTL input of a Microvitec Cub.

Also, don't forget the decoder marketed to allow OTA software downloads for the BBC Micro in a matching case. I only ever saw one, in Technical Investigations at work.

Posted by: @cathovisor

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Do you know if that's the case or are the pages generated by your various devices to the original Teletext standard?

Hi Mike,

You're mistaking me for someone who knows what they’re doing, the honest answer to that is I've no idea, I'm learning on the fly. I may discover more as I deep dive further, and my current belief flawed. All I can say at this point in time after reading the XM-11 book, is it seems to suggest the N700 board should display something more than the top line I was getting from the Rank board, but that's as it stood 40+ years ago.

As I find out more, and I'm not sure where or what to research next, things may change.  For now, I plod onwards with the above belief, we'll soon know if I can get this 7026 back into some sort of shape. It might be a foolish project, and I'll end up with egg on my face but nothing new there!

The old spliced in section of the remote control has been cut out, the spades removed from PL1, and the new end prepared for reconnection to PL1.

That's the remote handset rewired to PL1 and the cable now once again through the chassis with grommet.

Next those two failed switches

After that I will need to replace the Mullard tropicals on the other modules as all the ends appear to be falling off!

Probably not really relevant but the TiFax boards and the wired Labgear remote brings back memories.

Pye at Marrickville did a short run of teletext TVs in 1976 using the TiFax board and the Labgear remote, sporting a new decal.

It was made possible by the advent of the T30C chassis with its 63cm Toshiba self-converging CRT.

I got one of these TVs for Mum, who used it daily for nearly 40 years until she passed away. The TiFax failed after about 5 years.

The clip-in plastic bracket that had previously held the convergence control board held the TiFax and its little interface and power supply board as if it had been designed that way.

It was always a marginal performer, the antenna reception needed to be top-notch to get totally error-free teletext. ATN7 in Sydney had to replace their beloved 1960 vintage AWA transmitter for Teletext - with an NEC.

We only had one network in Oz that did anything much with Teletext and that was the 7 network. They had a very lucrative contract with the TAB (think huge govt owned betting shop - all competition was banned!) By the time that deal came along the TiFax couldn't handle the required feature set.

Minor Update

Remedial work complete and my guess at 1N4001 for D2 on the TIFAX board was correct (I removed the on the rank to verify), so with everything in place I connected up to a TV and ran through what I perceive to be the set-up procedure, based on the Radofin and AYR boxes.

At the rear, I selected 'TEXT', tuned the TV to the signal output from the Labgear. In this mode, I would have expected to see the top line, but nothing was present. A finger test on the TIFAX chips, they were warm, checked supply volts 5V present, I pressed onwards.

Flicking the switch up to 'TUNE' the blank signal locked returned to snow. I then selected channel 1 on the Labgear and began trying to tune into my signal. From one end to the other nothing, no test card or test tone to be seen or heard. I selected another channel on the Labgear and repeated the step, once again nothing.

I started to suspect that the tuner PCB might have a fault, maybe a supply voltage was missing. I checked the TIFAX again and the chips seemed cold this time, meter checked and the 5V supply has gone! Back at the supply I find the fuse on the PSU is open.

I so wish I had a circuit diagram for this piece of kit. There does not seem to be much to the PSU, a transformer, a couple of rectifiers, one is massive, 2xBD132 and 1x1.130 (no idea what this is), one unknown transistor (can't access to read), 6 electrolytics, 3 tropicals, 5 resistors, 1 diode and 1 zener. The 1A fuse (the one that blew) off one of the BD132.

Things to check, why the 1A fuse off the BD132 blew, and was that responsible for the inability to tune or do I have another fault.

Posted by: @crustytv

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There does not seem to be much to the PSU, a transformer, a couple of rectifiers, one is massive, 2xBD132 and 1x1.130 (no idea what this is), one unknown transistor (can't access to read), 6 electrolytics, 3 tropicals, 5 resistors, 1 diode and 1 zener. The 1A fuse (the one that blew) off one of the BD132.

I think if you undo the screw on the "I.130" the type number will be beneath.

As far as I can guess, that PSU will need to generate three rails: 33V for the varicaps in the tuner, 12V for the analogue side of things and 5V for the digital side. The fuse didn't die a violent death so it may just have been old age or something like a tant took it out. The discrete regulators look real "Boys' Own" designs so should be easy to follow - have a look at some of your TV circuit diagrams to get a feel for how they work. At their most basic, it's two transistors and a zener diode or if you're a real cheapskate, a zener and a beefy transistor as the pass element.

Posted by: @cathovisor

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I think if you undo the screw on the "I.130" the type number will be beneath.

oddly not

Posted by: @cathovisor

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that PSU will need to generate three rails: 33V for the varicaps in the tuner, 12V for the analogue side of things and 5V for the digital side

I had the 5V & 12V, I can't be certain, but I'm sure I had 44V on the tuner board on one binding post. Will need to check once I've stuck in a new fuse.

Posted by: @crustytv

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Will need to check once I've stuck in a new fuse.

To save a fuse, can you put an ammeter across the fuse holder and see what the current draw of the 5V rail is first?

Edit: now here's a thing. There was an SGS-ATES device called an L130 and it was a 12V regulator in a TO-126 package, which is what you have there. In fact, on re-examining the picture, that is an 'L' preceding the 130, it's just got a bit of the lower stroke scuffed off.

https://www.datasheetcatalog.com/datasheets_pdf/L/1/3/0/L130.shtml

Sorry Mike, I'd already put in a fuse before you posted that suggestion. Good find on the L130, you are a search ninja.

I can report that I seem to have all rails, but I am a little unsure of something. First, we do indeed have the 5V, 12V and 33V rails as you correctly surmised.

The one thing that's bugging me are the three green wires out of the PSU board to the television tuner, Aerial signal tuner and the colour switch, these all have 0V. Again, without a cct diagram I cannot make head nor tail of this, but something is nagging me saying this does not feel right, and feels like it ties in with the fault I'm experiencing with set up, i.e. no signals.

I mapped all the voltages on the PSU.

Posted by: @crustytv

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The one thing that's bugging me are the three green wires out of the PSU board to the television tuner, Aerial signal tuner and the colour switch, these all have 0V. Again, without a cct diagram I cannot make head nor tail of this, but something is nagging me saying this does not feel right, and feels like it ties in with the fault I'm experiencing with set up, i.e. no signals.

No, I suspect that's what's called "star-point earthing". The idea is it prevents earth currents flowing that may carry undesirable stuff like RF into places you don't want it, or hum in audio equipment. If you look at the circuit of the Bush TV1 you'll see that all the earthy connections are drawn converging into a single point. McMichael actually did this in some of their 1938 radios too, such as my monster motor-tuned 382.

Posted by: @cathovisor

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No, I suspect that's what's called "star-point earthing".

Every day is a school day, I'd never heard of that. Just checked continuity and indeed they all return to chassis. 👍

Time to attempt set up again, the fuse may have gone as I was attempting the last one.

Here's an illustration of what I mean, taken from my TV1 manual - this is in the vision receiver:

Before we go delving into the 5V supply and why the fuse keeps opening, it might be instructive to see if you get a variable voltage to the ELC1043 when you alter the relevant pot from the channel selector? You should have 12V and the tuning voltage on that PCB - and is that a sort of stand-alone Philips IF unit on the board in the big screening can?

I'm just off to have a wash, but I'll be back...