Microvitec Monitor 1451MS4
BBC Microcomputer TELETEXT Project
Viewdata, Prestel, Philips
Philips Model Identification
1976/77 Rank Arena AC6333 – Worlds First Teletext Receiver
PYE 1980s Brochure
Ceefax (Teletext) Turns 50
Philips 1980s KT3 – K30 Range Brochure
Zanussi Television Brochure 1982
Ferguson Videostar Review
She soon put that down
1983 Sanyo Brochure
Wireless World Teletext Decoder
Unitra Brochure
Rediffusion CITAC (MK4A)
Thorn TRUMPS 2
Grundig Brochure 1984
The Obscure and missing Continental
G11 Television 1978 – 1980
Reditune
Hitachi VIP201P C.E.D Player
Thorn 3D01 – VHD VideoDisc Player
Granada Television Brochure, 1970s
Long Gone UK TV Shops
Memories of a Derwent Field Service Engineer
PYE Australia Circa 1971
Radios-TV VRAT
Fabulous Fablon
Thorn TX10 Chassis
Crusty-TV Museum, Analogue TV Network
Philips N1500 Warning!
Rumbelows
Thorn EMI Advertising
Thorn’s Guide to Servicing a VCR
Ferguson 3V24 De-Robed
Want to tell us a story?
Video Circuits V15 – Tripler Tester
Thorn Chassis Guide
Remove Teletext Lines & VCR Problems
Suggestions
Website Refresh
Colour TV Brochures
1970s Lounge Recreation
CrustyTV Vintage Television Museum
Linda Lovelace Experience
Humbars on a Sony KV2702
1972 Ultra 6713
D|E|R Service “The Best”
The one that got away
Technical information
Microvitec Monitor 1451MS4
BBC Microcomputer TELETEXT Project
Viewdata, Prestel, Philips
Philips Model Identification
1976/77 Rank Arena AC6333 – Worlds First Teletext Receiver
PYE 1980s Brochure
Ceefax (Teletext) Turns 50
Philips 1980s KT3 – K30 Range Brochure
Zanussi Television Brochure 1982
Ferguson Videostar Review
She soon put that down
1983 Sanyo Brochure
Wireless World Teletext Decoder
Unitra Brochure
Rediffusion CITAC (MK4A)
Thorn TRUMPS 2
Grundig Brochure 1984
The Obscure and missing Continental
G11 Television 1978 – 1980
Reditune
Hitachi VIP201P C.E.D Player
Thorn 3D01 – VHD VideoDisc Player
Granada Television Brochure, 1970s
Long Gone UK TV Shops
Memories of a Derwent Field Service Engineer
PYE Australia Circa 1971
Radios-TV VRAT
Fabulous Fablon
Thorn TX10 Chassis
Crusty-TV Museum, Analogue TV Network
Philips N1500 Warning!
Rumbelows
Thorn EMI Advertising
Thorn’s Guide to Servicing a VCR
Ferguson 3V24 De-Robed
Want to tell us a story?
Video Circuits V15 – Tripler Tester
Thorn Chassis Guide
Remove Teletext Lines & VCR Problems
Suggestions
Website Refresh
Colour TV Brochures
1970s Lounge Recreation
CrustyTV Vintage Television Museum
Linda Lovelace Experience
Humbars on a Sony KV2702
1972 Ultra 6713
D|E|R Service “The Best”
The one that got away
Technical information
Hi Rich, yes there's a lot of that on this set.
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Some thoughts, observations and ramblings for a very wet and windy Thursday morning.
I was nudged to remember that all I need info on the Tifax teletext interface to this receiver is documented in the RRI engineer's guide to teletext. Indeed, this is true, everything over on that huge board has a "14" prefix. However, where I have difficulty making sense is where it interfaces with the decoder, all those flying leads, not the PL1/PL2 interface to Tifax.
The decoder layout plan is not included in the engineer's book, and thus far the textual description seems not to mention them, I may have missed it so need to reread. The service manual of the Z718 just shows the standard decoder, all those additional connection posts for those flying leads do not exist. I do have a standard non text decoder, so a long-drawn-out process can be actioned to map exactly where it is all hooking up.
I removed all the wires to gain access to the decoder as I'm trying to fathom the odd intermittent fault I have around the TCA800, where I have a perfect picture but overlaid with that odd pulsation and the black bar riding down the screen.
Having first taken notes and photos before removing all the teletext flying leads, they were removed one by one, this revealed a problem on the brown wire connection on the decoder, huge corrosion! I seriously doubt whatever is connecting from the teletext board to that pin was making contact. I've also no idea why that seems to be the only post in that condition.
I followed the removed brown wire over to the teletext board, and it ends up on a post that shares a pad with pin6 of 14IC2 a "7405" ic, also on the pad is a post with a yellow wire, that directly connects to PL2/20 on the Tifax module. I've read and reread the section "Tifax Module Interfaces", and cannot find any mention of the function of 14IC2.
Looking at the back of that corroded post on the decoder, there are a couple of diodes and a resistor. Using the non teletext decoder, I should be able to figure out what that post is interfacing with.
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Just a thought on the corroded pin, could it be caused by decomposing PVC wire? I have seen it happen where just one wire starts to go, whilst all other colours are unaffected. You can normally tell when it’s going, as either the insulation starts to go crispy, or you see green goo coming out of the wire ends.
Further update:
I cleaned the plug then continuity checked it back to the teletext board, all OK. From there you will remember I said it shares a pad with 14IC2/7 and a post with a yellow wire that goes to PL2/20 on Tifax, I checked, and it has continuity all the way to Tifax. All good.
I then cleaned the badly corroded post on the decoder then checked continuity through to the print side and those diodes, all was OK. Again all good.
I then reconnected all the flying wires from the teletext board back onto the decoder, powered on and tested teletext. If you remember, from my previous test of teletext this resulted in the expected black picture, teletext page loading in full text mode, with only the header and the page number counting continuously at the top.
This time when I pressed teletext on the remote the test card just disappeared, I was just left with a blank grey screen as if the set was not on. I found this most odd and for some reason I adjusted brightness up, I could then see teletext had loaded but in mixed mode, but the test card was now black and white, I could see the teletext header at the top but garbled.
So with the connection of that wire we've moved forward but sideways, I think it also suggests that the truth table for the interface has been ignored. The following extract from the RBM Teletext guide states the functionality, and I've highlighted in red the crucial clue.
Whenever the "Picture" instruction is given, the output pin 11 of 14IC10 goes low during the 12us "Tifax" strobe pulse. Via 14D19 this takes pin 16 of 14IC5 low to "clear" the shift register; i.e. all flip-flops set to low. Thus channel selection is "enabled" whenever the "picture" instruction is given regardless of the previous state of the shift register.
When the receiver is first switched on, 14C25 charges relatively slowly through 14R2. Thus, initially, the cathode of diode 14D6 is held low, again "clearing" 14IC5 to ensure that channel changing is not arbitrarily inhibited at switch-on. The slow charge of 14C25 also clears 14UC6 at switch-on by holding pin 5 low.
14D7 discharges 14C25 at switch-off. A further switch-on function is carried out by 14D22, which is held in conduction before 14C25 charges up. 14D22 thus holds the "D" input line low initially, which is interpreted as a "Clear" input instruction to the Tifax module (see truth table below). This ensures that the Tifax module always starts up by displaying "picture" rather than "Mixed", which it might otherwise do.
Clearly my set Tifax is loading up in "mixed" mode, ignoring the "truth" table, which is one fault. However, I also seem to have another fault on top of that, in "mixed" mode it results in the TV picture being very poor (lack of brightness/contrast) and loss of colour.
Based on all the above, all I can do is check the components listed as they would appear to be critical in the function of the "Truth" table.
The normal TV side of things when stable, gives a very good account of itself now, here's the latest.
Posted by: @lloydJust a thought on the corroded pin, could it be caused by decomposing PVC wire? I have seen it happen where just one wire starts to go, whilst all other colours are unaffected. You can normally tell when it’s going, as either the insulation starts to go crispy, or you see green goo coming out of the wire ends
Interesting Lloyd, however, I checked my wire, and it's totally dry, flexible and no apparent degradation in its structure.
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Well, I spent the afternoon recapping the RBM teletext board, carefully and methodically, to not make any mistakes. Sheer desperation I accept, but it was worth a try. As expected but dearly hoping the opposite, it failed to resolve the previous error.
Also, once more the odd problem came back, as shown in the previously uploaded short video clips. I decided to remove the teletext board entirely from the set and see if it would run without it. Well, no surprise the intermittent fault that I thought related to TCA800 was gone, the picture way better too, the teletext board was the cause all along!
This proves there are not one, but two, maybe even more faults on that teletext board that are not only causing the teletext side to not function, (even if it can with the old Tifax) but also impacting the operation of the set as a normal receiver.
For now, the board will be disconnected from the TV to allow it to function, until such a time I can study and seek advice/help on figuring out the faults the teletext board is presenting. Maybe now the board is removed, I can power it via two bench supplies, then armed with a scope, meter and logic probe, see if I can fathom what's going on. It could also be a foolish endeavour due to the old Tifax board being incompatible.
This now concludes the Teletext anniversary project, with some final shots, showing the resultant test card and with a teletext set-top box connected to the TV via the IMOGen (teletext page generator) showing what might have been had its own teletext board functioned. I have to say the test card in my opinion is stunning. Now sitting beside its brother, the BC6378A (T20A) in the lower half of the museum.
Thanks to everyone for their input and following along. Now on to another project........
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As usual you have made a fantastic job of that Chris! The tube looks as good as new. It would be interesting to see if the emission readings on the B&K are now in the green...
I can't say I was that impressed with the pictures on the Z718 'back in the day'. That said the ones I saw were getting on a bit so were probably a bit past their best. The ones I looked after were fairly reliable and predictable despite their age, which is more than can be said for the early T20s !
Your set demonstrates that if the tube is good and the set adjusted correctly they are capable of very good performance. and again... Which is more than can be said for the late Varicap tuner A823AVs with its grotty AFC action and Hanover blind prone decoder!
A very well done for sticking with it!
Posted by: @slidertogridI can't say I was that impressed with the pictures on the Z718 'back in the day'. That said the ones I saw were getting on a bit so were probably a bit past their best. The ones I looked after were fairly reliable and predictable despite their age, which is more than can be said for the early T20s !
I must be lucky as my white cabinet Z718 gives a great picture as well.
My T20A also gives a good picture too, though it has now developed an EW fault, which you can see the start of in this picture.
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The T20 did give a good picture when it was working it was the reliability of early sets that was the problem. They often didn't work or work for long straight from the box. later models were better the T22 better still and the 30AX T26 was fine.
The Z718s that I looked after were early models I think as they had the mechanical tuner as used in the mono sets and the A823 early models.
The performance of Bush Murphy sets seems to have been a bit of a lottery, some sets were better than others. Some A774 sets were frankly awful, smeary pictures with dark scenes cloudy and bright scenes looking washed out. Others were better and the odd one fairly good. Same with the A823 varicap with the two chip decoder some were OK, others went off tune when you switched on the AFC (the fact that you had a little switch on the front - AFC on/off suggests that they knew it didn't work properly) Often it was best left off! Decoder performance varied and drifted the reds looking a bit purple realigning would cure the problem for a while. And again, some sets were surprisingly good!
I would guess that the crap ones are long gone and only the best were kept and survive... Maybe? I certainly did my bit in reducing the numbers!
Set looking good Chris after 40 years in my loft.
That's because they didn't have the black level clamp that the A640 chassis did - any set that uses AC coupling to the tube will do that.Some A774 sets were frankly awful, smeary pictures with dark scenes cloudy and bright scenes looking washed out.Posted by: @slidertogrid
Don't know how I missed this by Bob Fisher from May 1979 Television, on the Rank AC6333 & BC6333 Teletext receivers. The article is superb and should help greatly when I get back to trying to diagnose the two faults related to the teletext side. The TV works flawlessly without the text board installed, but when installed it causes the odd video error when in normal operation mode, and I have just a single garbled text line in text mode.
https://www.radios-tv.co.uk/rank-ac6333-bc6333-teletext-receiver/
I've now added ii to the main websites servicing the sets directory page, if you've not seen it, here it is
https://www.radios-tv.co.uk/vintage-electronics-blog-forum/servicing-the-sets/
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