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Rediffusion MK4

 
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Rediffusion MK4

Post by Lloyd » Sun Nov 01, 2015 11:24 pm

Here's something you don't see everyday! A Rediffusion MK4, 22" colour TV with teletext and remote control. It also has an AV input on the back, with a switch hidden behind a flip down panel that also conceals the headphone socket. Unlike most sets of the time, the channel display is on-screen, not on a separate LED display. Tuning the set is done by little push buttons hidden behind another flip-down panel, which come up on screen telling you what UHF channel each preset is tuned to. All the other controls are behind this panel too, the only non-hidden control being the power switch. There are also 2 green LED's on the front panel, one to tell you the remote signal is being received, and one to tell you the set is muted. The remote control is almost the size of a brick! it uses a 9 volt battery (PP3), and has all the controls for teletext, as well as the other user pre-settable controls.

I came across this set at the NVCF, left out in the car park not far from my car, with a note on it saying 'free to a home', it didn't specify that it needed to be a good home, so I thought I'd better take drastic action, and put it in the car... It did look like it was about to rain, and chipboard doesn't fair well in the rain, better to move it out of the possible rain, or it would turn to a horrible pile of mush and ruin the car park! I did ask 2 guys nearby if they knew anything about the set being there, but they didn't know who left it! I thought I'd not be able to get any picture on it at all when I saw it, as I thought all Rediffusion sets were cable only sets that required some odd signal, but closer inspection revealed it was a UHF set, so would be fine with nothing more than a modulator.On getting it home, I was hit with the reality that there was no room in the workshop for it! Nor was there for any of the days purchases...

I opened the set up in the kitchen, just to see what was in it, to my surprise, I found the mains lead coiled up inside, with a plug fitted. I also noticed some odd green goop on one of the PCB's, and some strange white powdery stuff growing on all the little black pots. I eventually got the set to the workshop, where it lived in the doorway on it's side. I checked for shorts on the mains lead, and plugged it in. When I finally got enough courage (or should I say distance between me and the set!) I hit the switch, there was a 'POP' and a 'BEEP' from the speaker and a crackle of static, and it came to life! Once the CRT had warmed up there was some very out of focus snow on the screen. Once the freeview box was connected it was obvious there was a problem with the picture focus, much like a Bush TV22 with a low emission EY51, the picture would bloom when the brightness was turned up. With the back off I wound the focus control from end to end, but could not focus the picture. I decided the set was a dud, so left it alone for a while. Then I saw mention on UKVRRR about a MK4 with a duff tripler, causing bad focus, this all sounded familiar! I then kept an eye out for a new tripler on ebay, but never found one, mostly because I didn't really know what I was looking for... I did come across someone selling a load of Rediffusion remote controls, so I bought one, and asked if he happened to have any triplers for the same set, to my surprise, he said yes, he had one left!

With the new tripler and remote, I set about making some space to work on the set. Off came the back, unclipped the timebase panel, and removed the screws holding in the tripler. De-soldered all the wires after making sure the new tripler had the same connections, one was missing, from one end of the focus pot. It looked like the piece of wire from here had broken off long ago, there was just enough pad left at the end of the track to attach a new wire to. The new tripler was then fitted to the set, and that funny looking white powder brushed off the pots. I left the green goop in situ since it seemed fairly solid after poking it with a stick. I powered the set up whilst standing well back! This time when it came on the EHT rustle sounded a bit more violent than previously, and once the tube had warmed up a nice bright and not too out of focus picture had appeared. A quick tweak of the focus control bought a good sharp picture. The back was re-fitted and the set tested for a good few hours with no more problems.

One remaining thing to do is to find a way of testing the teletext decoder, as when text is selected it does some funny things! random stuff appears whenever the signal is interrupted, either by turning off the modulator, or selecting the test pattern. I did try to get some off a VHS tape, which I had managed previously, even though most of it was garbled, you could actually get some info from it. it would certainly be enough just to test it anyway. This TV also has an interesting party-trick, when the remote is used to turn off the TV, it operates a solenoid somewhere behind the main power switch, which unlatches it. On an old worn set like this, the power switch cap can come a little loose, and the effect of the rather violent unlatching of the power switch causes the button cap to get fired out the front of the set! I had hours of fun with this...

I haven't cleaned the set out yet, it's a bit disgusting, but since it's now being packed up ready for a house move, it'll have to wait until I get a new workshop! This will be the last TV repair I do in my current workshop!

Thanks for reading :cca

Lloyd.
Attachments
IMG_3335.jpg
As found, still with the note on it!
IMG_3338.jpg
Inside..
IMG_3340.jpg
Found a QA label, is that a date of 1982, or 83?
IMG_3531.jpg
First test, looks a bit fuzzy
IMG_3743.jpg
Just a wee bit out...
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Playing with the teletext before fitting the new tripler
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Remote and new tripler have arrived!
IMG_3765.jpg
Focus pot surgery
IMG_3767.jpg
New tripler fitted, ready for testing!
IMG_3768.jpg
Looking at the test card in the mirror after first switch on with new tripler

 
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Re: Rediffusion MK4

Post by Lloyd » Sun Nov 01, 2015 11:26 pm

One other thing I found in the set is a service report card, that looks to be dated /8/83. Looks like it never went wrong, or nobody bothered to fill out the card when it did!
Attachments
IMG_3769.jpg
service card
IMG_3770.jpg
other side

 
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Re: Rediffusion MK4

Post by crustytv » Mon Nov 02, 2015 12:53 am

Hi Lloyd,

What a great and opportune find and It certainly did find a good home with yourself. Tripler failure seem to be the common theme with a lot of early colour and obviously later sets such as your MK4.

Regarding the tripler failure, I guess this is something we are going to have to deal with more and more as time passes. Some of the early ones can be rebuilt as I did with the BRC 2000 and Baird M702, the early BRC 3000 ones can also be done. If the tripler is silicone potted then it's a simple candidate, if it's resin potted then another method needs to be found. I see no reason why we cannot then fashion our own containers, potting the diodes and caps in the correct silicone compound.

Good luck with your move, will you have a new workshop ready to use or are you going to have to start from scratch?

 
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Re: Rediffusion MK4

Post by malcscott » Mon Nov 02, 2015 1:33 am

Nice find, should come better with a few days running. Best to remove the psu and resolder all of the suspect dry joints. The large cap next to the BU326 transistor can cause major problems if dry jointed. I have one or two of these sets in my collection, the crt and tripler are about all that ever failed on them, Malc.

 
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Re: Rediffusion MK4

Post by Till Eulenspiegel » Mon Nov 02, 2015 11:01 am

There was a mention in one of the service bulletins informing service engineers to resolder the pins of the switch mode transformer. This was in response to the reported fault of failure of the line output transistor.
I had dozens of Mark 4 sets on rental, initially under the Doric name and later as Murphy.

Till Eulenspiegel.

 
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Re: Rediffusion MK4

Post by malcscott » Mon Nov 02, 2015 11:14 am

The pin that caused the problem was the one on the bottom left, it would knock out the BU208a and the frame o/p ic. Later panels had better soldering to prevent it, Malc.

 
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Re: Rediffusion MK4

Post by davegsm82 » Mon Nov 02, 2015 11:57 am

I do like the MK4's, probably because they were my Bread-and-butter for years and years.

Pretty much all of the ones I used to repair were branded 'Doric', various incarnations, but by far the post popular were the 25/26" sets with Teletext and power on the remote control. We were still selling them by the bucketful even when LCD technology came along. It wasn't just the older generation buying them either because of familiarity, people would come into the shop and see them side by side and compare them to the newer plastic cased sets which usually had inferior picture quality but a higher price tag.

Occasionally we would turn up at the shop and there would be one dumped outside the door, with or without a note attached, people used to just discard them there because they thought we could use them for parts etc. Truth be told any necessary repairs were trivial and they would soon be on sale again :aad

Common problems from what I remember were failure of the L/O transistor due to dry's on the tuning capacitors, SMPSU transistor failure from defective controller IC's (TDA4600 springs to mind, probably wrong), hundreds of tripler failures, occasional LOPTX burn up around the EHT solder tag and many more. The smell of Ozone from a damp set and the occasional 'SNAP' as the EHT arced to the Dag, those were the days.

I seem to remember some tubes were better than others on these sets, one particular style had side contacts for the pins on the tube base, these ones used to streak badly on bright scenes when nearing the end of life. Some responded reasonably well to an extra turn on the LOPT for the heater or just shorting out the resistor on the tube base or timebase panel (wasn't fitted to all models IIRC).

I do have a hankering for one myself but I don't really have the space for large sets any more :aaf

Dave.

 
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Re: Rediffusion MK4

Post by Rebel Rafter » Mon Nov 02, 2015 7:24 pm

Hi, folks, RR here. I once had a Rediffusion Mk4 set, one of the more basic models, a 20" 90 degree one. There were some with 110 degree tubes too. I gave it to my mum who had it for years without any failures. A good set it was, low energy consumption and a nice bright picture. The PSU control chip in these chassis was a TDA1060, an unusual one, an industrial one apparently. The TDA4600 was used in later Thorn TX9's like mine. The if strip in the Mk4 is a bit similar to the TX9 and 10's but with a different tuner but with the same chips and SAW filter. Later versions use a TDA2541 instead because a different tuner was used which needed AGC with the opposite polarity. I've not seen the power button popping off but then I've never had a set with remote off. It does remind me of something I once read in an article in "Television" magazine about Grundig 2x4 super VCR's (remember them?) which when they have a certain fault can apparently fire a cassette right across the room! So watch out if you have one of these. The tubes in the Mk4's were apparently S4's, weren't some of these also fitted in some Thorn TX10's, as I remember they were always shorting internally and sending the picture all green so a lot of TX10's used to get the tube base panel changed so the 30AX 540 series tube could be fitted instead. RR.

 
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Re: Rediffusion MK4

Post by Lloyd » Mon Nov 02, 2015 7:39 pm

Hi Chris,

I did think of rebuilding the tripler, it's potted in some sort of resin, but I have found with some types of epoxy resin you can soften it with heat, then it goes sort of rubbery and becomes easy to pull bits off. I'll keep the dead tripler for later experimentation!

There is no new workshop to go to yet, it will have to be built from scratch, although the work bench and shelving will all be coming with me. At the moment, everything is being packed up and sent to my Nan's house in Newark.

I shall have a look at the soldering on the PSU sometime, make sure all is good. I did accidentally leave the set on for 2 days in the workshop before I fitted the new tripler! I must have left the modulator on with no input, so the screen looked like the set was off. I only noticed when I went back to the workshop and could hear some 625 line whistle, I thought it was just me hearing things, but then went to the set to turn it on, and found it already was on :ccf

Dave, this one has the side contact CRT base, I think it's made by Videocolor. Good to hear plenty of good things about these set's!

Regards,
Lloyd.

I'll try and get a better off-screen shot from it ttt:

 
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Re: Rediffusion MK4

Post by Katie Bush » Mon Nov 02, 2015 8:00 pm

Hi Lloyd,

Teletext? No problem.. All you need is a SKY box (and a dish connected of course) and tune to "British Eurosport" (channel 410).. You can have all the teletext you need to test and demonstrate the set.. The real beauty is, you don't even need a SKY subscription, if all you want is the text.

You should find that the Teletext will decode whether fed via RF or AV...... Unless of course, the AV is inserted downstream of the text decoder in a set of this age?.. :aaq

Marion

PS..
Just as an aside, the teletext stream from British Eurosport also carries the EPG data for "Guide Plus" enabled PVRs, DVD recorders, VCRs and etceteras.. Also, can be used to set and maintain "auto setting" clocks on most VCRs that derive their time control via teletext.

 
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Re: Rediffusion MK4

Post by Cathovisor » Mon Nov 02, 2015 8:11 pm

Katie Bush wrote:You should find that the Teletext will decode whether fed via RF or AV...... Unless of course, the AV is inserted downstream of the text decoder in a set of this age?.. :aaq

It shouldn't be. Typically the decoder of choice of this era - the TDA3560 - decoded the composite video signals sent to it either from the vision detector or the AV input, and the text switching was carried out post-decoding in the RGB outputs within the chip. This allowed the "mix" mode to function.

Incidentally, the only set I ever saw where the mix mode was legible was my Mum's Salora: rather than just reduce the overall picture contrast and insert the text, the Salora seemingly increased the amplitude of the text signal which made it 'punch through' the background far better.

 
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Re: Rediffusion MK4

Post by Colourstar » Mon Nov 02, 2015 9:03 pm

We had a shiny new Mk4 as our skool telly sometime around 1984, when it replaced a Mk3 whose most memorable feature was to produce a sickly bright green picture at switch on, taking several seconds for things to calm down to a more natural balance. Perhaps they all did that, sir.

As to the Mk4, in 1982 Television described at as 'technically one of the most advanced chassis to have gone into production in Europe so far. Sets using the chassis are marketed in Rediffusion shops as 'TVRMs' - Television Receiver Monitors - since all versions being produced at present are fitted with video and audio input sockets for direct video connection to VCRs, video games, computers etc'

Steve
Attachments
Rediffusion.jpg

 
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Re: Rediffusion MK4

Post by Till Eulenspiegel » Mon Nov 02, 2015 9:08 pm

Rebel Rafter wrote: The tubes in the Mk4's were apparently S4's, weren't some of these also fitted in some Thorn TX10's, as I remember they were always shorting internally and sending the picture all green so a lot of TX10's used to get the tube base panel changed so the 30AX 540 series tube could be fitted instead. RR.


After Thorn Consumer Electronics was sold to Thomson in 1987 to form a new company called Ferguson Limited it was inevitable that Videocolor CRTs would be the first choice for the new firm. Those Videocolor tubes used S4 technology.
However, didn't Rediffusion source their S4 tubes from RCA? An American company which was in due course acquired by Thomson.
The Mark 4 colour decoder, was it the Motorola TDA3300?

Till Eulenspiegel.

 
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Re: Rediffusion MK4

Post by Rebel Rafter » Mon Nov 02, 2015 10:27 pm

Hi, from RR. The Mk4 colour decoder was indeed the TDA3300B and some versions apparently had a small sub panel with a TDA3030A for SECAM. The TDA3300B was chosen as it allows contrast and brightness adjustment of the RGB inputs for teletext which as I've already discovered with fitting SCART to my TX9 is not possible with the old bog standard TDA3560. RR.

 
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Re: Rediffusion MK4

Post by malcscott » Tue Nov 03, 2015 1:35 am

The first Mk4 sets did use RCA tubes which in my opinion were much better than the Videocolour S4 offerings.

 
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Re: Rediffusion MK4

Post by Lloyd » Tue Nov 03, 2015 8:50 am

I wish the chassis in my MK4 was as clean and shiny as the one on the front of that Television mag!

I'll have to get hold of a Sky box and dish, I did get given a Sky + box once, but I sold it as I had no dish!

As promised, some better off-screen shots of the set!

Regards,
Lloyd.
Attachments
IMG_2425.jpg
Close up
IMG_2427.jpg

 
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Re: Rediffusion MK4

Post by Doz » Tue Nov 03, 2015 7:53 pm

malcscott wrote:The first Mk4 sets did use RCA tubes which in my opinion were much better than the Videocolour S4 offerings.


There was a definite "knack" to bumping those videocolour tubes. I had a simple G8 mains TX, a diode and a pigmy bulb for these. The B&K would bop them, but they'd fade away quickly.

Hooking up the bulb-bopper, using microhooks to the base, and tapping the cathode connection against the relevant cathode briefly, you'd see a spark from the gun undergoing this abuse. This was repeated until there was a bigger spark. Never go past this point! Worked every time, and the tube would go on for years after. This process was also used with great success on the similar (if not identical) ITT tubes of the same era. No more nasty focus bars on contrasty bits!

 
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Re: Rediffusion MK4

Post by Katie Bush » Wed Nov 04, 2015 12:29 pm

We had a bulb bopper - built from a design in 'Television' way back in the late 70's, and it got a lot of use too, but you could just as easily destroy a tube as you could revive it.

I can recall a large(ish) transformer, dropper section, diode, a fair sized capacitor, resistor and bulb, plus a couple of swithces for mains on, and heater boost (for stubborn CRTs that wouldn't start bopping), all buit into (of all things) a budgie nesting box!

Definitely killed a few tired CRTs in G6s, and made mincemeat of a Thorn 3000 (my own set!) - but mono CRTs seemed to survive quite well... :bba

Marion


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