Rediffusion mk4 chassis overhall
I wonder if there is any love out there for the Rediffusion Mk4 yet? Considering these sets are very nearly 40 years old they still feel surprisingly modern, well to me anyway! We used to sell loads of these in the early 90s as ex rental sets and I always enjoyed working on them. A very straightforward and common sense design and lay out. So this set was rescued recently and I could not resist seeing if it could be coaxed back on to life. Currently it's completely dead as the power supply has suffered a chain reaction failure due to a dry jointed snubber capacitor in the collector circuit of the chopper transistor. So far 9 bits replaced. I've powered the board up from a stabilised 15v supply but no output from the switch mode controller IC so suspect that is dud too. New one ordered from eBay so fingers crossed that will be the last of the psu issues.
I wonder if there is any love out there for the Rediffusion Mk4 yet?
I added one of these to my collection last year and I do like it. I just had to have one in the collection as they were made just four miles from where I live and I visit the old factory site once a week as its now a Tesco superstore. Like yours mine was also D.O.A. when I obtained it. A relatively simple fix, R2 was open on the PSU and THY1 the 2p5m Thyristor had failed. A replacement Thyristor had it up and running once again. The repair blog is here
I’ve got a Mk.4 here too! Did a thread on it a while back, when I found it in the car park of the NVCF with a note on it saying free to a home. It’s the remote controlled teletext version, and it works! Just needed a new tripler.
I've got a Mk4 as well, its a 110° 22inch, looks like the one in the photo is a 90°, same line scan panel PCB but with some components not fitted. I used to work on lots of these in the mid 90s, dry joints and burn up on the scan board was a problem, I'd always deal with PSU faults out of the set with a 60W lamp on the bench, the board could be replaced in a minute and I kept spares. The manual tuner button unit was poor, but most of the ones I worked with used the CVBS input. Very easy sets to work on with easy to understand circuitry and standard components. I'd like a 20" teletext, nice looking set.
I've finally found some time to finish the Mk4 and its come up rather nicely.
I left off last time with a power supply that was very sick. The possible culprit for the destruction was the snubber capacitor 4c27 which had clearly been dry jointed at some time. The arcing at the pin had overheated the leg going into the capacitor causing it to bulge at the point of entry. I ordered up the required resistors and transistors along with the thyristor for the start-up supply for the switched-mode IC and its associated zener diode and carefully replaced them all, about 11 parts in total. With the PSU removed from the chassis and on the bench, a stabilised 15v DC supply was then connected to 4D6 which is the diode responsible for supplying LT to the SMPS IC and chopper transistor driver circuits once the power supply has been kick-started by the thyristor. Doing this should allow an output to be scoped from the IC and through the driver stages to the chopper transistor. If the 15v is also connected to across the main reservoir capacitor the power supply should run, all be it at a low level giving nice collector pulses on the collector of the chopper transistor. I find this a handy test as everything runs at a safe level with no destruction if there is a fault.
In this case, there was no output from the IC. A replacement was sourced from Bulgaria of all places1 Once it arrived it was fitted and the PSU once more tested. Success, and nice waveform at pin 15 of the TDA1060 and nice healthy pulses on the collector of the BU326A chopper as can be seen in the attached pictures.
The power supply was refitted to the set and full power was supplied but it just squealed, but no bangs or fuse blowing thankfully. Disconnecting the input to the tripler allowed the set to power up with a healthy arc being available at the overwind of the LOPTX. A replacement tripler was fitted and now we had a raster with no red!
The tube tested excellent, so clearly a video fault, which was quickly traced to a dead spot on the wiper of the red video drive preset. I changed the presets in all 3 channels as they were all a bit noisy. with a test card displayed, a few more tweaks provided a very pleasing picture. A replacement on-off switch was also required as the original one would not stay latched in for long before switching it's self off without warning. Once that was replaced and the channel selectors cleaned up as they kept jamming on channel change due to an accumulation of gunk the set worked very nicely indeed, with a tube that looked as good as new. However the case did not look new, it was dirty and shabby looking. A thorough clean ensued removing years of dirt and grime. the plastic surround at the front edge of the case had a very worn finish. This was cleaned up and resprayed and looked so much better for it.
All in all an enjoyable renovation of what was an unloved 80s ex-rental set. It still has a blue paper label on the back cover which was applied by Granada when they too over Rediffusion Tv rental in the mid-80s with its serial number details etc. there are also some drilled holes in the back cover to allow a coin meter to be fitted. I should think coin meter rentals must have been coming o the end by this time.
Although a little modern for my usual collecting I have decided to keep this set as its only a 20" model and looks quite smart now and works superbly. Being built in June 81 it's not too far off 40 years old so I think it can be considered vintage.
Remove IC holder and solder ic direct to pcb. Official factory mod, Malc.
I didn't like the Rediffusion Mk4 very much at first - my only experience being a loan set from Granada while they got to the bottom of the startup and LTB problems of the ITT set we were renting, and it too developed a fault before the ITT set was back. However, I later lived in a students' hall of residence in Sheffield where we had one. Someone vandalised the set, but I managed to reassemble the control panel and tuner, almost perfectly, and certainly adequately for the situation. They were actually rather sound sets.
One of our hall wardens acquired a teletext equipped version of the Mk4 in 1993 - therefore we also ended up with his old GEC C2002H in the lounge - that needed a new aerial socket, and had one failed channel selector I didn't manage to fix.
"Yes, a bit of wet string may get you a good TV signal here on four channels, but you'll have to dry it out to get Channel 5!"
I used to buy loads of these from Centrevision as a big of a bargain basement set to sell on ex rental when I had the shop. Used to fly out of the door at 40 quid. Most were easy fixes. A few had poor CRTs but made good donors.
I used to think I never threw anything out, now I'm sure;
Memories of Centrevision !
I'd get £60 for the non-remote Mk4s !
The teletext sets on the other receipt were not Mk4s but 20" ITTs, these would go out at about £100 with a new remote and would need repair of the back cover where a coin meter would have been removed.