Marconiphone 4714 (Thorn 3500)

[Tazman1966]

Hello all!

A project that’s been gathering a thick layer of dust has finally seen the light of day. It is a 22” Marconiphone 4714 fitted with the Thorn 3500 chassis and not too dissimilar to the one in Marcus’s thread (click here) although mine has the mechanical tuner fitted instead of the varicap.

First of all, it was pleasing to note that the cabinet was in pretty good condition and although not the luxurious (!) real wood finish of Marcus’s HMV, it is still wood – erm chipboard - with a dark wood veneer.

I first tried it out in Mikey405’s place from whence it was donated and although the tube heaters lit, not much else happened. I was going to take a few measurements such as the collector volts on the chopper transistor etc and also for some reason decided to plug the aerial in to the set. All of a sudden the house was plunged into darkness! Hmmm, I thought! There’s more to this than meets the eye. The RCD had tripped. That was as far as I got then and I brought the set home with me to St Albans.

A few days later I had the time to have a look at it again and after fixing the RCD tripping fault by re-making the aerial isolator with discrete components as it had been connected direct at some point by a previous repairer (no NOT Mike) I did some voltage checks and I soon established that the 12V rail was low and was also aware that R607 (100R) that supplies it was getting a bit on the hot side. Studying the diagram I suspected that the driver transistor VT605 or the diode in its emitter was leaky or short but this wasn’t the case. It was the deep mid winter at the time and the workshop/garage was cold so I abandoned the set and waited for warmer months (what a wimp!). I brought the line timebase panel inside to make sure it didn’t get damp though and it lived at the back of the boiler cupboard for the next couple of months or so.

Spurred on by the thought of all those lovely old tellies “up North” and having an overnight stay in “Hotel Bennett”, Mike suggested bringing the set up to his place and we (Mike, Lee and myself) could work on it which we did. There was a flaw in this plan however as being the air head that I am at times, I left the line timebase panel at home in the cupboard, doh! Mike had a nice 3000 chassis to try my PSU out in so that was okay then!

Studying the diagram again showed that there was a small decoupler, C608 (0.22uF) from one end of R607 but that was fine. Mike pointed out the smoothing cap, C607 (1000uF) for the 12V and 30V rails. It looked absolutely fine but a ‘scope check on the supply rail showed an incredible ripple and upon checking, it was found to be open circuit. He just happened to have an exact replacement in his extensive supplies and in a trice it was fitted and we had full PSU operation. I checked my decoder panel out in his set as well as his had no chroma and on firing up his set, it promptly shut down and a wisp of smoke appeared from R350 (12R) on the decoder. It is through this resistor that the 30V is supplied to that panel and it didn’t take long to find that the 4uF tantalum capacitor that decouples it was dead short. A nice shiny new 4.7uF standard electrolytic and we were in action.

When I came back to St Albans on the Sunday, I re-assembled my set and fired it up. Oh dear what a mess! The picture, what there was of it was awful, dark, hardly any green, brighter one side of the screen than the other, unlocked chroma, bad purity and convergence brightness control not working and to top it all, the tripler was hissing and fizzing like mad and lines were moving over the top of the picture in time with the hissing.

First and foremost the tripler was replaced with a nice new one supplied by Glyn (Welsh Anorak). This put a stop to the hissing and fizzing! The brighter one side of the picture than the other was traced to L205 which supplies the line pulse to the panel. One of the wires had come off its post. C221 (1uF) was also replaced in that part of the circuit as it had leaked.

The picture was a bit better but still grim. The tube looked quite poorly but I knew that it wasn’t as I’d already tested it and it was in fact, a goody! The known stock fault for this of course is leaky A1 switches on the convergence panel and sure enough, all three were leaky especially the green, just like Marcus’s one! These were whipped out and wire links replaced them. We now had full brightness and a half decent picture. The brightness control was still not working though and after some fruitless searching, I discovered that I’d forgotten to plug in the little edge connector on the beam limiter board. With that sorted out, the brightness was adjustable.

Time to run through the whole purity and convergence procedure but before I could do that, the screen went blue! This was traced to R277 (12k wirewound) the blue output transistor’s load resistor going open. On my set it is part of a thick film resistor unit but I replaced it with a 7W wirewound and cut the print to the thick film unit in case that repaired itself.

Once the purity and convergence was set up I had a pretty good monochrome picture and a tweak on the reference oscillator R315 and locked colour came flooding back. A few other adjustments and the set was now looking good.

I then tried to tune in the other buttons and found that the BBC2 button wouldn’t latch properly and when it did, it wouldn’t release. I knew exactly what it would be. The plastic collars on the tuner. I did my usual “Rawl Plug” mod and all was well. A final check on the HT voltage which I reduced from 65V to 59V for greater reliability (the range in the manual is 58V to 65V) and the electrical work was complete. A thorough clean to remove years of nicotine and the set was looking as good off as it was on!

It made the grade to take pride of place on the sideboard in the kitchen/diner but the story doesn’t end there…

Whilst we were watching Eastenders – Sara’s choice, not mine – there was a click and the set switched itself off. The cut-out had done just that! I re-set it but it popped out immediately. It was removed to the workshop again until today when I discovered that the chopper supply diode, W602 (BY127) was dead short. I replaced it with a hefty 1N5404 diode. That won’t fail in a hurry! Another tickle up with the convergence as I wasn’t completely satisfied and all was well again. There is a small amount of tuner drift when you first switch on but other than that, for a television at 35 years of age I’m well pleased with it.

Sorry that the tale was so long but I think there were some useful faults found for anyone else who fancies having a go at one of these. Surely there must be a good few still about as I believe they were one of the most common sets of the very early seventies.

Here’s some pictures of the inside…

[Tazman1966]

...and here's some of the outside with the set working.

Hope you like it as much as me and thanks once again to Mike and Lee for their help earlier on.

Anyone else got any Thorn 3000/3500 sets to share stories?

Cheers,

[neil29]

Hi tas. that is a nice set. and it has a very good picture too. i have a 26" ferguson version which did work ok before its journey home. but since then its decided it did not want to co-operate with me. and all i can get is a very grainy/non existant picture. and to top it off it weighs a ton so manouvering it about isnt easy either. cheers neil.

[Dr-Watts]

Originally Posted by Tazman1966

for some reason decided to plug the aerial in to the set. All of a sudden the house was plunged into darkness !

Ha ha ha, We've all be there Tas !
Great telly mate, well done

Dr. Dave

[chipp1968]

Its looking good .I like the cigar buttons

[Hybrid tellies]

Wow this looks a very good example of a 3500, it brings back a few memories of happier days. Has it had a regunned tube fitted at some time?
You need to check for a wirewound resistor on the convergance board which goes open cct and causes a burn up that can damage the panel itself. This resistor should be replace by a higher watttage one of the same value. I cannot remember off hand the number but will try and look it up over the next few days. Unless of course someone beats me to it.

[Baggrus]

Nice.. I think we tend to forget how good these 3500's were. I know of at least 2 which are still working. The only real weakness seemed to be the 5th harmonic cap on the lopt, and setting the sit-up in the field stage with no scope... Broke a couple of those little skeleton pots in my time.

I lived with a salvaged one for quite a while (ultra badged), wish I had thought to reduce the HT a little.. One night watching The Prisoner (1992-3 ish) a huge crack from somewhere inside and the cut out popped out.. Power off and reset.. gingerly power up and nothing amiss.. hmmmmm.. bit of a big bang for nothing to happen.. No inclination to investigate further due to being rather damp from the full pint of homebrew that I had been just looking forward to lubricating my tonsils with...
A few days later flyback lines traced to A1 resistor burning up.. and a rather large and very fried spider welded to the focus spark gap.. big bang cause found.

Nice set, good tube.. should be good to go for another 35 years These are one of those sets, they either go on and on and on, or are a right royal pain...

[captain_70s]

I must say its a very nice looking set in my opinion.

Its a shame I could only buy a mint condition one as I can't fix the things!

[Mikey405]

Well done Tas, that looks fantastic!

[Hunts smoothing bomb]

Nice one Tas, that tube does indeed look a corker!
A few faults sorted there, you should be dead pleased!

Cheers
Lee

[Welsh Anorak]

Well, that looks very nice, obviously due to the extremely high quality tripler installed.
Not often you see the Marconi version of these - quite a pretty set. I must pester someone who has a 26" Fergy (I think) in their house which I hope is still there for me! Would certainly make a change from this **** Philips plasma I'm taking a break from.....
Well done as usual....
Glyn
PS Years ago I remember having major problems on a PSU on one of these - not unusual, I hear you cry! I phoned Thorn and had a very long discussion with an engineer about the PSU, the design, and how every stage worked and why. I congratulated him on his knowledge of the product - turned out he was head of the 3000 design team who had just happened to answer the phone....

[Tazman1966]

Thanks for the replies guys

Neil - check the aerial socket first. It doesn't take much to yank the wire off the isolator at the back.

Dave - LOL!

Paul - What do you mean "cigar buttons"?

Simon - I can't find a label on the tube at all but I assume it's a re-gun. It was certainly pretty clean and therefore I would imagine a replacement considering the rest of the set was nicotine yellow

Bagruss - Nice story! How 'bout some pics of the two you have working?

Captain - Start with a mono set first and you'll eventually graduate to colour.

Mike - Thanks for the donation in the first place - well I suppose I should thanks Sid as well! Your collection of 3000's is as good as well though don't forget.

Lee - cheers mate! And thanks for the encouragement.

Glyn - Thanks for the tripler! Unless we have a heatwave (very unlikely) and I run the set 12 hours a day, that tripler should last for ever. You know you want to dig out that 26" version.

To all though: The tuner does drift quite markedly in the first 10 minutes or so. So much so that the pictire is B&W to begin with unless you adjust the tuning but then it drifts so that there is no picture. It drifts up frequency not down and once it's been on for 10 minutes it's stable. Any ideas? I suppose I need to get a can of freezer

Look forward to your thoughts.

Cheers,

[chipp1968]

Cigar Buttons Ive always called them that , really cant rememeber where i heard it from . I mean the type of channel buttons on this period of set which have the long cigar shaped clunky arrangement, useually about 4 in a row .Very distinctive of these sets . At least that what they look like in your pics.

[brunel]

To all though: The tuner does drift quite markedly in the first 10 minutes or so.

If you remove the cover on the tuner, plastic lid type?? you should see the ganged tuning cap, the rotor is earthed where it passes through the sections by silver plated flat springs that will have a layer of solidified grease on.

Clean off the grease using a small brush that has has the bristles cut down.

Re-lube with a spot of fresh grease.

Make sure your cleaning fluid is removed from between the vanes with a good shake of the tuner.

If you are really enthusiastic you could try removing the springs to clean.

[Mike Phelan]

This used to be a stock fault in all Thorn UHF tuners, even when they were comparatively new; we thought that the earthing springs weren't tensioned sufficiently, and removed and washed them, followed by curving them a little more.

When greasing the springs and gang, use Vaseline.

[gec2110]

Hey what a brilliant example it looks so good
the bbc 1 testcard looks great

[repairman 1234]

Hi Tas and all,

What a super job!

A great looking set and a brill write up too.

Another set saved.


Chris.

[Steve_P]

I've only been away a week! Superb set as always there. These sets are sometimes given the brush off because of that power supply, but it's easy really.

Try Mike's trick for the tuning drift, and if it does not work come back and I'll get the circuit out!

Cheers,

Steve P.

[Tazman1966]

Thanks guys for the suggestions about the tuner. I'll hopefully have a bit of time on Sunday to check that out.

And Chris, GEC and Steve, it IS as good as it looks in the pictures. I'd forgotten how hot they run though

Cheers,

[Steve_P]

Not original, but I have come across a couple with PC fans on the back to keep the PSU cool. Sometimes, originality comes second to problem avoidance.

Those PSU's can be sods when the transistor fails...

Cheers,

Steve P.