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Old 5th Nov 2019, 10:01 pm   #1
MickMcmichael
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Default Marconi 4610. Thorn 850.

Several years ago myself and Rich (Slidertogrid) were offered the opportunity to 'clear some old tellies' from a ramshackle and heavily woodwormed premises up in Yorkshire. We analysed some online pictures sent by the owners who were clearing the family home following a bereavement. We had to make a rough offer for the sets based on the pictures and a brief description offered to us by the executor of the estate - the son of the bereaved.

There were other interested parties lining up if we didn't wish to go ahead with the clearance. We were able to identify a few specific models from the pictures but an offer was difficult to work out for over 30 items quoted by the estate.

Well, two days later we were in a hired long wheel base Transit and heading up the A1 armed with foldover cheese sandwiches and packets of wine gums.

We arrived at around midday. The overgrown back garden sported an American Dodge from the 1970's and several other vintage American vehicles. They were in such a bad state that not even Tim and Fuzz would have wanted to restore them!

Up three storeys were the tellies, radio's and parts. They were all stacked around the edges of the loft. Nobody dared to put any weight in the middle of the small space due to the badly rotted rafters and floor boards.

The owner, a portly chap in his forties, demonstrated the trampoline effects of jumping on the floorboards as a warning to us to take care.

It's worth mentioning at this point (as this thread is all about a Thorn 850 TV) that I'd been looking for an 850 for years and never found one. I used to buy these sets for 25p a time from a local TV shop in the mid 70's. My Dad had to fetch them in his Vauxhall Viva for me but never more than two at a time! My mate Rich had wanted a 'blue front' GEC d/s for years and never found one, but we had identified both models in the pictures. That's why we were here!

Both were easily spotted after Jumping Jack Flash had finished putting the eebie jeebies up us on the loft rafters. He went back downstairs for a bacon sandwich (or two) and left us to cart these many tellies down the several flights of stairs and to the Transit.

By 4pm we were done. Amongst many other sets, Rich was happy with his GEC blue fronter and I had THREE 850's. Lovely! We left behind several piles of wood dust which were radio's destroyed by worm. All of the highly varnished TV sets and others had escaped the creepy crawlies.

Talk about a junk bomb going off. This house had it all!

Some five years down the line and I've decided to get one of the 850's up on the bench. The G6 I'd started earlier this summer has had to go back into hibernation for the winter due to cataclysmic bodgery and innumerable headaches.

So here's one of the 850's. It's a Marconiphone 4610. Hardly ever worked on. A nice original chassis with what I call a 'proper' dual standard chassis - one with a valve UHF rotary tuner.

I've brushed out the inside and wound it up slowly on the variac.

Tube a bit poorly (a bit?...jeeezzz) and cramped frame. It will get better. Very soon.

Time is limited but updates will be forthcoming. Might have to look out my one spare heater tranny with the 20% extra tapping.

Will update again later this week.

Cheers All,

Mick.
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Old 6th Nov 2019, 1:02 am   #2
slidertogrid
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Default Re: Marconi 4610. Thorn 850.

Blimey! You got round to running one of the 850s up in the end then mate!

I can't talk... After all the time I wanted a GEC dual standard and then finally getting one it has been displayed in my dining room but has never seen power in fact I don't even remember having the back off...
Hopefully the tube will get better with a run, at least there isn't much else wrong and after all those years of disuse it's quite remarkable!
I will follow your progress with interest!
Rich.
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Old 7th Nov 2019, 9:35 pm   #3
MickMcmichael
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Default Re: Marconi 4610. Thorn 850.

It was going to be difficult to get the frame right with such a dismal looking picture. A quick PCL85 replacement showed no improvement in the frame at all. The height pot did little and the two linearity controls only made things worse.

I was convinced I had a heater tranny somewhere in the workshop with a 20% boost tapping. As usual I couldn't find what I was looking for. I know I've seen the damn thing three times over the past month but where, gawd only knows!

Plan B. The B&K!

As you can see, the emission was right down. I only tickled it with a clean and balance which has brought it up nicely.

Back to the frame. The 680k in series with the height pot was up to 1.5m; the cathode resistor had doubled in value and I replaced the cathode decoupler too. The result is more frame but the two linearity controls still don't do a lot - but more than they did!

There's a black Hunts and a blue Dubilier in the linearity circuits so they will be next.
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Old 8th Nov 2019, 1:42 pm   #4
slidertogrid
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Default Re: Marconi 4610. Thorn 850.

That's certainly an improvement Mick, A few capacitor replacements should see it right. How does the LOPT look? Any signs of distress? maybe a few short runs would be best at first just to allow any moisture to escape...
Is the UHF tuner a valve one ?

Lets hope the tube holds up, I have always been a bit cautious of commercially built tube reactivators but as you say you only gave it a gentle bop.

I had a Videocircuits booster years back and it was always a bit aggressive. It was very easy to make the emission worse. I got rid and built a "bulb bopper" from one of the Television magazine articles, that worked very well on mono and delta colour tubes.
A few years back I built another one. I have only used it once on a 20" mono tube, it did the trick and years later the tube is still holding up!


Rich
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Old 8th Nov 2019, 2:38 pm   #5
FERNSEH
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Default Re: Marconi 4610. Thorn 850.

Hi Mick,
Is that a transistor UHF tuner? Yours is the later version of the Thorn dual standard chassis. In the first versions the output from the UHF tuner was sent straight to the IF board, the result vas very poor performance on 625. With the 850 the signals from the UHF tuner are further amplified by selecting a position on the VHF tuner. Only disadvantage if my memory serves me was that you have the ritual of selecting to the 625 line standard by the switch on the cabinet left side and then selecting "UHF" on the VHF tuner. Customers didn't like that very much.
Actually for the early dual standards sets a little transistor IF amplifier was made available from the BRC service department. Plugged into socket SKT304.
Watch for bad capacitors in the frame timebase. These are plastic cased components but they still can go leaky.
Nice compact cabinet.

DFWB.
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Old 8th Nov 2019, 3:32 pm   #6
Heatercathodeshort
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Default Re: Marconi 4610. Thorn 850.

The grey .01uf behind the lin pot will cause that bad linearity but I suspect most of the caps in the frame will be either leaky or double the stated capacitance. Check also the 22k resistor behind the 6/30L2. J.
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Old 8th Nov 2019, 4:28 pm   #7
MickMcmichael
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Default Re: Marconi 4610. Thorn 850.

Yes it's definitely a valve UHF tuner. The aerial socket for the UHF is broken so for the moment I've been on 405 only. The 625 raster is up and present when you operate the system switch on the side of the cabinet.

I shall do a few cap changes later and see how it goes. I know from doing a super little 19" 950 a few years ago that most of those blue Dubiliers will be pretty poor.
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Old 8th Nov 2019, 9:22 pm   #8
MickMcmichael
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Default Re: Marconi 4610. Thorn 850.

Getting better now!

I replaced C119 (small black Hunts .01) and C118 (blue Dubilier .03) and all frame controls are now fully functional. Contrast and brightness controls both work but could do with a bit more variation than they're giving at present. This will probably improve when I replace a few of those grumpy looking old caps on the IF board. The other thing that will help is finding the correct attenuator to use on the Domino for this particular set. There's a lot of signal.

Going back a couple of posts, I notice that there is a U position on the 405 tuner. It doesn't switch standards as there's a 405/625 system switch on the left hand side of the cabinet.

What a palaver eh! Back in the day, to get BBC2 and look at the lovely Victoria in The High Chapparal, you had to switch to 625 on the left hand side and then put the 405 tuner to U and then fine tune the rotary 625 tuner.

Happy days.

More soon.
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