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Old 3rd Feb 2018, 12:34 pm   #1
1100 man
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Default Marconiphone VT161 17" TV

Good morning,
This TV has had a rough time of it over the past 20 years mainly due to being owned by me! It's been dragged from pillar to post, moved in various vehicles, had stuff piled on top of it, stored in a woodworm infested barn and finally a very damp loft. However, it's never been mortally wounded but has collected a few more battle scars!
It belonged to a friends' Grandmother and spent it's life in Felixstowe, so technically I am only the second owner!

20 years ago, 'getting it working' was more my thing rather than overhauling it with any thought of long term reliability. All sorts of odd ball components have been tacked in, most of the originals are still there with one wire cut. It did get to the point of producing very good pictures and was used a fair bit.

Back then, a standards converter was a distant dream: they were available but expensive. A friend, who was an electronics designer, had converted a 525 line B/W camera (1 inch tube) and a Sharp monitor to work on 405. The camera, pointed at a 625 line picture, produced remarkably good 405 pictures. A channel 1 modulator was built 'rats nest' style and a usable system created.

A few years ago, I decided that an Aurora was too good an opportunity to miss at 200, so I bought one. Without thinking, I dragged the Marconiphone out of the loft and fired it up. The poor thing had been up in the damp for 5 years and being powered up was too much for it: it worked for a few minutes and then died. I should have remembered from my years delivering TV's & VCR's in the winter that powering them up was a disaster unless they had warmed up first.

There was not much activity from the line stage, so I disconnected the EHT overwind and could then get a healthy spark from the PL81 top cap. I concluded, wrongly, that the overwind had developed shorted turns. I couldn't justify spending the money on getting it re wound, so it was put to one side with thoughts of a LOPT transplant if something suitable should come up.

Last year, I was experimenting with 'ringing' LOPT's and looking at the results on a 'scope. Confidently expecting the one from this set to be dead, I was amazed to see a perfect ring trace! Putting one turn of shorted wire round the transformer, killed the ring completely, so the transformer must be OK electrically.

So last month, I put the transformer back in the chassis and powered it up. The transformers in these sets seem to go through a whole series of wheezing, gasping and generally strange noises before settling down to a steady whistle. Just as it was getting into it's stride, a small plume of smoke and an arcing sound occurred. This seemed to be coming from the inboard end of the overwind near the core.

As detailed here https://www.vintage-radio.net/forum/...=142919&page=2 post 33, the wax was removed from the overwind and the former removed. A charred area was found which was conductive so the winding had been arcing through the former to the ferrite core.

This was cleaned up and temporarily re assembled to prove the point. Amazingly, it all worked perfectly and a really good picture resulted. I had forgotten how good the picture was on this set- the Mullard tube is in great shape with pin sharp focus.

I think the set now needs the TLC it deserves. Not a total rebuild, but a sympathetic change of necessary components and all the odd capacitors that have found their way in there. I can also do a proper repair & re insulate on the LOPT.

All the best, Nick.
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Old 3rd Feb 2018, 9:20 pm   #2
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Default Re: Marconiphone VT161 17" TV

Hi Nick,
The Marconi VT161 employs an unique Thorn hand wired chassis in which the metalwork was used in certain export TV receivers.
Link to the restoration of a VT161 by HKS: https://www.vintage-radio.net/forum/...i+VT161&page=1
My contribution to the topic can be found in page 2. Post No. 24 shows the replacement mains dropper resistor. A slab like component used in later Thorn models.

DFWB.
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Old 3rd Feb 2018, 11:28 pm   #3
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Default Re: Marconiphone VT161 17" TV

Hi David,
I found & read your thread last week when I started experimenting with the repaired LOPT. It's praise indeed that John rates it as one of the best chassis ever produced! I must say that the picture I saw when I was testing it was amazing- very sharp & contrasty. It will be even better when the dodgy caps in the frame stage have been replaced and the set given a general going over. I seem to remember I had a 'breathing' picture on it years ago, so I will investigate the main cap for leakage & value.

Such a small mains dropper: nothing like the immense amount of volts & heat that has to be got rid of in my Pilot!! It has a much higher valve count and a valve rectifier so much less heater dropping required. The Pilot is pared down to the bone when it comes to valves used!
I suspect, being 90 degree, that the HT current will also be less than the 110 degree Pilot. I will measure it just out of interest.
Right, time to pull the LOPT apart again and start the final repair/ re- insulation process. This will take a week or so by the time the varnish dries.
I would be very interested to know what our LOPT rewinders (MB & ED) use to insulate the final overwind.
I am really looking forward to getting this set running perfectly as I know it is capable of such good results.
All the best
Nick
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Old 4th Feb 2018, 12:40 pm   #4
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Default Re: Marconiphone VT161 17" TV

"I am really looking forward to getting this set running perfectly as I know it is capable of such good results."
Hi Nick. The VT161 is fitted with the best domestic monochrome CRT of all time, the Mullard AW43-80.
Actually the VT161 was not a cheap set to make. A similar looking set was supplied to British Relay Wireless. A year later a Ferguson hand wired TV set was made which employed very similar metalwork, the model 505T, a hi-spec fringe area receiver.

DFWB.
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Old 4th Feb 2018, 1:12 pm   #5
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Default Re: Marconiphone VT161 17" TV

The test card need a little re centering but it gives some idea of the quality.

Many of these VT161s and the BRW BR217s required very little servicing throughout a long service life. Maybe a PCC84 in the tuner, a PL81 LOP valve or more likely a PY32 HT rectifier. That was about it.

An almost identical circuit to the later Ferguson printed circuit 500 series 110 degree models. I suspect that these high quality hand wired 'specials' were produced at the Hainault factory.

Nice to see a few have survived. John.
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Old 7th Feb 2018, 12:49 am   #6
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Default Re: Marconiphone VT161 17" TV

Good evening,
I have now repaired the charred hole in the EHT overwind's former with a small drop of Araldite. After removing all the remaining traces of carbon, the former was replaced inside the overwind and the process of building up a dozen or so coats of insulating varnish began.

While this was happening, I turned my attention to the chassis and the motley collection of components that had found their way in there.
I must say, from an access point of view, it's a dream to work on. The top panel, which contains all the signals circuitry, hinges up & clips in position. The side panel with the controls & Fireball tuner, hinges outwards: they have even marked the screws to remove with arrows cut out of the metalwork. The whole thing, complete with tube, sits nicely on its side, giving excellent access to the underside wiring. All very nicely thought out: it's a bit ironic that a set with such good access rarely seemed to need fixing!!

Have a look at the first picture below. On top of the Mazda valve box is a grey object made of thick card and of a strange shape. This was taped to the top of the chassis near the speaker transformer. Does it have anything to do with this set and if so what? It looks like some sort of special tool.

Whilst dismantling the LOPT, I discovered that it's windings were not exactly as per the VT161 circuit (in R&TV servicing). It seemed to have an extra winding and was more similar to the VT157/ VT160 from the same book. The scan coupling cap was a 1.0mfd and the tuning cap a 800pf, which are also the same as the VT157/60.
Interestingly, the VT157/60, uses a very similar circuit but a slightly different valve line up. It uses a PL36 rather than the tiny PL81 but the entire thing is on two PCB's. Rather a brave move I would think!
Being in the same R&TV book, is it reasonable to assume that both chassis were in production simultaneously?
My VT161 has a large hole for a PL36 base but a plate with a PL81 valve holder has been spot welded over the top. I imagine the original design used a PL36.
All the best
Nick
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Old 7th Feb 2018, 2:03 am   #7
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Default Re: Marconiphone VT161 17" TV

Hi Nick,
The tool shown in the first picture is a template and is used for positioning the line linearity sleeve. Information from the servicing notes for the Ferguson 406T:
Line Linearity Correction A closed loop, magnetically coupled to the deflector coils and critically positioned in relation to them, is fitted to the picture tube neck. If a new tube is fitted, the loop must be transferred to it from the old one. To assist positioning the loop, a template is provided and will be found taped to the top plate of the chassis assembly. fig 1 illustrates the use of this template.
It's most likely the VT161 employs a similar scanning assembly as the 406T, the Marconi version of the 406T was the VT157.
With regard to the line output transformer, the addition winding is used to supply the reference pulse to the flywheel sync circuit incorporated in special fringe area receivers. Not used in the VT161.

DFWB.
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Old 7th Feb 2018, 11:25 am   #8
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Default Re: Marconiphone VT161 17" TV

That's very helpful, David. Many thanks. Amazing that's stayed taped to the top of the chassis for nearly 60 years!
Is the linearity sleeve supposed to be adjustable in service or is it just pre set when the scan coils are fitted? Looking at the yoke assembly, I can't see the linearity sleeve.
Am I correct in thinking that the adjustable bit with the knob, immediately behind the coils, is for picture position?
Also is the ring just next to the tube base an ion trap magnet?
In fact, thinking about it, what focus adjustment does this set have? Good grief, so many things I didn't know that I didn't know about this set & tube!!
Many thanks
Nick
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Old 7th Feb 2018, 12:09 pm   #9
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Default Re: Marconiphone VT161 17" TV

Hi Nick,
According to the circuit diagram it would appear that there is no focus control is fitted in the VT161. The CRT focus electrode connection on the tube base is pin 6. The focus voltage comes from a potential divider across the boost HT supply, look for resistors R82 and R83, both 2.2Mohms.
It's possible some later production sets do have a focus control. Trace back the wire to pin 6 of the CRT to confirm if that is so.

Again, according to the service information that the linearity sleeve is once fixed in to position by using the template on the CRT neck requires no further adjustment. Assuming that it is fitted in your set. I often came across sets in which the linearity sleeve had be removed, possibly to increase picture width or a previous repairer didn't know what is was used for so simply discarded it.

DFWB.
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Old 7th Feb 2018, 6:40 pm   #10
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Default Re: Marconiphone VT161 17" TV

If you push it in too far it can cook the scan coils.

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Old 7th Feb 2018, 6:56 pm   #11
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Default Re: Marconiphone VT161 17" TV

Hi David & John,
Yes, the focus circuit is as you describe with no adjustable control. In the above picture, is the ion trap magnet the ring at the extreme end of the neck next to the base? If it isn't, what is that bit?
Many thanks
Nick
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Old 7th Feb 2018, 7:00 pm   #12
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Default Re: Marconiphone VT161 17" TV

Yes it is Nick. Adjustment is critical. Move it slightly from side to side with a slight twist at the same time for maximum brightness. Do not use it to position the picture or improve the focus. Maximum brightness only. J.
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Old 10th Feb 2018, 11:23 pm   #13
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Default Re: Marconiphone VT161 17" TV

Well, I have to confess to being very naughty today Instead of spending the day working on the bungalow, I ended up just staying indoors playing with televisions: well it was raining!

During the last few days I've been building up 15 coats of transformer varnish on the repaired EHT overwind. I didn't do anything at all to the primary, working on the basis that if it ever does need repair it will be easier to take apart if it's not varnished!

I've tidied up the rest of the chassis, component wise & replaced the motley collection of caps. Strangely, the remaining black Hunts caps, usually notorious for being leaky, were in pretty good shape with no cracks in their cases! They tested OK for leakage & capacitance. I replaced some of the more critical ones, but left a few in circuit, notably the scan coil coupling cap which seems to work fine!!
I put the transformer back together temporarily for testing & once again sat it on the bench on long leads.
It all worked fine and a pretty good picture was obtained. Now the caps in the frame timebase have been replaced & the pots cleaned, the scan is now nice and linear.

The line sync is a bit tetchy and is very critical on signal level. It's very prone to line tearing and the setting of the hold control is critical. I wasn't too fussed at this stage as my focus was on the LOPT. I'll investigate that issue another day!
After it had been on for an hour, the transformer was mildly warm but not hot so that was good.
However, I could hear a gentle corona discharge which tended to get worse the longer it had been on. After some very prolonged investigation and probing, I discovered if the transformer core was left floating, all the discharge stopped. Reconnect the earth lead and the sizzling started again. I wonder why they earth the cores of LOPT's- it must put more stress on the insulation?

Anyway, using a plastic 'listening tube', the discharge seems to be between the former of the overwind and the brass clamping rod. There is a distinct 'hiss' at the top & bottom of the former. I wonder, therefore, if the former which was the original cause of trouble, is still faulty?
I shall take it apart again and investigate further. I might have to replace the former completely.
Still, I'm very pleased that the LOPT works as well as it does, especially as I had condemned it as needing rewinding!
All the best
Nick
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Old 17th Feb 2018, 1:28 am   #14
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Default Re: Marconiphone VT161 17" TV

All I seem to have done for the last week is take the LOPT apart and put it back together again:- I've lost track of how many times now!! Mind you, the wires are just tacked on to the terminals and the whole thing sat on the bench and connected with long wires to the set.
I suspected that the EHT leakage was due to the insulating sleeve between the overwind and the ferrite core. This was the original cause of the breakdown and I had repaired the burnt hole in it. I removed this sleeve from the overwind again and applied some varnish to the inside surface which is against the ferrite core.
Put the transformer back together again and it was much better, but not cured completely. I decided that the sleeve would have to be replaced, but it measured 17.5mm external diameter by 14mm internal. The nearest I could find on the 'net was 18mm which would need turning down- all too much trouble!

I made a temporary sleeve from some thin polycarbonate sheet, coiled up so it was several layers thick. Its natural springiness made it a very tight fit in the overwind.
That seemed to cure the problem completely, so I made a more permanent version and after a few more tests, finally reassembled the transformer properly and reinstalled it in the set.

I've got used to the variety of strange noises that the transformer emits as the valves warm up, but this time it just sat there with a rather feeble 1 Khz whine. After a few more seconds, the PL81 started to glow red. I switched off.
Why is it things that have worked for hours as a lash up, fail to work when you put them back together properly?!!

After a quick investigation, it turned out that the primary had gone open circuit between the PY81 connection and the rest of the winding This turned out to be one of the fine lead out wires broken inside the sleeving. Luckily, it had left me an 1/8 inch stub to connect to which was very civilised of it, I thought!
I effected a repair and the thing is working happily again connected by long wires. Whether it will still work when I re- install it again, only time will tell!
Another bonus is that the EHT has gone up by about 1KV.

All the best
Nick
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Old 17th Feb 2018, 4:34 am   #15
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Default Re: Marconiphone VT161 17" TV

The formers of transformers often go slightly conductive, some are only card, some soft paxolin. I have found that switch cleaner soaks in to them and makes matters worse.
The switches in Bush VHF radios do the same, the HT leaks across. I have stopped using switch cleaner as an aerosol, preferring to use it on cotton buds and just clean the contacts.
It could be that the solvent from the varnish did the same?
Nice fix, hope it gives good service after all the work.
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Old 18th Feb 2018, 9:43 pm   #16
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Default Re: Marconiphone VT161 17" TV

Hi Nick

I thought you might be interested in seeing some scans from the original Marconi sales leaflet featuring the VT161.

It's the Real Thing (apparently!)

Steve
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Old 18th Feb 2018, 11:16 pm   #17
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Default Re: Marconiphone VT161 17" TV

Hi Steve,
Thanks for that: it's always interesting to see these brochures from the time. It makes it all more real somehow!
It also answers another question:- why is my LOPT that I have been playing with different to that shown on the VT161 circuit?
The VT161 must have been the old model. Totally hand wired chassis and 90 degree, fat neck tube. The VT163 & 164 pictured have the all new 110 degree tube and are all on two pcb's!
I'd not found those models as they are in a different R&TV book, but my LOPT has the same winding arrangement as those so that's interesting (at least for me!)
Out of interest, is there a date on the leaflet?

The LOPT has finally been put back in the chassis- hopefully for the last time! It is working perfectly!! Apart from the increase in EHT mentioned above, one other bonus is that it runs cooler than before. Not that it got excessively hot before, but it is now cooler.
So I'm now wondering if the various LOPT's I've got that get way too hot, but seem to work, might also have a similar leakage problem? I've never yet got to the bottom of that problem.

Sam, yes, this former was a paxolin/ SRBP type of material. It was in a bad way when I first removed it and I repaired the burnt hole with Araldite. I guess once the material degrades, it becomes leaky with several KV pulses applied to it.

So far, the repair is working well! and I've now got a very good picture. I'm now turning my attention to the horizontal sync which is not all it could be. It's prone to line tearing and the H hold is critical. The verticals are slightly cogged as well. The problem can be cured by de- tuning slightly so that the picture is lower definition. I suspect that some of the higher frequency video components are getting through the sync separator and disturbing the line timebase- I can see it on the scope as noise on the line pulses.
I can also effect a cure by placing the meter probe on the grid of the separator- the picture goes smeary, but the tearing & cogging stop. More investigation needed!

All the best
Nick
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Old 18th Feb 2018, 11:22 pm   #18
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Default Re: Marconiphone VT161 17" TV

Quote:
Originally Posted by 1100 man View Post
Out of interest, is there a date on the leaflet?
Yes- issued in August 1959

Steve
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Old 18th Feb 2018, 11:40 pm   #19
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Default Re: Marconiphone VT161 17" TV

Ah. The date on the smoothing can in mine is May 59. I guess the 90 deg tube was getting a bit old fashioned by then, but I bet side by side it would have given the better picture!!
Cheers
Nick
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Old 19th Feb 2018, 11:03 am   #20
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Default Re: Marconiphone VT161 17" TV

After some probing about with the scope last night, I discovered that what started as nice clean line pulses at the anode of the PCL84 sync separator, got steadily noisier as the contrast was reduced. The video going in stayed nice and clean, but at minimum contrast the pulses coming out were lost in the noise.
A new PCL84 solved that- good clean pulses at minimum contrast and no line tearing. Bit of a shame as that was a 'Marconophone' branded valve so might well have been an original.
Next problem is random shifting of the picture from side to side by about 1/4 inch. Looks like the phase of the line oscillator is changing slightly so that's tonight's investigation.
Cheers
Nick
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