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B&W TV Marconiphone VT161

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Katie Bush
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Posted : 06/07/2021 5:01 pm
PYE625
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I must admit that is is rather alarming to be adjusting the line hold and have the line oscillation stop randomly. The effect in this case was a wiggly bright line down the centre of the screen, not too dissimilar to a bolt of lightning. Then the stage would start up again upon further minor adjustment of the control. Needless to say, a good PL81 will remain in the set to avoid this happening.

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Topic starter Posted : 06/07/2021 5:23 pm
Cathovisor
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Posted by: @katie-bush

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Posted : 06/07/2021 5:35 pm
PYE625
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I would guess, although I did not try it, that if the line stage was left in a non-oscillatory condition the PL81 would overheat and cause damage. I presume it would not just sit there happily doing nothing ?

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Topic starter Posted : 06/07/2021 6:16 pm
Cathovisor
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Posted by: @pye625

I would guess, although I did not try it, that if the line stage was left in a non-oscillatory condition the PL81 would overheat and cause damage. I presume it would not just sit there happily doing nothing ?

It depends on what the grid bias is in its inoperative state.

ETA: Hmmm. Zero bias on the PL81 in a no oscillation condition.

That won't do it or the LOPT any good whatsoever - the valve data suggests an anode current of 350mA (yes, you did read that right) with Va = 200V, Vg2 = 150V. I'm assuming there's a fuse in the HT?

 

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Posted : 06/07/2021 6:47 pm
Cathovisor
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See https://frank.pocnet.net/sheets/010/p/PL81.pdf for PL81 data; those figures quoted above from page five.

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Posted : 06/07/2021 6:57 pm
PYE625
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Nope, no HT fuse. The cathode is grounded as you might expect and the 1st grid returned to ground via a 1.8 meg resistor (just off the below circuit to the left).

Screenshot 2021 07 01 220540

 

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Topic starter Posted : 06/07/2021 7:11 pm
Nuvistor
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@pye625 
Looking through some Ferguson/Marconi circuits with similar chassis, the line osc was either multivibrator or blocking osc, perhaps they thought this was worth trying with a few less components but reverted back to the other type of oscillator.

Ekco/Ferranti used this type oscillator in the 368/370 chassis but with the triode of a 30FL1 and 30P4 as output/osc and U19 boost. I don’t recall this problem with those sets.

 

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Posted : 06/07/2021 7:58 pm
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PYE625
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I must stress it is only a problem with a well worn valve, the worst being the one that was fitted and had much reduced width too. All new PL81's (and I have about half a dozen) behave perfectly fine. Some used ones are a bit of a mixed bag, so I will certainly leave a new one in place. I am certain there is nothing wrong with the set.

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Topic starter Posted : 06/07/2021 8:21 pm
Nuvistor
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@pye625 

No, nothing wrong with the set, just interesting that the very similar sets had different oscillator circuits, perhaps trying which type would be best.

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Posted : 06/07/2021 8:32 pm
PYE625
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It is interesting and show's that wear in a timebase valve is often more apparent. A good example was the PCL82 used for the sound output. Ok it produced sound, but when placed into the frame stage socket, it's shortcomings were very obvious.... short being the word with a much reduced scan.

This article in general is interesting at "The Valve museum".... Valves and their habits, from Practical Television Aug/Sept 1965.

The PL81, among many others, is mentioned....

http://www.r-type.org/articles/art-012.htm

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Topic starter Posted : 06/07/2021 8:43 pm
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irob2345
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The EL81 / PL81, being the only attempt at a line output valve in a B9A envelope, was always a wimpy device. I've never seen it used for more than 70 degree deflection. No Aussie maker ever used it, preferring the 6CM5 / EL36, and neither Philips, Mullard or AWV ever made it here. I have a 1956 70 degree German TV that uses a PL81 and it only runs for about half an hour before the width comes in and the PL81 redplates. But my Ekco TX275 with its EL81 copes better - only 50 degree deflection and 5kV EHT!

Philips recognised it was a marginal valve - see the dissipation warnings.

EL81 PL81
Ekco TV

Better designs countered the problem of valve aging by using stabilisation in the line output and negative feedback in the vertical. Almost all valve based Oz HMV TVs have a current transformer in series with the yoke, the secondary of which is in series with the sawtooth cap. There is no vertical linearity control, nor is one needed. And the performance of the vertical is unaffected until the vertical output valve (usually an ECL82/6BM8 or ECL85/6GV8) drops below about 40% emission.

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Posted : 07/07/2021 8:37 am
Till Eulenspiegel
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In the UK the PL81 was employed many 110 degree CRT TV sets, Philips, Ferguson and Pye for example.  In use the valve gave a long service life not requiring replacement too often. Much has been written about the PL81 in other TV related forums.  In the valve's earliest form it was a true pentode but later on the valve was redesigned as a beam tetrode.

Getting back to the Marconi VT161. I don't think the line oscillator arrangement is entirely satisfactory but a similar circuit was employed in the first Thorn 110* models introduced in 1959 such as the 500 series chassis. Like the VT161 the line output valve was a PL81. The Thorn 600 series of 1960 employs a blocking transformer in the line oscillator circuit. Also worth mentioning the line output valve was changed to a PL36 which was more suited to the demands of 110* scanning. The use of a blocking oscillator simplifies the inclusion of line flywheel synchronising if required.

I'm considering modifying the line oscillator in my Marconi VT161 to use a blocking oscillator transformer. 

A discussion about the PL81: The PL81 TV line output valve. - UK Vintage Radio Repair and Restoration Discussion Forum (vintage-radio.net)

Till Eulenspiegel. 

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Posted : 07/07/2021 11:07 am
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PYE625
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I think the PL81 was ideal for the CRT types generally in use at the time it was introduced, but obviously over time more powerful valves had to be developed to cater for larger screens and more line deflection power. Hence the PL500/504. Another factor that must have played a part is keeping the cost of set's down. I guess it would have been quite possible to produce a no-expense spared ultimate design, but competion would make it a non-viable option I would imagine.

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Topic starter Posted : 07/07/2021 6:10 pm
Till Eulenspiegel
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As line output transformers became more efficient as a result of the high mu cores it was possible to continue with modest line output valves like the PL81. I found it is possible to operate the valve beyond the published ratings, the Philips 21TG100 I modified to use a Philips 210 line output transformer is testimony to that. The poor thing is drawing 200mA during the latter part of the forward scanning stroke. The mean current is 130mA.

Till Eulenspiegel.

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Posted : 07/07/2021 7:55 pm
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Cathovisor
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Posted by: @pye625

I think the PL81 was ideal for the CRT types generally in use at the time it was introduced, but obviously over time more powerful valves had to be developed to cater for larger screens and more line deflection power. Hence the PL500/504. Another factor that must have played a part is keeping the cost of set's down. I guess it would have been quite possible to produce a no-expense spared ultimate design, but competion would make it a non-viable option I would imagine.

You have to remember that most, if not all, UK set manufacturers worked on the premise that "a penny spent is a ha'penny wasted" and that the market was cut-throat.

ITT/KB used the PL36 in the VC52 chassis IIRC.

 

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Posted : 07/07/2021 10:20 pm
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Cathovisor
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Posted by: @till

The poor thing is drawing 200mA during the latter part of the forward scanning stroke. The mean current is 130mA.

Till Eulenspiegel.

Which I think is still within the dissipation limits though?

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Posted : 07/07/2021 10:23 pm
PYE625
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I also replaced the PCC89 (plus the PCF80 whilst I was at it) in the tuner due to low gain on Band III. This is now much better as a result.

There is an important thing that I should have noticed earlier but paid little attention to until considering it now. Due to the open circuit dropper sections, a couple of resistors had been fitted in a previous repair. The thing is, they were of the wrong value by quite a margin. There should be a total of 197 ohms in series with the heaters on the 240v mains setting. The resistors were of 66 and 33 ohms. This is nearly 100 ohms too low. I was using a lamp limiter during initial testing of the chassis so the valve heaters were not unusually bright. It was not until the new dropper was installed that I applied full mains. At this time, the heater drop would then be correct.

This would explain why several of the valves were in a poor condition... They had all been overrun for possibly quite some time during the service life of the set. The only valves that are perfectly ok are the EF80's, the CRT fortunately, a newish PCL82 and ECC82 in the frame stage and the PY33. Oh, and the EY86 but of course that's not in the heater chain.

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Topic starter Posted : 07/07/2021 11:06 pm
PYE625
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At the time of repair, it is a top priority to get a picture back on screen as cheaply and as quickly as possible. But at what future cost? No one thinks of that, and if the set was getting on a bit, they probably never really cared.

Easy to criticise now, but at the time, an engineer would be under pressure to achieve what he could with whatever spares he was carrying. Oh, and just stick in whatever value of mains fuse you happen to have...

IMG 5927 50
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Topic starter Posted : 08/07/2021 10:52 am
Till Eulenspiegel
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Posted by: @pye625

At the time of repair, it is a top priority to get a picture back on screen as cheaply and as quickly as possible. But at what future cost? No one thinks of that, and if the set was getting on a bit, they probably never really cared.Easy to criticise now, but at the time, an engineer would be under pressure to achieve what he could with whatever spares he was carrying. Oh, and just stick in whatever value of mains fuse you happen to have..

That's right, it was always a case of "do as it cheaply as you can" and if you were making a living from only repairs and no chance of a sale or rental of a new set you just had to accept those terms from the customer. For one reason or other the Marconi VT161 was often a victim of mains dropper bodges. The correct dropper would be fitted and then you found the CRT and and many valves needed replacement. The customer sees the horrible dim picture and then you have to explain why the picture is so bad. Of course the reply was "well it was al right until you did something to the set" One of the reasons why I concentrated on rental rather than have to rely on TV repairs for a living.  

Till Eulenspiegel.

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Posted : 08/07/2021 12:35 pm
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