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Vintage Television and Video Vintage television and video equipment, programmes, VCRs etc. |
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13th Dec 2021, 4:07 pm | #1 |
Octode
Join Date: Jun 2004
Location: Reading/Fakenham, UK.
Posts: 1,320
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TV22/24 metrosil
I'm trying to tackle the fading and de-focussing on my TV24 mkI.
On first switch on after picture has settled, brightness and focus are pretty good. But after 30 mins or so the focus starts to deteriorate and the picture starts to loose contrast, but doesn't balloon. Picture geometry is fairly constant. While the picture is good, all voltages on the tube base (measured using a decent Fluke multimeter) are near those listed. But after 30 minutes or so the 1st anode voltage starts to fall from 223V to 160V. The meter may be pulling it down to lower than it really is, but it's still showing it as falling. (Putting the meter on A1 temporarily dims the picture.) I'm assuming this fall in A1 voltage is the cause of the picture fault; the heater voltage is stable at 6.05V so am assuming there is no cathode short. So... would a failing metrosil cause this problem? I've looked through the forum and it does seem likely, but why gradual decline I wonder? All capacitors have been replaced. I have the two circuits for metrosil substitution if necessary - the one by Dr H Holden and the one posted by Beery. Just waiting for confirmation or other ideas before starting... Thanks. Ian |
13th Dec 2021, 4:35 pm | #2 |
Dekatron
Join Date: Jun 2005
Location: Warnham, West Sussex. 10 miles south of DORKING.
Posts: 9,145
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Re: TV22/24 metrosil
More likely to be LOPT saturation. The Metrosil is very reliable but there is always one..
A check on the EHT will prove if the LOPT is the cause. Very common. John. |
13th Dec 2021, 5:09 pm | #3 |
Octode
Join Date: Jun 2004
Location: Reading/Fakenham, UK.
Posts: 1,320
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Re: TV22/24 metrosil
Thanks. The EHT is more or less ok as measured with an EHT probe and the EHT transformer was rewound a couple of years ago by Mike Barker.
EHT has fallen a bit over the past hour or so, but not too much. Just checked and with the brightness to minimum it's about 5.8 kV. And the picture hasn't ballooned which if it did is a sign of EHT falling isn't it? |
13th Dec 2021, 5:43 pm | #4 |
Dekatron
Join Date: Jun 2005
Location: Warnham, West Sussex. 10 miles south of DORKING.
Posts: 9,145
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Re: TV22/24 metrosil
The transformer lacks drive when this annoying saturation takes place. It appears to reduce the scan and the EHT so they even things out.
Mike carries out very good transformer rewinds/rebuilds so maybe a check of the PL38 may be in order. The chassis has obviously had all suspect caps replaced. Hope you get it sorted. John. |
13th Dec 2021, 6:27 pm | #5 | |
Hexode
Join Date: Sep 2019
Location: Crystal Palace, Bromley, London, UK.
Posts: 417
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Re: TV22/24 metrosil
Quote:
What exactly is this LOPT saturation? Is it mostly due to losses in the hot core, or is it more related to the overwind? How does it manifest exactly and what can be done to mitigate it (e.g., reinforcing/replacing insulation, cleaning up rust, rewinding, ...). Thanks, Helder |
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13th Dec 2021, 9:23 pm | #6 | |
Octode
Join Date: Jun 2004
Location: Reading/Fakenham, UK.
Posts: 1,320
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Re: TV22/24 metrosil
Quote:
While it's out I'll do some other minor mods and maybe swap the speaker. It sounds OK on tone and fairly undistorted on the 'scope, but some sound tracks are quite muffled on this set while others are ok. Not noticed that with the TV22... Thanks again, Ian |
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14th Dec 2021, 10:39 am | #7 |
Dekatron
Join Date: Jun 2005
Location: Warnham, West Sussex. 10 miles south of DORKING.
Posts: 9,145
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Re: TV22/24 metrosil
Helder, it's basically 50 years + of damp ingress in the transformer windings. The main problem being the overwind. There is masses of information on the Forum and I feel it is unnecessary to repeat it.
Ian you may find that it was slight damp that has dried out. This was never a problem when receivers were in use every day. Time will tell. John. This may help explain. https://www.vintage-radio.net/forum/...ad.php?t=24655 |
14th Dec 2021, 12:18 pm | #8 |
Nonode
Join Date: Feb 2005
Location: Worcestershire, UK.
Posts: 2,525
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Re: TV22/24 metrosil
My understanding is this. I welcome any corrections.
There are two sides to the problem: the windings (mainly the overwind) and the magnetic core. The wax coating and impregnation of the windings can absorb water, just like our old friends the 'waxy' capacitors, and then 'leak' - particularly under the influence of high voltages. This causes an effect like partially shorted turns. The efficiency of the transformer diminishes, with the excess energy appearing as heat in the afflicted winding. The drive output, both for the line scan and EHT, then is diminished. So, whereas the picture may get softer and dimmer, its width doesn't necessarily change. However, the reduced EHT may also reveal itself as increased height, since the vertical scan has not been reduced (unlike the line). As for the core, a little rust coating on the laminations may do no harm at all and indeed reduce eddy currents. However serious rust is another matter. The iron is eaten away, reducing its capacity to follow the energising currents and saturation occurs too soon. The result is again heating and lack of efficiency in the transformer. Well, that's my take on it. Steve
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14th Dec 2021, 1:17 pm | #9 |
Octode
Join Date: May 2009
Location: Stevenage, Herts. UK.
Posts: 1,515
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Re: TV22/24 metrosil
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14th Dec 2021, 2:58 pm | #10 |
Octode
Join Date: Jun 2004
Location: Reading/Fakenham, UK.
Posts: 1,320
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Re: TV22/24 metrosil
Yes, I wholeheartedly agree with the the contents of the above link. I usually completely rebuild most of my restored radios and tvs. All new capacitors and resistors - unless the wire-wound ones are OK. I thoroughly check wound components and re-hotwax them if appropriate. Rewiring with silicone wire is part of the job.
Some members think this is quite over the top and not usually necessary and I would agree that it would be sacrilege to do that to a very rare or interesting set. But it does give years more reliable service and gets them working at least as swell as they did when they left the factory. Ian |
14th Dec 2021, 3:03 pm | #11 |
Octode
Join Date: Jun 2004
Location: Reading/Fakenham, UK.
Posts: 1,320
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Re: TV22/24 metrosil
Back on topic:
I have a note showing a modification for the screen grid of V8 of the TV22 having two 160k resistors in parallel with 0.1uF across them inserted between V8 pin 8 and the horizontal form inductor. I think it relates to the MKI although this part of the circuit is identical in the MKII. Can anyone tell me where this came from (I didn't write the source down!) and what advantage it's meant to have? Thanks. Ian |
6th Jan 2022, 11:44 am | #12 | |
Hexode
Join Date: Sep 2019
Location: Crystal Palace, Bromley, London, UK.
Posts: 417
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Re: TV22/24 metrosil
Quote:
Helder |
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6th Jan 2022, 11:54 am | #13 | |
Hexode
Join Date: Sep 2019
Location: Crystal Palace, Bromley, London, UK.
Posts: 417
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Re: TV22/24 metrosil
Quote:
Thank you very much for your detailed reply. It makes total sense to me. As I mentioned in my answer to John above, in my the potential changes in the dielectric or water ingress in the overwind do not appear to be behind the issue I have always experienced with my TV22 from the early stages of its restoration. The EHT decreases a bit over time and then stabilises after one hour or so, with the image getting slightly narrower (doesn't change a lot, but the deflection still depends on the square root of EHT in a set with flyback-derived EHT, I think) and taller, as you describe. This however appears to be mostly a temperature-related effect, as the EHT tends to increase back to its initial value when I blow some cold air at the line output stage. As I mentioned in my reply above, I will soon be able to check whether cleaned and treated laminations will result in smaller losses, less heating and hopefully better EHT stability, but right now I have no reason to suspect the problems described by John in his post about LOPT saturation. Helder |
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6th Jan 2022, 12:02 pm | #14 | |
Hexode
Join Date: Sep 2019
Location: Crystal Palace, Bromley, London, UK.
Posts: 417
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Re: TV22/24 metrosil
Quote:
Helder |
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6th Jan 2022, 12:54 pm | #15 | |
Hexode
Join Date: Sep 2019
Location: Crystal Palace, Bromley, London, UK.
Posts: 417
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Re: TV22/24 metrosil
Quote:
This may be useful for an aged valve, but its usefulness in general is debatable, as it reduces the capability of the PL38 to act as a switch. Helder |
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6th Jan 2022, 2:24 pm | #16 | |
Nonode
Join Date: Feb 2005
Location: Worcestershire, UK.
Posts: 2,525
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Re: TV22/24 metrosil
Quote:
Could the same be true of magnetisable materials? In other words, the variable causing the issue here would actually be the core temperature. This would first have been raised by unacceptable inefficiencies in the core, as discussed, and can be artificially reduced if desired. I can vouch for the benefit of temperature reduction, having fitted little fans in the TV22 LOPT can tops many times. Steve
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6th Jan 2022, 4:25 pm | #17 | ||
Hexode
Join Date: Sep 2019
Location: Crystal Palace, Bromley, London, UK.
Posts: 417
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Re: TV22/24 metrosil
Quote:
Good point Steve. The magnetic permeability of most materials, namely iron, and hence the magnetisation, does depend on the temperature. This short and highly pedagogical video illustrates this well with a very basic transformer: https://www.youtube.com/watch?v=pLPUZBQvbDI It may be possible to reduce the core heating by reducing its losses (up to a point, of course). This should improve the steady-state (thermalised) operation point of the stage. I think the poor EHT stability I have been talking about (i.e., EHT reduction due to increased beam current) may be due to the same losses, so hopefully by minimising the loses one should gain both on the final / steady-state EHT value and on its stability. Helder |
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6th Jan 2022, 6:24 pm | #18 | |
Hexode
Join Date: Sep 2019
Location: Crystal Palace, Bromley, London, UK.
Posts: 417
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Re: TV22/24 metrosil
Quote:
Helder |
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6th Jan 2022, 6:59 pm | #19 |
Nonode
Join Date: Feb 2005
Location: Worcestershire, UK.
Posts: 2,525
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Re: TV22/24 metrosil
I take your point about the high Curie temperature indicating that losses in the windings themselves could be the dominant factor in raising the temperature here. However, these losses would not be primarily resistive but due to the 'partial shorted turns effect' caused by leakage in the degraded waxy encapsulation (as previously mentioned). Otherwise, a new LOPT would suffer just as badly.
Please correct me if I have misunderstood you. Steve
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6th Jan 2022, 7:01 pm | #20 | |
Hexode
Join Date: Sep 2019
Location: Crystal Palace, Bromley, London, UK.
Posts: 417
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Re: TV22/24 metrosil
Quote:
Helder |
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