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Vintage Television and Video Vintage television and video equipment, programmes, VCRs etc. |
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16th May 2022, 9:07 am | #21 | |
Hexode
Join Date: Sep 2019
Location: Crystal Palace, Bromley, London, UK.
Posts: 417
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Re: EHT reservoir cap - anything wrong with small ceramics?
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I am aware of the various voltage ratings, capacities and prices of the blue ceramic caps, and merely got 200nF ones (20 of them for 5 quid on eBay) so that I could increase the capacity more gradually. You can get them up to 1nF, like yours, and 30kV. - Helder |
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16th May 2022, 10:09 am | #22 |
Dekatron
Join Date: Jun 2005
Location: Warnham, West Sussex. 10 miles south of DORKING.
Posts: 9,147
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Re: EHT reservoir cap - anything wrong with small ceramics?
The capacity between the inner and outer 'plates' of a typical tube is around 500pf, more than enough to smooth the 10kc/s line ripple.
I have some .01uf 15kv capacitors and just as an experiment, a while back connected one across the EHT supply. No improvement in EHT regulation, it just held a charge for a lot longer. If you are experiencing poor regulation at normal viewing levels of brightness and you have replaced the EY51, PL38 and PZ30 valves it is likely that the LOPT is showing it's age. Deteriorated laminations, low Q windings aggravated by years of damp are likely causes. It is much harder to obtain good regulation with the TV12 series. The TV22 are usually very good. They are not computer monitors and a slight increase in picture size with maximum brightness [never used] is quite normal for a TV from 1950. Regards, John. |
16th May 2022, 9:11 pm | #23 | |||
Hexode
Join Date: Sep 2019
Location: Crystal Palace, Bromley, London, UK.
Posts: 417
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Re: EHT reservoir cap - anything wrong with small ceramics?
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- Helder |
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16th May 2022, 11:41 pm | #24 |
Dekatron
Join Date: Nov 2009
Location: Lincolnshire, UK.
Posts: 5,000
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Re: EHT reservoir cap - anything wrong with small ceramics?
You won't overload the LOPTx - the EY51 will easily limit things, it's a dreadful valve at the best of times and doesn't have a long life anyway. It could even be the EY51 that causing the problem, but from what you've now said in the previous post I would much more suspect the LOPTx. You say that you've sealed it in pitch and/or varnish, but the trouble is how certain can you be that every last microscopic drop of moisture was completely removed from those overwind windings. The trouble with sealing the windings with modern products is that there's a risk of sealing in forever any slight moisture that may still remain. It's best to give the set a good running for a few weeks before sealing to make sure that the transformer is completely dry and to make sure you don't have a problem like the one you now report. I know you say that you've dried them out, but I suspect that you haven't quite achieved it.
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17th May 2022, 12:52 am | #25 | ||
Hexode
Join Date: Sep 2019
Location: Crystal Palace, Bromley, London, UK.
Posts: 417
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Re: EHT reservoir cap - anything wrong with small ceramics?
Quote:
Quote:
- Helder |
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17th May 2022, 7:26 am | #26 |
Heptode
Join Date: Jun 2006
Location: Eindhoven, Netherlands.
Posts: 642
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Re: EHT reservoir cap - anything wrong with small ceramics?
Helder,
Are you sure it's not the CRT? Relatively easy to check by using the EHT from a known good set. Check the EHT of this set to power a CRT of the known good set to make sure it is problematic. Jac |
17th May 2022, 7:49 am | #27 | |
Hexode
Join Date: Sep 2019
Location: Crystal Palace, Bromley, London, UK.
Posts: 417
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Re: EHT reservoir cap - anything wrong with small ceramics?
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I'm not sure if the CRT is to blame or not as I don't know how much beam current is considered too much beam current for the MW22-16. I suspect this may be the case, even though the overall image quality is really very good and bright. From my readings, this is not uncommon with slightly soft tubes though. I don't have a known good set. My whole "collection" amounts to 3 TV22s of different vintages (Mk.2, Mk1.5 and a recently acquired late Mk.1) and only the Mk.2 has been restored and is working. Testing the other two MW22-16s with the working Mk.2 did cross my mind. Their heater resistances measure ok, which is promising; they will surely need to be left working for some time to "wake up" but this should not take more than a few hours. I must confess that I feel more tempted to do this than to finish treating the 48 laminations of my presently disassembled LOPT. There is still a chance that all three tubes are soft and the results are inconclusive, but as long as its fun to do and nothing gets necked, well, that's one of the main reasons why I'm into this hobby. - Helder |
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17th May 2022, 9:11 am | #28 |
Dekatron
Join Date: Jun 2005
Location: Warnham, West Sussex. 10 miles south of DORKING.
Posts: 9,147
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Re: EHT reservoir cap - anything wrong with small ceramics?
I didn't find the EY51 that bad! Many old receivers still have their original EY51 valves still working fine. It was used in the Philips G6 colour chassis till 1971.Now if your talking about the Mazda U25, that was a bad one!
Giving the massive difference in EHT voltage between normal and high brightness it may be the tube but if it gives a very good focused picture when viewed normally I would suspect the LOPT first. |
17th May 2022, 9:17 am | #29 | |
Hexode
Join Date: Sep 2019
Location: Crystal Palace, Bromley, London, UK.
Posts: 417
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Re: EHT reservoir cap - anything wrong with small ceramics?
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- Helder |
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17th May 2022, 2:34 pm | #30 |
Dekatron
Join Date: Jun 2005
Location: Warnham, West Sussex. 10 miles south of DORKING.
Posts: 9,147
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Re: EHT reservoir cap - anything wrong with small ceramics?
This is one of my TV22s. It has it's original cracked and baked LOPT and the EY51 looks original. First picture with blank screen 9kv. Second with average brightness level pic which is excellent but the camera does not show it. 9kv and the last is complete white out with brightness on full, just under 7kv. Tube is original DAG coated Mullard MW22-16 with ion trap.
If yours is reducing to as little as 3kv it's my guess that the LOPT has lost it's grunt. Regards, John. Last edited by Heatercathodeshort; 17th May 2022 at 2:40 pm. |
17th May 2022, 2:41 pm | #31 | |
Hexode
Join Date: Sep 2019
Location: Crystal Palace, Bromley, London, UK.
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Re: EHT reservoir cap - anything wrong with small ceramics?
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The last picture is saturated of course, due to the high brightness of the image, but how much has the image changed geometrically? Could you please take a picture with a smaller aperture/exposure to avoid saturation, if I'm not asking too much? - Helder |
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17th May 2022, 2:50 pm | #32 |
Dekatron
Join Date: Jun 2005
Location: Warnham, West Sussex. 10 miles south of DORKING.
Posts: 9,147
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Re: EHT reservoir cap - anything wrong with small ceramics?
If the EY51 is failing after just a short amount of use it may be overrun. A small resistor was shunted across the heater to absorb excess current. I must admit in over 50 years repairing old TV's I never had any problems with the EY51. It used to go O/C but no more so than the B9A EY86.
Some manufacturers that had originally wound their line output transformers for the 2V Mazda U25 often modified replacements to take the 6.3V EY51. This usually put a stop to EHT rectifier failure. The U25, fatter than the EY51 failed in various ways. O/C heater, internal flashing and gassy creating a bright pink/purple glow. John. [PS It remains the same size. you can just about make out the edge of the test card on the right.] |
17th May 2022, 3:11 pm | #33 | |
Hexode
Join Date: Sep 2019
Location: Crystal Palace, Bromley, London, UK.
Posts: 417
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Re: EHT reservoir cap - anything wrong with small ceramics?
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Your same-size ultrabright image is very impressive! Having struggled with the low EHT and image/focus distortions ever since I started this hobby 3 years ago, I cannot but be impressed. And this shows that it is indeed possible to have very good EHT regulation on the TV22, in spite of the lack of active regulation in this basic TV. When I have some time, possibly next month, I'll try my other CRTs with the present TV22 and LOPT, and in case this fails I will finish treating the laminations of my presently disassembled extra LOPT. In case this fails too, I'll have to live with it as the problem may well be in the overwind, which is hard/expensive to rebuild. I don't think anyone here in the forum can do it, but it could in principle be done elsewhere, e.g., by this company, although I have no idea how much a custom job like this would cost (btw, the photo in their website below is very interesting and definitely reminiscent of the overwind in the TV22 LOPT): https://agw.co.uk/news/article/new-m...winding-at-agw - Helder |
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23rd May 2022, 11:33 am | #34 |
Hexode
Join Date: Sep 2019
Location: Crystal Palace, Bromley, London, UK.
Posts: 417
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Re: EHT reservoir cap - anything wrong with small ceramics?
Yesterday I tried the tube on one of my unrestored sets (the Mk1.5) and the results were similar to the original tube, although the decrease in EHT with brightness is a bit less. This of course depends on image content, but for Test Card C, the EHT drops from 8.5kV at normal brightness to a bit below 7kV for maximum brightness. So, as John (Heatercathodeshort) suggested, the bottleneck here seems to be the LOPT. The good news (for me) was that the tube from the Mk1.5 seems to be in excellent condition, and gives a very bright and focused image. One thing that surprised me quite a lot though: I was expecting to have to adjust the focusing magnet x-y position, as well as the H and V linearity and height controls for the new tube. However, I didn't need to touch a thing as the image was linear and well centred in the mask. Either I should play the lottery today or the manufacturing tolerances of the MW22-16 are really tight. AFAIK, these tube require a lot of glass turning and spinning, coating is partially done by hand, etc., so I was believed the differences in e-gun positioning and angle would require readjusting the external field to optimise the image.
Ayway, the next step will be to try to improve the insulation in the laminations. It also seems that energy is being lost in the primary section and not the overwind, as I can perceive a reduction in the brightness of the EY51 filament... - Helder |
23rd May 2022, 1:15 pm | #35 |
Hexode
Join Date: Sep 2019
Location: Crystal Palace, Bromley, London, UK.
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Re: EHT reservoir cap - anything wrong with small ceramics?
In my previous post, I meant "Yesterday I tried the tube *from* one of my unrestored sets..."
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