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Old 3rd Sep 2023, 4:33 pm   #1
EF80TVVALVE
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Default Bush TV12 dark contrast/low CRT?

Hi all,

I've been restoring this Bush TV12 over some time and it's nearing completion. I've noticed the contrast isn't great, darks are very dark, there's not much visual information to be seen in shadows for example, the image is fairly bright though. If the contrast is increased to improve the image then flyback lines soon appear, the focus becomes soft and further increasing causes complete focus loss and ballooning.

I'm going to check the EHT voltage and see what is present at the final anode along with the other CRT voltages but could this also be a sign of a low CRT? The set and tube have evidently done many hours over it's life and I wonder if it was put away when the tube became tired and by that time it was probably just better to invest in a new set.

I wouldn't say its awful, the test card shown for example looks quite acceptable but it is definitely noticable compared to a set with a good CRT.
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Old 3rd Sep 2023, 4:39 pm   #2
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Default Re: Bush TV12 dark contrast/low CRT?

When you say compared to another tv, do you mean another tv12? I'm not sure the TV12 was known for its sparkling performance especially with such low EHT.
Contrast wise, the rf alignment is everything, I stagger-tuned mine originally as per the manual and had dismal results, but then did it again by eye only and got a far more contrasty, though slightly lower resolution pic.
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Old 3rd Sep 2023, 4:41 pm   #3
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Default Re: Bush TV12 dark contrast/low CRT?

Tube looks fine to me.

Sounds like lack of EHT, ie, rectifier or overwind problems.

Is this the set that had the hum bar/breathing fault running up or down the screen that never reached any conclusion in the particular thread it was discussed in? Standards converter power supply was the last thing I seem to remember being blamed.
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Old 3rd Sep 2023, 4:52 pm   #4
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Default Re: Bush TV12 dark contrast/low CRT?

Maximum EHT in the TV12 was only 6KV, and that was the later models.
Early production sets it might be <5KV.

DFWB.
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Old 3rd Sep 2023, 5:21 pm   #5
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Default Re: Bush TV12 dark contrast/low CRT?

Thanks all for your thoughts. Kevin, I have a D18T and by comparison it isn't as good contrast wise. The RF alignment was a bit off on this one with poor picture response and a lack of sound. I carried out an alignment by eye too but focused more on getting the 2.5mHz bars good and of course more volume, the picture is much better than what I started with, 2.5mHz lines are clearly displayed but like I say, not as good contrast wise as the D18.

Techman, I'll have a measure of the EHT and see what it comes out at. I did dry out the lopt on this one so it'll be interesting to see what it currently measures. This isn't the same set as the hum bar fault, that was a TV22 which I cured with another PSU for my Aurora, strangely it was the only set that suffered that fault when using that power supply.

Thanks for the EHT data David, from memory this is a TV12A, I don't know where that sits in the line of production dates but the smoothing caps were dated late 1948 from memory.
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Old 3rd Sep 2023, 6:47 pm   #6
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Default Re: Bush TV12 dark contrast/low CRT?

Also, I've noticed with vertical lines that they turn slightly diagonal when the brightness is increased as photoed showing a gradual increase in brightness, also the same when part of the picture changes, such as on the sky menu when the small picture in the top corner changes the lines down the menu bars will become diagonal in places. I also noticed this when showing the F1 grand prix, the vertical information strip on the left hand side of the screen pulled horizontally as the cars were shown driving around corners etc. Focus too needs further attention, whilst it is focused at the centre of the CRT it is quite unfocused as you move away from the centre and the extreme edges are very blurred.
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Old 3rd Sep 2023, 6:58 pm   #7
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Default Re: Bush TV12 dark contrast/low CRT?

Sounds more like the tube is a bit flat, quickest way is to swap the tube from a good set and see if the TV12 has improvement, it saves looking for a fault when none exist.
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Old 3rd Sep 2023, 6:59 pm   #8
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Default Re: Bush TV12 dark contrast/low CRT?

Thanks Stephen, I've already considered swapping with the one from the D18 just to see if there's any difference! I'll take measurements tomorrow to see what I'm working with and if all seems fine there then I'll swith over the CRT and see what difference it makes.


And a little more on my previous post, also showing that it affects line spacing as can be seen here, along with the lower part of the presenters face being shifted further rightwards.
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Old 3rd Sep 2023, 7:23 pm   #9
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Default Re: Bush TV12 dark contrast/low CRT?

I have to say it does sound like the EHT is struggling under load, ie, higher beam current. It could well be the EHT rectifier struggling with emission. I tend to think that a leaky overwind would start to affect the picture width on normal running if it was down to that - fingers crossed it's not that. I've got a bit of a personal theory about these little three wire rectifier valves in that they're of so small a volume compared with most other valves that even the slightest leakage around the wire to glass seal over many decades is going to cause the valve to start going soft and I think this will even be the same for new ones that have been in storage for many decades. It's the sort of thing that doesn't show up so much in valves with a larger glass bulb. It's probably lucky that they've only got three wires going through the glass rather than seven or nine. They never were a good valve and constantly needed replacing back in the day.

It's hard to say regarding the other fault other than it's possibly some kind of sync issue.

Good to hear about what was causing the hum bar on the 22. There seems to be no logical answer to why it only affected that one particular set, although I expect if we sat down and thought about it hard enough there's probably a logical explanation. My explanation is probably that the PSU was a switcher and was letting past some unfiltered muck that a certain stage in that set was susceptible to - perhaps it was beating with an oscillator or something similar, but just guessing really.
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Old 3rd Sep 2023, 7:49 pm   #10
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Default Re: Bush TV12 dark contrast/low CRT?

As an addition to the above, it's likely that the tube will be past its best (as many are in these old sets), but you've got to see it in person to know the typical symptoms. I think it could be a bit of both - mainly EHT struggling, combined with an ageing tube. You could temporarily 'up' the heater voltage from a separate transformer to prove how the tube is, also check the voltage across the heater as it is in the chain to make sure it isn't partially shorted. ie, you'll be reading a lower than stated voltage if it is - AC, of course.
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Old 3rd Sep 2023, 8:31 pm   #11
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Default Re: Bush TV12 dark contrast/low CRT?

Thanks Techman, yes it'll be interesting to see what the EHT measures at and I'll get the figure for that tomorrow. I'm sure the EY51 could possibly even be the original, before any work was carried out it looked as if the LOPT and surrounding circuitry had never been touched. The heaters come in at about 5.9v so slightly low but not dire and I don't think enough to cause such problems.

Regarding the other fault, it's a bit of an odd one as it only happens with the brightness turned up more, not something I've experienced before but I'll investigate EHT/poor tube and once that's cured see how bad it is then.
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Old 4th Sep 2023, 12:51 pm   #12
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Default Re: Bush TV12 dark contrast/low CRT?

Suspect the EY51, but it is also worth trying a fresh PY31 and different PL38 valves. I have found PL38s to behave differently in different sets.

Generally the pictures on the TV11/12 are not especially bright, the EHT transformer is very reliable, but the line output transformer in the metal can has been known to fail.
The line output transformer is very easy to rewind as it is not wavewound.
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Old 4th Sep 2023, 2:10 pm   #13
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Default Re: Bush TV12 dark contrast/low CRT?

Thanks Andy, once I'm home tonight I'll measure what EHT voltage I have at the CRT. If it's low I'll look at replacing the EY51 as they aren't expensive and I'll also test the PL38 and PY31 to see what their emission is like. It would be good to get the picture a little better and I'm sure it can be done, it does just look a little too flat at the moment and I'm sure the little TV12 is capable of something better!
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Old 4th Sep 2023, 2:42 pm   #14
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Default Re: Bush TV12 dark contrast/low CRT?

I didn't know they had separate transformers. I saw a lot of old sets of this type, often used as second sets, but strangely never came across any of these Bakelite Bush types, now they seem to crop up all over the place. Years ago where I used to work there used to be one stored up on a high shelf and it was considered a bit of an 'icon' and 'antique' even back then, but never got to see the innards, or even knew why it was there.

I wouldn't be completely happy with that 5.9 volts on the tube heater (that tube needs all the help it can get), so I would take heater readings on a couple of other random valves in other parts of the set on that heater chain that are easy to get to just to confirm whether it's the 'chain' that's generally a bit down or an issue with the CRT heater itself. It could be that your mains is on the low side of where the dropper tapping is set or perhaps there's a resistance increase somewhere - easy enough to check so you may as well do it.

If the whole heater chain is generally down it'll tend to affect the performance of the set in general. The HT could also be a bit down, although there might be an argument that this could give an ageing set a slightly easier time.
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Old 4th Sep 2023, 6:16 pm   #15
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Default Re: Bush TV12 dark contrast/low CRT?

did you do anything with the EHT smoothing capacitor?
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Old 4th Sep 2023, 7:09 pm   #16
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Default Re: Bush TV12 dark contrast/low CRT?

Well I've managed to take some further readings and it does look like it's a bit of both. Kevin, the capacitor has been replaced with a 'doorknob' type though it is a pull from a scrap set so whether it is up to spec or not I can't say, I'm looking to replace it with a nice new one at some point soon. I checked the heater voltage for the CRT again today and it was at 5.4v this time, definitely too low. I checked a few valves and they were all set spot on at 6.3v so it looks like the now increasingly common heater short on the 9" mullard tube. EHT came in at 4.2kv on a blank screen and rose to 4.5kv with test card C displayed. A little below David's data he gave so some attention here may improve things but really it's going to need another tube for the best results.
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Old 4th Sep 2023, 8:31 pm   #17
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Default Re: Bush TV12 dark contrast/low CRT?

Might be worthwhile checking any heater chain decoupling capacitors for leaks.
The CRT heater is high up in the chain, connected between the heater of the EL42 frame output valve and pin A4 on the B5 plug and socket.
A separate 6.3volt heater transformer can be considered, however, if the CRT heater is partially shorted there's always the possibility the heater might go open-circuit. But before doing anything like fitting a CRT transformer confirm that the heater voltages are correct on the valves on the the upper deck. The heater chain thermistor and it's parallel 300ohm resistor should be checked.
Various gun assemblies were fitted in MW22-14 CRTs, tubes might have an M type heater and others a double M heater.

DFWB.
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Old 4th Sep 2023, 9:33 pm   #18
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Default Re: Bush TV12 dark contrast/low CRT?

Thanks David, will definitely check decouplers. I have had success in the past with using a 6.3v heater transformer in a Marconi set with a 12" Mullard tube fitted, that was much worse at only 3v when supplied by the chain and it was fine running from the transformer. I should have a spare transformer somewhere but I may sub for the D18 CRT if that one is okay, there's much more space in the cabinet for the transformer in the PYE than the Bush, an ideal situation would be for a scrappy 9" receiver to turn up and donate it's tube but they're getting thinner on the ground nowadays.
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Old 5th Sep 2023, 9:05 am   #19
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Default Re: Bush TV12 dark contrast/low CRT?

I would definitely replace the EY51, I always see an improvement in EHT once replaced with a NOS valve.

The picture was never very bright on the TV12, the TV22 is noticeably brighter.
The D18T is also brighter than the TV12.

I would certainly try running the CRT at 6.3 volts, I am sure it will make a noticeable improvement.


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Old 5th Sep 2023, 10:04 am   #20
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Default Re: Bush TV12 dark contrast/low CRT?

When I was restoring vintage TVs I always used to pretest all the valves, and fit new ones at any sign of below par performance. I'd just change that EY51, as you say, it looks original.
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