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Old 20th May 2020, 7:25 pm   #21
Heatercathodeshort
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Default Re: Safe to apply mains?

The LOPT should be OK. They are very reliable but do suffer from the old saturation problem, loosing width after approx 30 mins use. It is completely curable with the BT302 but don't worry about that yet. Just thought I would give you a bit of reassurance.

The tube label is 7405A A GEC number but it is in fact a Mazda CME1703. GEC ceased making their own tubes around 1958 buying their requirements from MAZDA and having their name in place of Mazda on the familiar rectangular label.

Good luck with it. They are easy to repair or should be..regards, John.
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Old 20th May 2020, 8:07 pm   #22
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Default Re: Safe to apply mains?

That's definitely reassuring, it does look cracked, is that normal? My G8's one did have a crack in, but I was told it was fine, but again, this is a slightly older set, so better to be safe than sorry.

Thanks for the luck, hopefully i get somewhere with it soon

Thanks
'77
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Old 20th May 2020, 8:18 pm   #23
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Default Re: Safe to apply mains?

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Originally Posted by 19Seventy7 View Post
How would that (table lamp) be set up? I've a few that can be used.

Thanks
'77
wire it in series with your set, like this:
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Old 20th May 2020, 8:26 pm   #24
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Default Re: Safe to apply mains?

Quote:
Originally Posted by rambo1152 View Post
Quote:
Originally Posted by 19Seventy7 View Post
How would that (table lamp) be set up? I've a few that can be used.

Thanks
'77
wire it in series with your set, like this:
Thank you, i'll see if i have any suitable bulbs and get to having a go.

Thanks
'77
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Old 20th May 2020, 10:43 pm   #25
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Default Re: Safe to apply mains?

If you search for 'quick CRT test' on the internet you may find the antiqueradios.com web page.

They use a 6V DC power supply to energise the heater.

In addition 6V positive goes directly to the G1 pin.

The 6V negative goes to the cathode via a 1k resistor. A Voltmeter is wired across the resistor to effectively measure the current through the cathode.

They say that a 4V reading can be expected from a good CRT and 1 Volt from a well used tube.

It should work with 12V DC. However the interpretation of the results will be different.
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Old 21st May 2020, 10:51 am   #26
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Default Re: Safe to apply mains?

Post 4 of this thread https://www.vintage-radio.net/forum/...ad.php?t=65435
shows another very simple emission test where the heater is powered and an AVO set to measure resistance is connected between cathode and grid.
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Old 21st May 2020, 4:51 pm   #27
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Default Re: Safe to apply mains?

Thank you.

I'll definitely try those set ups, although it doesn't really matter too much as I wont be using this set as much as a 625 colour set, so it'll be worth it just for the experience and to learn from it.

But as I've said, the set does look relatively low hour, so there's hope.

Thanks
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Old 9th Mar 2021, 10:19 pm   #28
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Default GEC BT302 Television restoration

Hi all

Sorry if this is pretty novice question but I've never dealt with a valve set before, only solid state, so this is all very new and exciting to me.

I've got a GEC BT302 which I've slowly been restoring over the past year or so. I've not yet got a standards converter, so there's been no real rush to get it done.

I got first light with the set a few days ago, and it still needs work, as seen in the photos, but I'm chuffed to have got something on screen at all. It turns out a valve (or several) isn't sitting quite right and isn't making connection with the socket, which is why I hadn't been getting much further with it.

Some of the valves seem very loose in their holders, and the metal that's in the holders is free to move. Can this be fixed or do I need to replace those particular holders? One holder also has a couple bits of metal lower than the rest, which isn't flush with the top of the holder. I did do a search online and found how to re-tension them but that's not seemed to have done much for me.

Edit: Dunderhead forgot the images

Any ideas appreciated

Thanks
'77
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Last edited by Cobaltblue; 10th Mar 2021 at 8:50 am. Reason: New title to merge threads
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Old 9th Mar 2021, 11:31 pm   #29
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Default Re: Television valves

Update: re-tensioning the holders did work. I went through and did all of them, other than just the loose ones. Got continuity again, though I realised I've missed out a 0.5uF Hunts capacitor in the HV cage, so it wont get run up again until that's been replaced - I've another on its way.

(Mods, if anyone sees this I realised I had another thread open for this set, maybe it'd be a good idea to merge them and retitle? https://www.vintage-radio.net/forum/...d.php?t=166927)

Thanks
'77
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Old 10th Mar 2021, 8:54 am   #30
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Default Re: GEC BT302 Television restoration

Threads Merged and re-titled

Cheers

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Old 10th Mar 2021, 11:26 am   #31
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Default Re: GEC BT302 Television restoration

A great deal of work was carried out on another BT302 on this forum. It might be worth a search to find it and have a read. It may save you a lot of tiresome repetition.

The GEC tube fitted in your BT302 is in fact a Mazda CME1703 with a GEC label. It looks OK from your pics posted earlier and should give a reasonable picture. J.
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Old 10th Mar 2021, 3:41 pm   #32
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Default Re: GEC BT302 Television restoration

Hi

I’ve read through a fair few GEC BT302 threads which have definitely helped and will probably give more help when needed. I thought it’d be best to merge the threads should I have any more questions which I’ve not found an answer to (and I do have one I forgot to ask) I also wanted to document it as this is my first valve and 405 line repair, so where better to put it

The CRT does look a little dim in person, even with full brightness and contrast, I’m hoping that with use the CRT will wake up a bit, not too worried if it’s not the brightest anyway as I cant see me using it daily.

The question I meant to ask is about the mains switch. When I turn it on all is fine, but when I switch off, the control locks into place, but it doesnt turn off until a few seconds later when something inside clicks (it can be anywhere upwards of 2 or 3 seconds after) Could this cause problems? I’ve never seen/heard of it happening before

Thanks
‘77
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Old 10th Mar 2021, 3:50 pm   #33
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Default Re: GEC BT302 Television restoration

I presume by the late '50's, Mazda had sorted out their reliability issues? My various sets are from about 1957 onwards and have a mix of both Mazda & Mullard tubes. Both are perfectly capable of suffering low emission, but usually respond well with a little, er, 'encouragement'!

I've certainly not found Mazda to be any worse and I've not had other faults such as shorts or air leakage with any tube.

I would be interested in John's comments about Mazda tubes from this date - were they as reliable as Mullard or have I just been lucky?

Hi '77,

Well done on getting this far! From your two pictures, it looks like a classic case of poor capacitors in the frame timebase giving little or squashed scan. I assume you haven't changed these yet? I seem to remember you were also having trouble with a damaged linearity or height control?

It looks like you will have to start saving for a standards converter as you are not far off from a decent raster now!

There is something particularly satisfying about watching a decent 405 line picture and I hope you enjoy it when you get there!

All the best
Nick
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Old 10th Mar 2021, 4:09 pm   #34
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Default Re: GEC BT302 Television restoration

The CME1703 Mazda 110 degree CRT is nothing like as reliable as the Mullard AW43-88. The Mullard tube will replace the Mazda. The heater voltage differs, the Mazda has a 12.6v heater and the Mullard 6.3v but in a series heater chain 6v makes no difference to the operation of the chassis.
I have never seen an example of a Mullard AW43-88 low emission and that goes for it's 21" big brother, the AW53-88 and the later AW47-91 19" series.

These later Mazda tubes start off OK when new. After a couple of years they dropped off in emission by about a 1/3rd and sat at that level for around five years. The sparkle had gone and the focus was slightly off but by no means badly enough for the customer to request a tube change. When new, Mazda aluminised screens were the best in the industry and gave a first class picture.

The later 19" CME 1901 of 1961 can only be described as dreadful often requiring expensive replacement within a two year period. Something must have gone wrong!

The raster on yours is in focus but don't expect a lot of brightness from the BT302 chassis with no signal applied. I think it is quite capable of displaying a decent picture once the frame linearity problem is solved you may discover it is capable of sound and vision. John.
PS The mains switch probably needs a squirt of switch cleaner.
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Old 10th Mar 2021, 5:10 pm   #35
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Default Re: GEC BT302 Television restoration

Quote:
Hi '77,

Well done on getting this far! From your two pictures, it looks like a classic case of poor capacitors in the frame timebase giving little or squashed scan. I assume you haven't changed these yet? I seem to remember you were also having trouble with a damaged linearity or height control?

It looks like you will have to start saving for a standards converter as you are not far off from a decent raster now!

There is something particularly satisfying about watching a decent 405 line picture and I hope you enjoy it when you get there!

All the best
Nick
Hi Nick

Cheers! Well I’ve actually replaced all the hunts caps, other than that 0.5uF one i mentioned, (Some might say more money than sense, but I don’t have that much money either ) but I’d expect some resistors to be out of spec too, and probably the lack of a working linearity pot doesn’t help either. Once I’ve replaced the pot and the HV cage cap, I’ll have another look and go from there.

I’ll definitely be getting a converter soon! I’m getting pretty excited to see 405 lines in person now! I enjoy 625 line b/w so hopefully the 405 line experience going to be even better!

Quote:
The raster on yours is in focus but don't expect a lot of brightness from the BT302 chassis with no signal applied. I think it is quite capable of displaying a decent picture once the frame linearity problem is solved you may discover it is capable of sound and vision.
That was interesting to read about the tubes, especially sitting at 2/3 of the original emissions. Why would it drop then flatten out for a while?
that’s a relief to hear about low brightness with no signal. I was a little bit worried that it might not be that great in general. I’m hoping it can produce a picture and sound. I’ll get a standards converter in the next few months and we’ll find out then!

Quote:
PS The mains switch probably needs a squirt of switch cleaner
Why is it I never ever think of the obvious and always the complex??

Thanks
‘77
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Old 10th Mar 2021, 5:43 pm   #36
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Default Re: GEC BT302 Television restoration

Argghh. You have probably been watching too much Star Trek or Doctor Who. John.
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Old 10th Mar 2021, 7:55 pm   #37
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Default Re: GEC BT302 Television restoration

Believe it or not, I’ve never watched a single episode of either, other than doctor who (which oddly enough was about vintage televisions) I think I’ll have to watch Doctor Who when I get this set up and running how it was meant to be watched!
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Old 11th Mar 2021, 8:48 pm   #38
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Default Re: GEC BT302 Television restoration

Update

Replaced the Hunts 0.5uF cap. Couldn’t find a 0.5uF so used 0.47uF. This seems to be doing okay. I also fitted the 100k pot that Sideband very kindly sent to me.

Adjusting all the pots and controls I’ve managed to double or even triple the height, although it’s still not full.

Slowly but surely getting there
‘77
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Old 11th Mar 2021, 10:18 pm   #39
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Default Re: GEC BT302 Television restoration

Wow, your latest set is a find! And I have to agree that it does look low hours from the pics. Good luck with it and do post if and when you get it going.

As you are obviously interested in TVs, I wonder if a LOPT tester might be useful. They used to be common but obviously are not really available anymore. I did find this kit on ebay that you could make which would give a rough indication of whether a LOPT is going to give you grief: https://www.ebay.co.uk/itm/AnaTek-Bl...kAAOSwBLlVDzSA

Good luck!
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Old 11th Mar 2021, 11:11 pm   #40
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Default Re: GEC BT302 Television restoration

Thank you, it definitely does look it to me, a couple of areas with a bit of blackening but I'd expect that. Will 100% post. Hopefully I'm not too far off getting it going.

A LOPT tester would be great, I've just been going at sets hoping the CRT and LOPT are alright. Thanks for linking that, i'll definitely look into it. It'd save a lot of hassle.

Thanks
'77
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