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Old 18th May 2020, 8:11 pm   #1
19Seventy7
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Default GEC BT302 Television restoration

Hi all,

Just a quick question, hope it's okay to ask.

I've got an old valve set that I want to try and repair, but before I spend any more money on it, I'd like to know that the CRT has fairly decent emission.

I'll snip out the mains smoother before switch on. I did think about a lamp limiter, but this will be the only set I'll be working on (That hasn't had power applied recently) for a while, and haven't got the money right now for a variac, and don't particularly want to spend money on making a lamp limiter for one thing, if it's not essential.

It's a GEC BT302, if that helps

Any advice appreciated.

Thanks
'77
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Old 18th May 2020, 8:58 pm   #2
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Default Re: Safe to apply mains?

I'd advise you against powering this up if it's unrestored. It's likely to be full of leaky capacitors, so is pretty unlikely to even produce a raster, and it may go bang.
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Old 18th May 2020, 9:17 pm   #3
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Default Re: Safe to apply mains?

I think you will find their is a high probability that the selenium rectifier is faulty or will die pretty quick. I have had similar sets on on power up the H.T. has been low due to the rectifier Andy
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Old 18th May 2020, 9:35 pm   #4
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Default Re: Safe to apply mains?

You have to try to work out why the set went out of use. It may be that the tube went soft, but it's also possible that it developed a completely unrelated fault, or even that it was simply put aside in the 70s as UHF 625 lines became almost universal. If you don't have any info on the history, then all you can do is test it.

As you've probably worked out, the only way to meaningfully test it is to put a fair bit of work in.
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Old 18th May 2020, 9:55 pm   #5
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Default Re: Safe to apply mains?

It does look to be a fairly low hour set, to be honest, so I do have relatively high hopes for the tube, but also expect it to be low.

I'll get to replacing those Hunts caps, and go from there

Thanks
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Old 18th May 2020, 10:01 pm   #6
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Default Re: Safe to apply mains?

i know nothing about that particular model but.....i dont see much harm in powering all tube series string sets directly for a couple of seconds just to get a baseline...if there is a hard short then there is no power transformer to blow up, only the fuse will blow...if there is high voltage and a raster thats great, recap it....if not then think...either troubleshoot first or shotgun it with caps...
i like to troubleshoot first to avoid spending time and money on dead sets
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Old 18th May 2020, 11:01 pm   #7
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Default Re: Safe to apply mains?

Hi 77,
It really is a bad idea to just apply power to an un-restored TV of this vintage. It is guaranteed, 100%, not to work and more importantly, components that could be salvaged with the correct procedures can be destroyed.

THE most critical and delicate item is the line output transformer. Members have developed a process of slowly removing the absorbed moisture thereby giving a better chance of survival.

Even then, failures occur all too often. Without exception, ALL the LOPT's on the sets that I have restored have caused me issues in one way or another. They can be rewound but this is expensive, so trying to prevent failure in the first place is better!

CRT's on the other hand are not usually a problem in '50's & '60's sets and will generally work well enough to give a picture. The emission often improves with use especially if it's not been used for 50 years!

The main smoothing capacitors can usually be reformed- it's rare I've had to replace one- but again there is a process for doing this.

Don't forget that this is a 405 line TV, so you will need a standards converter to be able to watch pictures on it. That will set you back the best part of 200.

These are great looking sets (I've been after one for a while) and probably a good TV for a first restoration, but be prepared for a lot of work and a long journey- there are no short cuts!

You also need to be fully aware of the safety implications of working on Valve TV's and 'live chassis' sets.

Good luck
Nick
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Old 18th May 2020, 11:13 pm   #8
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Default Re: Safe to apply mains?

Hi,

I did think about the LOPT, and did consider taking it out of circuit for a while, just to see if any valves glow, and if the CRT heaters glow, as that's what I have been told previously (Although for a Philips G8)

I've always read that mains smoothers should be removed or replaced before power was applied, for any reason. I did actually have one go off with a very loud bang and lots of smoke, won't be forgetting them any time soon.

I'm hoping to get a converter some time soon, hopefully not too long either.

I was lucky to find this one, it was in my town and I got it for 15, the cheapest 405 line set I've seen for sale on that site. It'll definitely be a good way for me to learn more about the basics of a proper TV set.

I've been following the other BT302 thread, so I'll try use that as a guide where needed/possible.

Thanks
'77
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Old 18th May 2020, 11:45 pm   #9
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Default Re: Safe to apply mains?

Quote:
Originally Posted by 19Seventy7 View Post
Hi,

I've been following the other BT302 thread, so I'll try use that as a guide where needed/possible.
I would ignore that thread completely as it will totally confuse you! There are much better ways of proceeding and getting to the point of at least producing a raster (blank but illuminated screen).

15 was a really good price especially if it was local to you. That's the sort of price these things should be!

You can wait until you have got to the point of getting a decent raster before rushing out and getting a standards converter!

The Aurora converter has been a really great piece of kit, but having searched about for a while, I'm not sure they are still available. However, the Hedghog converter is equally fantastic and forum member (Freya) was offering them as built units at a great price.

The first thing is to get a copy of the circuit diagram and then to post some pictures of the internals of the set.

Ah, I see you have already done that- well done!
There is lots of information on this site about the BT302. (type it into the search box at the top of the site) Have a look at some of the older threads. That should give you more of an idea of what you are dealing with!

Cheers
Nick

Last edited by 1100 man; 18th May 2020 at 11:54 pm. Reason: more text added
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Old 19th May 2020, 7:03 am   #10
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Default Re: Safe to apply mains?

I normally do a quick but cautious 'viability check' on vintage radios before I start spending money on it.

However I have never tackled a valve TV.

Apparently you can do a simple cathode emission test on a monochrome CRT (assuming 6V heater) using a 6V power supply, a 1K resistor and a multimeter.

Does anyone on this forum do simple tests like this before starting a full TV restoration?
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Old 19th May 2020, 7:29 am   #11
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Default Re: Safe to apply mains?

It sounds like the CRT is the original GEC, but most likely to have been made by Mazda (as were most of the GEC branded valves in the TV). Mazda CRTs have a poorer reputation for long life them most Mullard types. So, I would certainly try to check if the CRT is viable first.

I expect this CRT will have a 12.6V heater.
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Old 19th May 2020, 8:19 am   #12
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Default Re: Safe to apply mains?

CRT's that have not been used for many years seem to loose their emission. It's almost like the cathode gets covered in dirt & cobwebs!

Before starting a restoration, out of interest I usually hook the CRT up to the tester. Almost always they read zero, or very poor emission. If I'd have taken that at face value, I would not have proceeded with any restorations!!

Once the set is working well and the tube can be left with a picture, the emission often improves over several hours. If it doesn't, then I am quite happy to give the tube a gentle 'tickle' with the reactivator or failing that a full 'boost'

In all cases, my sets have ended up with perfectly watchable tubes.

My Sobell 1000 is a case in point. The emission read almost zero with the needle barely moving on the tester. It actually produced a perfectly watchable picture even without rejuvination- for 10 minutes that is until the LOPT went pop.....!

So I never worry about the CRT in the early stages because I know that the LOPT will give me way, way more aggravation!!

Cheers
Nick
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Old 19th May 2020, 5:41 pm   #13
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Default Re: Safe to apply mains?

Doesn't it just! I have to admit it's been useful for me, especially with the lists of parts, as I'm not great at reading diagrams, and a fair few of the caps in my set are hard to read.

I've been looking at getting a 405 line set for some time, but the prices were always so high, so 15 was a no brainer for me, especially being a 2 minute drive away!

I was planning on getting a 625 to 405 converter back around Christmas, but decided against it as I didn't actually have a 405 line set at that time, but now I do, there's no question of me getting one.

The older threads will definitely come in handy, if needed, photos attached.

Quote:
I normally do a quick but cautious 'viability check' on vintage radios before I start spending money on it.

However I have never tackled a valve TV.

Apparently you can do a simple cathode emission test on a monochrome CRT (assuming 6V heater) using a 6V power supply, a 1K resistor and a multimeter.

Does anyone on this forum do simple tests like this before starting a full TV restoration?
I'm much more cautious with TVs than radios and such, because of the LOPT and EHT. Much easier for something to go wrong, in my opinion.

Have you any more information on the DIY CRT tester? Perhaps something I can do before going any further.

Quote:
It sounds like the CRT is the original GEC, but most likely to have been made by Mazda (as were most of the GEC branded valves in the TV). Mazda CRTs have a poorer reputation for long life them most Mullard types. So, I would certainly try to check if the CRT is viable first.

I expect this CRT will have a 12.6V heater.
I'd heard that about Mazda, it'll be my first Mazda tube, so i'll be curious how they compare to my other sets' tubes, although they're about 20 years newer than this set.

Would a 12V power supply work for the DIY tester? I'll just account for a lower reading, but it'll still give me a rough guess.

I've hope for the CRT, as I say, the set looks relatively low hour, and one of the better examples of the PCBs I've seen from other threads (No burns etc) so I guess it's fingers crossed until it's safe to power it up.

Perhaps OT, but I'm just curious, how hard would it actually be to rewind a LOPT? I'd imagine it'd be the same as a normal transformer?

Thanks
'77
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Old 19th May 2020, 6:20 pm   #14
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Default Re: Safe to apply mains?

Sieve head here forgot to attach the photos...

Apologies
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Old 19th May 2020, 10:05 pm   #15
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Default Re: Safe to apply mains?

Never heard about damaging a good flyback from just powering up a set a couple of seconds with the proper precautions
what i do hear is people damaging output tubes from using a dim bulb on tv sets wich is NOT advisable. if the oscillator doesnt start the tube will redplate very quicly. plate current on the tube will jump from say 50ma to 200 300 or more
just remove the horizontal output plate cap
if the mains smoothing filters are OK nothing will blow or pop, if it does, who cares right-? nothing important will be damaged because no tubes are conducting yet.
if you are afraid just reform and test those caps first. they are the main source of fuse pops and bangs on a first powerup BY FAR
when the tubes warm up, get a screwdriver and touch the plate cap to the horizontal output. make sure brightness and contrast are at maximum
if you dont get a raster quickly or if you hear any weird noise from the flyback "screaming" just take the cap off again. there is NO way you will damage the flyback even if there is a dead short on it or around it. moisture is a problem yes but i like to tackle it only after set's viability is proven. i have seen enough people here and on other forums destroying good flybacks by messing too much with them, just search here and they will show up. the intetion is good but they are old brittle components that should be left alone even brushing off too much dirt can do harm.


i have plenty of restored sets, usually with a good sucess rate, this is a good method and i would not advise it if i would not have proven it. just act with caution to avoid personal injury and also try to avoid frustration and time and money wasting. i know what is like to spend 20 or 30 caps just to have a bad vertical output transformer in the end or a bad flyback etc etc

i have a crt tester and i use it prior to tackling a set but many people dont have one so another reson to go the route "doing the minimal to get a raster" first

after i get a raster first part i replace is the boost filter cap and that is by far the part that can do more damage to a flyback if it leaks or shorts out
very often if you do a bulk recap a mistakewill be made somewhere and it will be VERY difficult to trace. if the set is able to be powered up you can change 3 or 4 caps and keep testing, if a fault appears just go back a couple of steps
VERY often after bulk recapping faults will appear instead of faults cleared. that story is heard over and over again everywhere

for example i had a set that worked perfectly with all the leaky caps and the vertical collapsed with new caps....weird right? reason was the cathode bypass cap on the pcl84 was shorted and the cathode resistor overheated and got open when the set was being used in the past,so the leaky shorted cathode cap was making the turn of the resistor and making the vertical work perfectly. then after replacing it with a new cap it stopped working because the cathode was left floating...just an example. the fault was very easy to find because the recap was made in steps with testing in between... just imagine a number of simultaneous faults that could be a nightmare...

wish you best of luck with your set and will help if i can

Last edited by PortugalTV; 19th May 2020 at 10:24 pm.
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Old 20th May 2020, 1:41 pm   #16
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Default Re: Safe to apply mains?

That's one of my queries, although. Do I have a good LOPT or not? We're not to know until power up. I'd rather not stress the set out as it's probably been sat dormant for the best part of 50 years. I'd rather try once there's newer components in, that'll be kinder to the set.

I've no means of reforming a capacitor, so that doesn't seem likely for me to do, unfortunately.

Being my first valve TV, and only, I want to do this right, so i'll hold off with power for now, until it's in a better state.

Thanks
'77
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Old 20th May 2020, 4:44 pm   #17
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Default Re: Safe to apply mains?

Has anyone suggested you make a lamp limiter for powering up valve radios and TVs for the first time?

It's very simple, just a 60 or 100 watt incandescent bulb in-series with the mains supply to the set. Search this forum for details.

It takes some of the electrical "strain" off the set and allows the main electrolytics to reform without overheating.
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Old 20th May 2020, 5:09 pm   #18
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Default Re: Safe to apply mains?

I was actually suggested to, but I can't get to any stores to buy a plug socket to make one with, as I don't drive, and they're too far away to walk, being the other side of town.

I couldn't even get a lift as no one I know is willing to drive for anything but work and/or food, so that's a snag.

I'll happily wait until after lockdown, when it's easier too. I was just curious if I could do it now, without a lamp limiter.

Thanks
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Old 20th May 2020, 5:54 pm   #19
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Default Re: Safe to apply mains?

I use a table lamp (when SWMBO is not looking). It's the incandescent bulbs that are in short supply in this house.
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Old 20th May 2020, 6:03 pm   #20
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Default Re: Safe to apply mains?

How would that be set up? I've a few that can be used.

Thanks
'77
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