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 Post subject: Increasing CRT filament voltage
PostPosted: May Tue 28, 2019 3:34 pm 
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Joined: Jun Wed 13, 2018 9:18 pm
Posts: 60
So I picked up a pretty TV a year ago. I got the TV home, tested dead and had a brightener.

Yesterday I did a bunch of reading around and I ended up trying out a CRT test circuit and turning the 6.3v filament supply up to 9v for an hour and then turning it back down and letting it stablize. It stablized at a higher cathode current than it was before, from about 337uA to about 450uA. After seeing this result I decided to leave it at 9V for a few more hours and the results continued improving. I opted to leave it at 9V overnight and the results are still improving with each minute.

How long can a filament handle power like this? It's at almost 150% nominal voltage which I'm not too confident about. I want to keep burning in the cathode until the results stop improving, but I want to know if there's some danger point I may be approaching. I don't want the filament to open.

Tube RCA 16GP4 metal cone CRT

Edit: tube number

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Last edited by SixFiveZeroTwo on May Wed 29, 2019 3:57 am, edited 1 time in total.

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 Post subject: Re: Increasing CRT filament voltage
PostPosted: May Tue 28, 2019 3:38 pm 
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Location: Detroit, MI USA
There is danger with anything.......and by doing that you may have already ruined the tube for future operation at the specified voltage IF you are expecting the highest emission level.

The old timers used to say that when CRT's were rebuilt, and when new ones were manufactured, the filament was operated at up to twice the nominal voltage for a specified period of time to activate the cathode. Yes, there's danger of burning it out at any voltage higher than the rated voltage, because some have burned out at the rated voltage in normal use.

What will almost always happen if you run a tube that has lots of use at extremely higher voltages is the tube may show more emission at that voltage, but it will always drop off when dropped back to the specified voltage. Therefore after that, you have to run it at a higher voltage, usually with a brightener, to get a decent picture.

And you don't need much more than a couple hundred microamps emission on most B&W CRT's to get a very watchable picture. They don't usually need to even come up into the green area to work OK. I have seen plenty of them that work nicely at 100 microamps or so.

Most sets that have an old CRT which has been run for a long time already with a brightener will not be able to successfully operate without one, but may get significant extra useful life with the brightener.

Remember those older sets were designed to be watched in dimly lit rooms, particularly the non-aluminized CRT's which includes the metal types. Under proper conditions, they are very watchable. Later sets will be watchable in moderate lighting, but the earlier ones won't be.

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 Post subject: Re: Increasing CRT filament voltage
PostPosted: May Tue 28, 2019 3:42 pm 
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I don't know what the "overheating" does to improve cathode performance, but I can promise you that it will shorten the life of the filament. If you are going to use the set on a daily basis, you are eventually going to need a new CRT---thus there is maybe nothing to lose by the "rejuvenation". OTH, if you just want to demo it once a month, I'd consider leaving it alone.

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 Post subject: Re: Increasing CRT filament voltage
PostPosted: May Tue 28, 2019 4:11 pm 
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Location: Santa Rosa, CA
Rejuvenators were popular in the late 1960s. We had one at Dr. Bob Casey's TV Hospital in Chicago.

http://antelopearcade.com/files/BK-Prec ... 28b%29.pdf

This was only a last resort on a CRT that would have been junked. Like re-forming capacitors, it's pretty much beating a dead horse. No idea how long it will last.

Rich

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 Post subject: Re: Increasing CRT filament voltage
PostPosted: May Tue 28, 2019 6:25 pm 
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Location: Montvale NJ, 07645
Your results will be very short lived. Let the tube sit a month, or less, and you will be back to square one. I have never had any sustainable improvement doing this.


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 Post subject: Re: Increasing CRT filament voltage
PostPosted: May Tue 28, 2019 9:37 pm 
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Location: Metamora MI, 48455
Just increasing the filament voltage will give a small increase in beam current, but it will not last. I destroyed a 19AP4 doing this back when I first started restoring TVs. The 19AP4 probably would have produced a decent, if slightly dim, image even weak.

On the other hand, temporarily raising the filament voltage very slightly and drawing a fairly large amount of current from the cathode in a controlled manner can, and often does, give a semi-permanent improvement beam current. This is exactly what my favorite CRT tester, the Beltron, does in operation. With the Beltron, the operator is the control element. You can be as aggressive or as gentle with the tube as you like. If you follow the directions to the letter, you aren't going to destroy a cathode with it. In fact, out of the countless tubes I've tried to resurrect with the Beltron, I've had only two that just wouldn't respond. On the other hand, it brought a very dead looking 24AP4 back to life, and it has been working fine, and testing fine, ever since. That was at least 5 or 6 years ago now.

The other commonly used method of rejuvenating a tube is not recommended, namely charging up a decent sized electrolytic and just "letting it rip". This more often than not finishes off a tube that could otherwise be salvaged with the Beltron. Eico, RCA, B&K, some Heathkits, and a handful of others were infamous for using this method.

The next natural question is why you would even want to rejuvenate a tube in the first place? For most tubes, this simply doesn't make sense. It's usually easier and less frustrating to track down a known good, strong replacement or simply live with a slightly dimmer picture.

Now if you have something that is truly irreplaceable, like a 15GP22, 21AXP22, or a DuMont 20BP4 or 30BP4, etc, rejuvenation is probably your last good shot at a working set. Otherwise, I'd be on the hunt for a new tube. In fact, should you decide to replace the 17JP4 in your set there are two listed at the bottom of this list from the Early Television Foundation being sold by one "Allen Williams":

https://docs.google.com/spreadsheets/d/ ... It/pubhtml


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 Post subject: Re: Increasing CRT filament voltage
PostPosted: May Wed 29, 2019 4:08 am 
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benman94 wrote:
Just increasing the filament voltage will give a small increase in beam current, but it will not last. I destroyed a 19AP4 doing this back when I first started restoring TVs. The 19AP4 probably would have produced a decent, if slightly dim, image even weak.

On the other hand, temporarily raising the filament voltage very slightly and drawing a fairly large amount of current from the cathode in a controlled manner can, and often does, give a semi-permanent improvement beam current. This is exactly what my favorite CRT tester, the Beltron, does in operation. With the Beltron, the operator is the control element. You can be as aggressive or as gentle with the tube as you like. If you follow the directions to the letter, you aren't going to destroy a cathode with it. In fact, out of the countless tubes I've tried to resurrect with the Beltron, I've had only two that just wouldn't respond. On the other hand, it brought a very dead looking 24AP4 back to life, and it has been working fine, and testing fine, ever since. That was at least 5 or 6 years ago now.

The other commonly used method of rejuvenating a tube is not recommended, namely charging up a decent sized electrolytic and just "letting it rip". This more often than not finishes off a tube that could otherwise be salvaged with the Beltron. Eico, RCA, B&K, some Heathkits, and a handful of others were infamous for using this method.

The next natural question is why you would even want to rejuvenate a tube in the first place? For most tubes, this simply doesn't make sense. It's usually easier and less frustrating to track down a known good, strong replacement or simply live with a slightly dimmer picture.

Now if you have something that is truly irreplaceable, like a 15GP22, 21AXP22, or a DuMont 20BP4 or 30BP4, etc, rejuvenation is probably your last good shot at a working set. Otherwise, I'd be on the hunt for a new tube. In fact, should you decide to replace the 17JP4 in your set there are two listed at the bottom of this list from the Early Television Foundation being sold by one "Allen Williams":

https://docs.google.com/spreadsheets/d/ ... It/pubhtml



I did what you described. I put the filament voltage up to about 8.96V overnight and it had gone up to somewhere around 3.5mA of cahtode current, and it was still slowly increasing when I reduced it back to 6.3v. I'd rather break it trying to fix it than just settle.

I actually had the wrong tube number, it's a 16GP4, not 17JP4. I've looked around a little to find new CRTs, I've only found one guy in a 50 mile radius who has a small room full of NOS vintage CRTs but he doesn't have the one I need.

If you know of CRT vendors anywhere in between Minneapolis and Chicago, that would be helpful.

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 Post subject: Re: Increasing CRT filament voltage
PostPosted: May Thu 30, 2019 12:03 am 
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Joined: Jun Fri 19, 2009 6:34 pm
Posts: 8923
Location: Long Island
Quote:
On the other hand, temporarily raising the filament voltage very slightly and drawing a fairly large amount of current from the cathode in a controlled manner can, and often does, give a semi-permanent improvement beam current. This is exactly what my favorite CRT tester, the Beltron, does in operation. With the Beltron, the operator is the control element. You can be as aggressive or as gentle with the tube as you like. If you follow the directions to the letter, you aren't going to destroy a cathode with it. In fact, out of the countless tubes I've tried to resurrect with the Beltron, I've had only two that just wouldn't respond. On the other hand, it brought a very dead looking 24AP4 back to life, and it has been working fine, and testing fine, ever since. That was at least 5 or 6 years ago now.

The other commonly used method of rejuvenating a tube is not recommended, namely charging up a decent sized electrolytic and just "letting it rip". This more often than not finishes off a tube that could otherwise be salvaged with the Beltron. Eico, RCA, B&K, some Heathkits, and a handful of others were infamous for using this method.


I think you've got it backwards. The Beltron was a real beast, 220-uF capacitors hooked up in a voltage doubler circuit discharging between the first grids and cathodes of CRTs through a pair of 7-watt, 220-volt indicator lights in parallel to limit the current. The heater voltage would be boosted for several minutes in the "cleaning" mode (which also drew current from the cathodes) to get them as hot as possible, then the heater would be shut off while the "restoring" was going on with the high voltage and current. This ensured quite a fireworks display inside the necks of CRTs that had a lot of cathode contamination to burn off. This process removed a lot of the emissive material from cathodes so a CRT might last a year in constant, daily operation after the first rejuvenation, then perhaps several more months after a second visit to the Beltron. The third time would likely strip one or more cathodes bare (in color CRTs), after which the tubes were junk.

The problem for service shops was that there was no way to know how many times a CRT had been "belted" before you got the set, so it was really hard to judge whether you were going to improve the performance of a TV, how long any improvement might last, or if you were going to kill it altogether. The older B&K, Eico, and other CRT testers smaller capacitors, typically 8-uF or so, and would only throw a single tiny spark when their "rejuvenate" buttons were pressed. They also did not cut off the heater voltage. This would improve the performance of a weak CRT if the cathodes were not too bad, but it wasn't enough to burn through really heavy contamination. So there were plenty of instances where they failed to make weak tubes better, but they didn't make any worse. We learned through bitter experience which tubes should only be rejuvenated on the B&K and which had the best chances of good results with the Beltron.

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 Post subject: Re: Increasing CRT filament voltage
PostPosted: May Thu 30, 2019 10:20 am 
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Joined: Sep Thu 23, 2010 6:37 am
Posts: 11807
Location: Powell River BC Canada
Metal cone CRTs. Ouch was my first recollection.

I would have tested the CRT first. Then put a small test CRT in to look at the picture
if any. Answers this. Is CRT bad? Is TV good otherwise?

With the brightener I would look at the picture very carefully to see if there
was any tendency towards a shimmering to negative image. Poisoned cathode.

If it is bad, a trick. try a width sleeve under yoke, and if a smaller raster
with a lower brightness works better use it.

Also an isolating brightner might be needed since higher temps lead to
hk leakage.

Can you point to a schematic for your TV. ?

i remember open video peaking coils wound on resistors that
washed out the contrast.

Nice Topic. I forgot that mostly I was just a TV repairman.

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 Post subject: Re: Increasing CRT filament voltage
PostPosted: May Thu 30, 2019 5:48 pm 
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Joined: Jun Wed 13, 2018 9:18 pm
Posts: 60
radiotechnician wrote:
Metal cone CRTs. Ouch was my first recollection.

I would have tested the CRT first. Then put a small test CRT in to look at the picture
if any. Answers this. Is CRT bad? Is TV good otherwise?

With the brightener I would look at the picture very carefully to see if there
was any tendency towards a shimmering to negative image. Poisoned cathode.

If it is bad, a trick. try a width sleeve under yoke, and if a smaller raster
with a lower brightness works better use it.

Also an isolating brightner might be needed since higher temps lead to
hk leakage.

Can you point to a schematic for your TV. ?

i remember open video peaking coils wound on resistors that
washed out the contrast.

Nice Topic. I forgot that mostly I was just a TV repairman.


I don't mean to sound unthankful, but you talk like everybody has a TV repair shop in their garage. If I had an extra CRT I wouldn't be ruining the one I have.

Here's a link to the SAMS for my TV. https://elektrotanya.com/rca_6t71_t74_t ... nload.html

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 Post subject: Re: Increasing CRT filament voltage
PostPosted: May Thu 30, 2019 7:25 pm 
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Posts: 14390
Location: S. Dartmouth MA 02748-1225 USA
The remaining life in the CRT filament/cathode is essentially unknown despite $$$ rejuvenation/testers tell the technician.

I always gave fair warning while rejuvenating a CRT with the customer present. I never killed a CRT but half the time the CRT did not come back. I had one customer that had me rejuvenate the CRT several times over a period of months until they could replace the set.

The customer gets desperate as the TV keeps the kids in the household from tearing the place apart. A rather poor excuse for a babysitter...

Customer wanted to get the last of the life out of the set and not replace the CRT.

I still got my $$ for the service call :)

Point is if the CRT is too dim to watch, then a brighter is used. If still too dim then a rejuvenation. If the CRT dims out rapidly, there is gas and the ions are beating the crap out of the cathode coating, that CRT is essentially dead, a candidate for rebuild. A rejuvenated CRT can and often does last a long time. I have seen two boosters, series on a CRT combined with an isolation booster. Often gas will cause a non-reparable focus problem, electrons smash into gas molecule and get deflected.

YMMV

Chas

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 Post subject: Re: Increasing CRT filament voltage
PostPosted: May Thu 30, 2019 8:28 pm 
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Joined: Sep Thu 23, 2010 6:37 am
Posts: 11807
Location: Powell River BC Canada
Quote.

Idon't mean to sound unthankful, but you talk like everybody has a TV repair shop in their garage.

If this was 1958, maybe that might be so.

I did download the TV manual. Are you restoring the tv set, recap etc?

The set has DC restoration, and if that works properly, the CRT would a better chance
at a longer life.

That set also has syncro guide. A VTVM and old scope would be good be nice.
If a 100 mHz digital scope stuck it's nose anywhere close to the chassis, it get snuffed.


I don't collect, or restore TV sets. However if one of those sets ever darkened my door,
I might obsess for weeks just taking voltage readings.



Has the metal shell zapped you yet ?

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 Post subject: Re: Increasing CRT filament voltage
PostPosted: May Thu 30, 2019 11:01 pm 
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A couple of on the bright side things to consider...
CRTs that have been dormant for years can "fall asleep" the cathode is a reactivate metal similar to the getter and will act as a getter when there is no emission for a long time....When the cathode acts as a getter it develops molecular bonds with gas molecules and it's surface layer becomes an insulator. Emission burns that layer off so if you can get some emission and there is emissive surface under the layer the years have built then it will gradually wake up.

Any old schlub could buy a brighter and install it even if they didn't know the first thing about electronics.... brighteners worry me, but sometimes they were a failed last ditch effort by a DIYer to compensate for a different problem they weren't smart enough to fix...in those cases you can wake the tube, yank the brightener and have a good tube.

At 3.5mA I'm pretty sure you are drawing several times more current than it was rated to supply under normal use... might want to stop before you burn off the cathode.


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 Post subject: Re: Increasing CRT filament voltage
PostPosted: May Fri 31, 2019 6:33 am 
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quote

It stablized at a higher cathode current than it was before, from about 337uA to about 450uA. After seeing this result I decided to leave it at 9V for a few more hours and the results continued improving. I opted to leave it at 9V overnight and the results are still improving with each minute.

Thats .335 Ma.

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 Post subject: Re: Increasing CRT filament voltage
PostPosted: Jun Sat 01, 2019 4:30 pm 
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Chris108 wrote:
Quote:
I think you've got it backwards. The Beltron was a real beast, 220-uF capacitors hooked up in a voltage doubler circuit discharging between the first grids and cathodes of CRTs through a pair of 7-watt, 220-volt indicator lights in parallel to limit the current.


The restorers to avoid are the older ones that discharge a capacitor into the cathode with no current limiting at all. I have a B+K 465 that simply connects a pair of 50 uF 450v caps in series with each other across the cathode - g1. Before I knew any better, I did try it, and it almost always made things worse, and never had a lasting improvement.

The Beltron limits the current to approximately 100 mA with the lamps. It doesn't discharge the capacitors into the CRT, they're just the filter capacitors for the power supply. The problem with it is that it's a completely manual process and it's easy to overdo it. More sophisticated testers like the CR7000 do pretty much the same thing (connect a high voltage 100mA limited supply to the cathode), but have some automation to make it more foolproof.

The Beltron cleaning function is very useful since it runs at a lower current and the voltage is too low to cause fireworks (the current is limited by a #49 lamp). It's low risk, and is good for waking up slightly tired CRTs.


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 Post subject: Re: Increasing CRT filament voltage
PostPosted: Jun Sat 01, 2019 4:37 pm 
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Location: Grand Chute, Wisconsin
ac wrote:
The Beltron cleaning function is very useful since it runs at a lower current and the voltage is too low to cause fireworks (the current is limited by a #49 lamp). It's low risk, and is good for waking up slightly tired CRTs.

So the consensus I'm seeing is that the Beltron is the one to use for old CRTs like the roundie color ones?

If so, then this looks like it was a good deal: https://www.ebay.com/itm/Beltron-Pictur ... true&rt=nc

.


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 Post subject: Re: Increasing CRT filament voltage
PostPosted: Jun Sat 01, 2019 7:16 pm 
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wiscojim wrote:
So the consensus I'm seeing is that the Beltron is the one to use for old CRTs like the roundie color ones?

If so, then this looks like it was a good deal: https://www.ebay.com/itm/Beltron-Pictur ... true&rt=nc

.



It does a good job, but so do many others. Just avoid the old ones that discharge a cap straight into the cathode.


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 Post subject: Re: Increasing CRT filament voltage
PostPosted: Jun Mon 03, 2019 12:05 am 
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radiotechnician wrote:
Quote.

Are you restoring the tv set, recap etc?

The set has DC restoration, and if that works properly, the CRT would a better chance
at a longer life.

Has the metal shell zapped you yet ?



I'm going to replace all the capacitors that connect across significant voltage sources and put RCA composite video connection directly to the video amplifier. Literally no point in doing anything to the IF and tuner sections at all. And the filters are shorted, so I haven't turned the chassis on.

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 Post subject: Re: Increasing CRT filament voltage
PostPosted: Jun Sat 22, 2019 1:16 pm 
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Location: St. Louis, MO, USA
+1 on the “fall asleep” issue. I “woke up” a 10BP4 by just running it at normal cathode voltages. At first, it registered dead but gradually came to life. It took 2-3 hours to come to life but your mileage may vary.

Dennis


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 Post subject: Re: Increasing CRT filament voltage
PostPosted: Jun Sat 22, 2019 1:43 pm 
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Just wanted to mention that I had a mediocre 10BP4 in my 630TS. A long time repairman told me to go ahead and try a brightener and don't worry about ruining the tube. Using it sparingly as a collector piece, it's been displaying an excellent picture for many years now. I would bet that tube will outlive me! :)
There's a big difference in using and old set everyday as a primary TV, verses just occasionally as a collectible.


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