I've been doing some experiments lately on increasing emission of mediocre/poor CRT's. The set I've done this to is the 'unboostable' trinitron type, a KV1320. This set was pretty low having poor tracking and focus, just a nasty flat, faded greenish tinted pic was the best I could get.
The idea was given to me by something I've read somewhere of setting up the heaters and drawing a very small current for a number of hours. I elaborated on this, looking at the effective emissive area of the cathode, (Around 2-2.5mm) a 'good' one should easily pass around 5mA without damage using G1 as the 'anode', for a short period.
This was confirmed using one of those cheapo 6" B/W portables, popular a few years ago, that had remained unused, 4mA was drawn with the set up described below.
The set up was as follows:-
- A 6.3V heater Transformer and a 12V lighting transformer were used.
- The 6.3V supplied the heaters of the CRT to be worked on and also supplied into the 12V winding of the lighting transformer, one rated for 50W that I had around.
- The mains connections of the lighting transformer therefore derived around 115V which was fullwave-rectified and smoothed by a 47uF 450V cap.
- A Series Current Limiting resistor of 33K was connected to the positive of this 'HT' supply derived across the 47uF cap, which after rectification, smoothing etc, read 145V. I wanted to keep the voltage fairly low so as not to cause any of the usual arcing that destroys the cathode material like often seen with mains derived and some commercial 'boosters'.
- A Voltmeter was attached across the resistor to monitor current draw.
The current with the 33K is limited to a little over 4mA (4.4mA ish) - The heaters of the KV1320UB were connected and allowed to run for 10-15 mins to fully heat up.
- The negative of the 145V was connected to a croc-clip to connect to the cathodes of the three guns independently. The positive via the 33K was attached to the common G1.
On application of the cathode connection, the initial voltage across the limiter R was 20V. Very slowly this increased over a period of minutes to 80V odd, then started to go down. At this point, I disconnected and went on to the next cathode. Same again, although the highest volts was 98V before it started to fall again. Last gun did much the same, with 78V before the volts went down.
The tube was then left with only heaters for 10 mins.
On application of the cathode connection to the first gun 133.5V was found and the current was drawn through that cathode for 1 minute before going on to the next. Again 133.3V or thereabouts was read and again held for 1 minute. The last gun read 134.3V and the current drawn for 1 minute.
This time, the voltage did not fall with any of the cathodes, it remained above 133V.
The tube was then left for another 10 minutes and re-tested by seeing how much volts derived over the current-limiter. All three cathodes drew 133.4-134.3V, which equates to around 4mA. The tube was then left to cool without heaters or 'HT' applied. Then tube re-heated (6.3V, no over-volts) and re-tested. The same 133 odd volts was derived straight away indicating an increase in emission, over what the initial reading was.
Now here's the rub! The 3AT2 EHT rectifier has died in this set (O/C Heater) so will not be able to confirm whether theres any improvement or not until I receive a replacement. I ordered one from the USA this morning. The above was copied from a post I made on 'That Forum' where it gained apparently little interest apart from one reply.
Since then, Ive received the new EHT Rectifier valve, (3AT2) and fitted it earlier on. I tried out the set and cannot quite believe the picture quality. Where before, it was faded, green tinted and somewhat de-focussed with a little red flaring, it appears transformed, since setting up. The picture is I would say near what a new CRT would have shown. Bright, Sharp and grey-scale tracking perfectly.
Its a shame that the test is somewhat unequal, as the set also had a new EHT rec due to the original failing O/C heater. The set beforehand didnt show any signs however of an EHT rectifier issue like ballooning etc, the rec heater just quit.
I feel this 'non-destructive' method is worth a try as it places little strain on the cathode's oxide coating and no spark erosion of material which leads to either a very short life or destruction of the cathode entirely, also the important clearance between cathode and G1 is maintained due to no erosion.
As the above is a Trinitron type tube, the cathodes are very small. (About 1.5mm diameter) as illustrated above, I limited current to 4.5mA. A more usual CRT with its larger cathode surface area I'm estimating by size(About 2.5mm) should take around 8-10mA.
If anyone tries this out please let us know how it turns out.
I'm hoping that others have success with this, it would alleviate the strain on whatever NOS/good CRT's are still around and give life to an otherwise dead set. How effective this is on a Really poor/dead tube I've not tried. The Sony was pretty grim but not completely dead, and I have no other 'dead tubes' to play with!
Hopefully it isn't just a flash in the pan The Sony has been running now for a few hours with no signs of issues. I did at one point temporary reduce the heater-volts to 5.5V to see what if any effects there were. I didn't see any apparent picture issues at a reduced voltage.
... Hopefully it isnt just a flash in the pan ...
I thought that was what you were trying to avoid in the first place ...!
Excellent result!
When all else fails, read the instructions
The established theory of cathode emission-loss over time is the barium and thorium metal deposited on its surface evaporates, leaving the surface devoid of emissive components. The heating to a higher temp than normal is one way to drive the thorium and barium metals to the surface--and it stands to reason? that a high current flow at high temps would strip/evaporate this new material away.
The idea to draw only a very low current at say,20% overheat therefore would allow the emissive material once at the surface to remain there and accumulate. I have a theory that CRT's which always seem to operate at very low currents--only 100 odd uA form a type of 'cathode-interface', a known issue that first appeared during the days of early computing where some valves remained at cut-off for some considerable time. Special cathodes were developed to combat this for the 'SQ' type valves used in later years.
This sort of seems to be confirmed by the fact that worn tubes seem to fall off in the frequency --light to dark response as seen on a test-card grating for instance. My idea was to draw a comparatively high (in relation to normal beam-current) through the cathode, but not so high that would cause it any permanent damage, as is the case of a normal tube reactivator.
It may well be found a combination of both high heat of cathodes and low/medium current through cathodes will be beneficial for some tubes. Time and experimentation will tell. I'm also a firm believer in reducing the heater voltage of a CRT somewhat to extend its life. Those worried about 'cathode-poisoning' under this condition need not worry, that only occurs when the electron cloud is insufficient to keep at bay any Ions present, --which at such low beam-currents tubes run at, and good cathodes able to supply many 10's of the current of the beam, this effect doesn't present itself.
The Sony sets for instance often drive the heaters too hot--the voltage of the 1320UB for instance is/was 6.7V (derived from a mains Tx). I've placed a series-resistor to lower this to just 6.0v.
Definitely a tube by tube basis for current....
Start off low--If that does it, stop there, but if not try a little more being careful on the type/size of the cathode/s
I think the very most important bit is not ever to allow the cathode to arc, I can think this can only be done by keeping the voltage below say--200V, again depending on tube type. Big old mono guns with large clearances between C-G could well tolerate much more, but tiny delicate things say--a Rigonda 6" with a pin-head cathode I doubt would tolerate much more than 100V and 1 mA
Bring on the CRM121s ?
I feel a little box coming up with a small transformer & rectifier, a resistor (perhaps a 5 watt pot) and a milliameter or a couple of terminals. Hmmm!
Ah Great stuff!
Ive not run the Sony for a few days, Ive been messing with the Bush. I did think of getting hold of a tube-booster from ebay, summit like a B-K and modding it up. If I can get a cheapo one, I might just do that. It would be nice to see the improvement in the same 'scale' as an official tester has....
I have a selection of rather old (poor) 12 and 15" mono crts.
I am very interested in trying to bring these back to some form of life.
I was very interested in this post but my small brain is getting stuck on this quote:-
On application of the cathode connection to the first gun 133.5V was found and the current was drawn through that cathode for 1 minute before going on to the next. Again 133.3V or thereabouts was read and again held for 1 minute. The last gun read 134.3V and the current drawn for 1 minute.
Does this mean I need to connect the cathode and the gun together for this reading?
I would be very happy to try this out and post back my results.
Cheers
Rob
The only connections I used were the heaters, The Cathode and The First Grid. My reference to 'gun' was for the Colour CRT I was messing round with. ie, moving onto the 'next-gun' meant that I was then working on a different Cathode and Grid of the same CRT..
I think I started with the 'Blue' moved on to the Green, then the Red. For your Mono CRT's you'll only have the single Cathode-Grid to worry about.
Magic...thanks for that!!
My poor old brain is tired at the moment!
Cheers
Afternoon all.
Monday...Shop shut today....time to experiment!
Right I am now getting set up to test some VERY low old 9" & 12" round tubes. Nothing to lose! They all measure naff all on the CRT tester
1 x Mullard 31-18
1 x Cossor 121K
1 x Mazda CRM92
I'll report back tonight with the results.
Cheers
Rob
Here are the findings of today's fiddling, this method works! I am now the proud owner of a great 12" Mullard tube. It had been bopped years ago (little card on side of tube confirmed this). Popped in the box and marked knackered by the old TV shop they came from. Sadly the Cossor didn't fair so well but the emission increased from nothing to half way into the red. The mazda was the same but increased a little more.
So. It would seem that the Mullard tube seemed to fair best. These tubes were knackered so had nothing to lose. I kept them just in case I needed to send on to be re-gunned! I hope i have made sense in what I have written. Im naff at explaining things!!
I would like to give this ago with my CRM 92, but I really don't want to kill it off, as I have just restored the set that it's in. The set has a separate heater transformer, so I can use that, I have a variable HT supply that goes from 0-360v at 300ma, but I really need to be careful to limit the current, any advise?
Cheers
Mark
Here are the findings of today's fiddling, this method works! I am now the proud owner of a great 12" Mullard tube. It had been bopped years ago (little card on side of tube confirmed this).
Thats a good result on the Mullard! ?
You mention that particular tube had been 'bopped' in the usual way before too? I have an old Hitachi CSP680 thats been done a few times in the conventional way, maybe I'll give that one a try, Its Really very bad. Seriously doubt there's any cathode left in it though. Your other two, maybe increasing the heater a little at a time to start things off may help. Time it seems helps some tubes, just leave them with say, 7V heaters (assuming 6.3V normal) and 'HT' applied for a while and monitor them.
I would certainly consider it. Its a fairly low-risk process, there's no destructive arcing going on, it may take some time for anything to happen, The Sony tube I did originally took a little while to get going. Start off low, say max current, 5mA (33K, 150V) try that, then increase if no apparent improvement. I wouldn't go much above 15mA though, the cathode in any even big old mono tubes is quite small.
I would like to give this ago with my CRM 92, but I really don't want to kill it off, as I have just restored the set that it's in.
I think providing that you take your time, let the CRT warm up well say at least 10 mins before you try anything, (I think this pre-heating stage may be important with tired old cathodes) limit the current by resistor first to 5mA and keep the voltage at around 150, there'll be no arcing to cause an issue, I very much doubt it will kill it.
How bad is the CRT in the set? If the picture is reasonable and you haven't a source of, or a spare CRT, maybe enjoy the set for a while till it gets too bad.
Try running the really knackered ones (that have not improved enough to be used) now with NO HT and +25% heater for about 4 hours and repeat the HT procedure? What is there to lose?
How bad is the CRT in the set? If the picture is reasonable and you haven't a source of, or a spare CRT, maybe enjoy the set for a while till it gets too bad
The picture is dim but watchable, but does go negative with the contrast or brightness turned up, I am waiting for delivery of an EHT capacitor to replace the visconal, from experience this could give me an extra 1Kv of EHT, I also have a NOS EY51 to try as well, but if it is still dim I will give it a go.
Cheers
Mark
The heaters I'm assuming are tungsten so can put up with 2500 degrees. Worth keeping in mind the boiling-point of the barium metal that does the emission is around 2100 a bright yellow heat. Boiling implies to me evaporation (and loss) of this material. Maybe I'm wrong its all a bit too nuclear physics for my addled brain!
If evaporation does occur where does the stuff collect? Probably condenses on the cooler grid close by, what effect that has I'm not sure 'grid emission' maybe?
I think what we are trying to do is to drive fresh material from deeper in the coating to the surface, keep it there and also remove the interface layer that I Think causes resistive and presents a capacitive 'layer' between the surface and the metal of the cathode cap. Most 'boosters' do increase the heater-volts during reactivation by 25% ?, so guessing it cant be too destructive if done carefully.
I wish I had a few really bad unboosted CRT's to mess around with. All I have is an untested A56-120X, a good A49-91W, and that Sony which now is good.
Guess I like to err on the cautious side.
For want of something better to do and to find whether it was any good, I tried the procedure on the Mazda A56-120X CRT I rescued in the Thorn 3000 from the tip. Each cathode started out giving me 4mA ish. (145V, 18K, 6.5V heater) This quickly rose to 7.5mA within half a minute
I left the CRT cool, re-heated and re-tried. I got readings of 7.5mA on all cathodes immediately. So I have what looks to be now a good A56-120X CRT and No Philips G6 to put it in, Doh!
I have been following this with interest as I have an Ultra BBC only set with a Mazda CRMXXX in it, it is in the spare room awaiting it's turn. My main worry was that the crt would be flat, I should now be able to find out if there is hope! Reading back issues of Television magazine it seems the Mazda Crm series were going flat not long out of guarantee, there was even mention of this being designed in by Mazda!
I had a Bulb bopper back in the days of delta gun tubes and found that it worked well on Mullard 90 degree colour and most mono tubes. I found the best way to extend the life of PIL and 110 tubes was to overrun the heaters slightly, A lot of sets had a resistor in series with the heaters that could be dropped slightly, KT3 s K30s etc we would start with shorting half of a heater choke on the crt base...It was sometimes years before you were down to one and a half coils shorted! The ones I didn't have a lot of luck with were the Toshiba tubes as fitted to some GEC sets and Decca/Telefunken 100 chassis they seemed to go intermittent h/c short or flat very quickly if messed with! Just my 4d th!
Interesting thread! ?
Rich.
