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simpsons 21st Nov 2020 1:12 pm

CRT Rejuvenation orientation
Is there a "best orientation" for a CRT when it is being rejuvenated.

I'm I right in thinking that the cathode surface when being heated will migrate due to gravity? If so would either placing the screen down or the gun down be an advantage?

I've got both Delta and inline (Sony) CRT's which when boosted work well and so believe a measured clean/rejuvenation would be a longer lasting solution.

Thoughts or experience on the subject would be very much appreciated.


Heatercathodeshort 21st Nov 2020 5:21 pm

Re: CRT Rejuvenation orientation
I have never placed a tube other than conventionally when using a booster. It's all a matter of a gentle approach but I very much doubt if the position of the tube makes any difference..
Some but not all reactivators blast the cathode to bits literally! I bought a new professional booster due to old Bertha looking a bit of a Heath Robinson affair and I sometimes used it with customers present. The new one destroyed every tube I tested! [Don't ask me for the make and model] I have a very old tester/rejuvenator that I call Bertha. She is very kind and never damages a tube. It was built by an unknown telly guy from an article in Practical Television December 1957.
I must have been back in the 60's when I discovered it in a skip around the back of an old radio shop that was being demolished.
A correctly reactivated tube will probably last for years. Regards, John.

mark pirate 22nd Nov 2020 8:44 am

Re: CRT Rejuvenation orientation

It's all a matter of a gentle approach but I very much doubt if the position of the tube makes any difference.
I too take the gentle approach, I have both the B&K 470 & 467 CRT testers, both of which have a "clean & balance" mode.
I have never killed a CRT using this method, in fact I have never used the rejuvenate mode.

I have tried CRT's in the service position and in the face down position, the results seem to be the same.
The idea is to blow off the crud from the cathode to expose fresh emissive material, some testers are quite crude in design, and can destroy the cathode coating completely!

As we all know, good replacement CRT's are getting harder to find, especially the early round tubes, if in doubt, leave as is, better to have a dim picture than none at all!


peter_scott 22nd Nov 2020 8:55 am

Re: CRT Rejuvenation orientation

Originally Posted by Heatercathodeshort (Post 1312896)
It was built by an unknown telly guy from an article in Practical Television December 1957.
Regards, John.



Heatercathodeshort 22nd Nov 2020 10:50 am

Re: CRT Rejuvenation orientation
1 Attachment(s)
The HT rectifier in the original data is an old selenium type. This needs to be retained. It's replacement with a silicon diode is not satisfactory.

I think this may be its secret. A clapped out rectifier will have a high resistance and almost self regulating low current, ideal for the CRT reactivation process.

I have made up a series of CRT connector leads for the various bases including a switched gun colour one.

I can give some details of the reactivation process using Bertha that differs slightly from the PT text.
Regards, John.

Jac 23rd Nov 2020 8:41 am

Re: CRT Rejuvenation orientation

Originally Posted by Heatercathodeshort (Post 1313054)
I can give some details of the reactivation process using Bertha that differs slightly from the PT text.
Regards, John.

Hi John,

Please do!
Any additional information concerning CRT reactivation is more than welcome.

Best wishes,

dragonser 23rd Nov 2020 9:23 am

Re: CRT Rejuvenation orientation
yes please, any info on this subject is most, most useful.
I have a Willow Vale 5a tester ( but no instructions with it ! ) .

simpsons 23rd Nov 2020 10:13 am

Re: CRT Rejuvenation orientation
Thank you Heatercathodeshort for introducing Bertha to the conversation.

I have B&K 470 which I have had variable results. Either it worked or ruined the cathode on Delta gun CRT's.

It is interesting how the different approach to cleaning the cathode has developed. The B&K raising the heater voltage and zapping the cathode with 450 volts whilst at the same time switching off the heater, Bertha, from my understanding using around 250 volts and raising the cathode temperature until no further improvement in emission and finally, in a "Trinitron only" design keeping the heater at 6.3 volts and applying 115 volts until again the emission has improved.

All of the designs requiring a high degree of trust and really user experience when, should the procedure result in the CRT being worse than when just the heater voltage is raised, all is lost.

This brings me to the question. Which is better. Leaving the CRT with a raised heater voltage or try rejuvenation?

Heatercathodeshort 23rd Nov 2020 11:19 am

Re: CRT Rejuvenation orientation
This is my method that has proved VERY satisfactory. I had a long conversation with Bertha last night and she told me some of her secrets. She is now 67 years old but informs me there is still a good tune to be played on an old fiddle.

1 Connect tube with suitable base or mini croc clips if you prefer. Make sure you connect the croc clips if you use them to the right pins. It can hurt if you don't.

2 Select heater voltage with switch and fine tune with min/max selector.

3 At this stage top left selector switch must be in heater position.

4 Bottom left 'Test CRT' switch in M/a position.

4 Switch on and allow tube to heat up for 5 mins.

5 Switch to m/a with v/ma switch.

6 Meter will then give a reading usually about a quarter/third on meter. It may fluctuate at this point and that is a good sign.

7 To reactivate turn heater switch to a lower setting. For a 6.3v tube I suggest the 2v setting. Experiment with it.

8 Meter reading will reduce then 'kick up' and 'kick back'. At this point tiny sparks may be seen between the grid and cathode of the gun assembly. This gentle action will clean away muck that has formed a microscopic layer on the emissive surface of the cathode.

9 When all activity has ceased the meter reading will reduce to zero. Sometimes it is necessary to reduce the heater volts to zero with action 7 above.

10 Reset heater voltage to normal. Meter should rise quickly to centre scale and remain constant other than a few short kicks as tube settles.

11 The tube should now show a first class picture and remain so for a considerable period.

Notes. For successful reactivation the tube has to have some emission. Gradually increasing the heater volts with the fine adjuster may be just enough to stimulate initial emission. With some stubborn tubes particularly Mazda 2v triodes and tetrodes a pre boost may be required from a lamp type booster with a 15w filament lamp in the grid supply to the CRT. Once some emission is noted by the dim glow of the lamp you may well find that Bertha can do her stuff.
2v Mazda triodes have a very delicate heater. It is unwise to increase this more than 20%. Failing this the heater will go O/C.

EMI tubes can be hit and miss. I have brought a zero emission EMISCOPE TA15 in an HMV 1851 back to life with Bertha alone but I risked all by over running it's 4V thick heater by 100% to stimulate emission. There was nothing to loose.

No initial emission can of course be due to O/C electrodes usually the internal cathode spot weld.

A picture that generally lacks sparkle with a slightly out of focus muddy appearance but will good emission readings is usually due to screen deterioration and nothing can be done about that. It is very rare but can explain lack of punch with good emission readings. We have an example of this at the museum, a Mazda CRM121B in a Plessey/Regentone.

A gassy tube particularly Mullard tetrodes can give the appearance of large what appear to be 'convergence' errors very noticeable in the corners of Test Card C. They usually slowly fade away after 5 mins use as the getter struggles to clean up the mess.

Mullard tubes from the MW22-16 to the AW43-80 can suffer from a partially shorted heater dropping around 3-4v in a series chain. Little can be done about this and the heater will glow brightly when force fed from a transformer including of course Bertha's.

Don't reactivate a tube 'just to see if I can make it took like a modern one' unless you know it is definitely low emission or gives a reasonable picture. There is a small risk of failure but due to Bertha's gentle hands this is very rare.

Colour tubes reactivate well. Use croc clips or a switched duodecal base to connect.

I have never experienced any success with just over running the heater for an extended time.

Hope this may shed a bit of alternative light on CRT reactivation.

I can only speak of Bertha. Some tube testers/boosters have violent action and one I would consider to be an execution device. No names please but it used a fluorescent lamp starter in series with the grid supply from HT and when the internal contacts closed in the glow switch, POP! Diabolical and I paid good money for it. Just a bench clutter for years then it got binned. Beware!

Regards, John.

Jac 23rd Nov 2020 11:47 am

Re: CRT Rejuvenation orientation
Thanks John!

I'll study this with care and see what I can implement on my Müter BMR6.

Does this procedure work well for pre-war tubes as well?

And in time I'll see if some improvement can be made on the 6505A in my GEC BT2147. The picture now (or when I saw it last time - over 20 years ago...) is very dim.

Best wishes,

Heatercathodeshort 23rd Nov 2020 1:55 pm

Re: CRT Rejuvenation orientation
I have never tried it on a pre war tube. The 6/6 in my 900 is very slow to start but gets very viewable after 15 mins.
I have a very good spare and maybe sometime I will fit it and try Bertha on the original one.
Just maybe. It does give a good picture once it has warmed up and to follow my own code maybe better left well alone.
Bertha is trying to get her hands on it and promises to be very gentle with it but her actions could finish the old boy off! No guarantees. Regards, John.

Heatercathodeshort 23rd Nov 2020 2:34 pm

Re: CRT Rejuvenation orientation
Another thing to note is that reactivation often changes the cut off point of the gun assembly.
The effect of this which requires a slightly higher grid voltage to maintain a crisp brilliance level. Usually this is well within the range of the brightness control itself but on rare occasions a reduction in the value of the resistor on the top of the brightness control is required to maintain a good range.

I think the microscopic space between the grid and cathode is reduced during the reactivation process. [Tiny amount of cathode material removed].

Another rather odd effect that Mullard tetrode tubes appear to suffer from is a very high resistance internal short probably between the first anode and grid but I am open to suggestions.

The effect is an overall lack of 'blacks'. Plenty of brightness and the contrast control usually appears to overload the video amplifier as it requires a high setting to maintain a reasonable picture. Often increasing the brightness control above an acceptable level also appears to increase the contrast!

My trick here is to reduce the first anode supply by about a third.

This is easily carried out by removing the supply from the A1 pin. Connect a short wire from the A1 pin [pin 10 Mullard Duodecal] to the slider of a 2mohm preset with its top end connected to the original supply wire and its bottom end to chassis. Adjust for the best black level then if you wish the potential divider can be constructed with fixed resistors. It's a good idea to fit a 220k resistor between the supply and the pot.

OK it's a bit of a bodge but in many cases it results in a much better picture with these very old but fully operational Mullard tubes.

Just to add it is very difficult to test for internal leaks in CRTs. Even with the very best of meters the very high impedance involved together with the microscopic current required to display a fault symptom.

Heater cathode shorts are often intermittent but are easily visually diagnosed with experience. It is the only short that can be detected with a test meter and then only if it is practically a dead short.

I worked closely with one of the best tube rebuilders in this country [Central Tubes]. He rebuilt just about everything, EMI Pyrex, Bonded, Rimband and colour.

He rebuilt the coded M5 BBC colour monitor tubes the screens of which were constructed with what was then known as the 'rare earth' red phosphor. The BBC wanted to keep all the monitors with the same colour match.

Believe me the CRT could produce some really strange symptoms. They were exciting times. Very hard work but you never thought of that back then.

I hope these jottings may be of some help. Put those tube blasters in the bin! Regards, John.

Jac 23rd Nov 2020 2:53 pm

Re: CRT Rejuvenation orientation
Thanks for all this John!


G6Tanuki 23rd Nov 2020 5:11 pm

Re: CRT Rejuvenation orientation
I remember attempts at 're-activating' some old CRT computer-monitors that had been 'on' continuously for more than half a decade and were a bit tired.

We did them with the tubes mounted face-up, the idea being that any bits of cathode-material displaced would not fall onto the shadowmask and become lodged there. From memory we did about a dozen tubes and got a noticeable improvement on about half of them. Alas the 'noticeable improvement' merely served to highlight the text burned into the screen-phosphor. The monitors were scrapped.

Welsh Anorak 23rd Nov 2020 5:18 pm

Re: CRT Rejuvenation orientation
Contrarily, I used to do them face down so any debris would fall out of the way, though I do get your point. One thing I always found was that to get a lasting improvement the TV would need to be used immediately afterwards for a good while, and sometimes would make noises in the neck.
My experience of boosting tubes started with sets from around 1970 - I think earlier ones could have needed quite a different approach. My original tester was based on a Television design followed by a BK 467 which was very successful. However the age of reactivation came to an end in the late Eighties with the 30AX tubes - and woe betide anyone who tried to reactivate a Sony!

Heatercathodeshort 23rd Nov 2020 6:48 pm

Re: CRT Rejuvenation orientation
Yes Glyn. Sometimes for a minute or less after reactivation the screen would flash up bright for a few split seconds and then stabilize. It was just bits of dust from the cathode getting trapped and 'pushed' out of the way.

The early 30AX tubes were diabolical but Mullard sorted the problem and the Mk2 versions were excellent with a very long life and one of the best colour pictures.

I never had much luck with Sony tubes particularly the early 27" 2704 series. Every example that came my way was completely flat and a few suffered from an O/C filament on one gun. One of the worst tubes ever produced that failed within 4 years and I was quoted £400 by Sony for a replacement.

The other horror was the 29" tube used in the model that suffered from a dry joint frame chip. The model with the rounded back and weighed a ton. John

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