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Old 23rd Nov 2020, 11:19 am   #9
Heatercathodeshort's Avatar
Join Date: Jun 2005
Location: Warnham, West Sussex. 10 miles south of DORKING.
Posts: 7,874
Default 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.
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