Non Destructive Tube Reactivation
The following procedure has been tried by a number of members with great success.
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.