EM81 - Reactivation with High power Laser - UK Vintage Radio Repair and Restoration Discussion Forum

davegsm82 · 17th Nov 2015, 2:26 am

Or also could be titled, drawing lines on an EM81 Phosphor.

Tonight I was doing some investigation work on a Philco FM tuner which has a defective EM81, the output is very low and only visible in a darkened room.

I found a replacement which performs much better but decided to have a play with the stricken valve, a little bit of percussion to begin with, no scientific reason just to see the results which were nothing.

I got to thinking about why the valve fails, whether it's a gradual degradation of the phosphor or perhaps just failure of the oxide coating on the cathode. This led me to think in what ways I could possibly have an effect on the phosphor without breaking the vacuum.

The answer I came up with was a high power laser (500mW red CW Diode). Not the sort of thing you use to play with the cat or wave around in the night sky.

This is it setting fire to some black tape on the workbench;

http://i284.photobucket.com/albums/l...psdu2f6nla.jpg

The laser has a collimating lens on the output, to be honest you need a fairly small pinpoint to do any real damage to things you're trying to burn.

I focussed the pinpoint onto the phosphor inside the EM81 while it was running and the result was thus;

http://i284.photobucket.com/albums/l...psmdy7k4gq.jpg

I played the laser on the left hand side of the valve more than the right (as viewed in the picture) and you can see that there is a definite change in the phosphor, it's brighter and appears to have turned a slightly different colour.

Obviously this test is quite unscientific, however it shows that you CAN modify the phosphor from outside of the glass and there is a chance that you could get a bit more life out of an ageing EM8x valve (and probably others).

If I were to investigate this further I would probably attempt to build some sort of laser scanning device to accurately draw lines back and forward across the whole face of the valve such that it all receives the same treatment and doesn't look patchy, but at this point I would have to beg the questions, is it worth the hastle when Russian Equivalents are still available? How long will the effect last? are there simpler ways of heating up/burning the phosphor? (Microwave or RF coil etc?).

Dave.

GMB · 17th Nov 2015, 11:09 am

That's a really interesting result. It's something I had intended to try one day but never got a round tuit.

Before wasting time on scanning, I would try to determine how long it took to achieve the effect. If you could pulse your laser with a square wave and sweep it across the target you might see a dashed line and by sweeping at different speeds you might get an idea for the minimum time it takes, then if you can determine the spot size you could get the important figure of the power density needed.

From that it should be possible to estimate how hot it got. If only a modest temperature then RF heating might work, but it might be so hot that the top layer was evaporated.

17th Nov 2015, 2:26 am	#1
davegsm82 Heptode Join Date: Jan 2004 Location: Blyth, Northumberland, UK. Posts: 858	EM81 - Reactivation with High power Laser Or also could be titled, drawing lines on an EM81 Phosphor. Tonight I was doing some investigation work on a Philco FM tuner which has a defective EM81, the output is very low and only visible in a darkened room. I found a replacement which performs much better but decided to have a play with the stricken valve, a little bit of percussion to begin with, no scientific reason just to see the results which were nothing. I got to thinking about why the valve fails, whether it's a gradual degradation of the phosphor or perhaps just failure of the oxide coating on the cathode. This led me to think in what ways I could possibly have an effect on the phosphor without breaking the vacuum. The answer I came up with was a high power laser (500mW red CW Diode). Not the sort of thing you use to play with the cat or wave around in the night sky. This is it setting fire to some black tape on the workbench; http://i284.photobucket.com/albums/l...psdu2f6nla.jpg The laser has a collimating lens on the output, to be honest you need a fairly small pinpoint to do any real damage to things you're trying to burn. I focussed the pinpoint onto the phosphor inside the EM81 while it was running and the result was thus; http://i284.photobucket.com/albums/l...psmdy7k4gq.jpg I played the laser on the left hand side of the valve more than the right (as viewed in the picture) and you can see that there is a definite change in the phosphor, it's brighter and appears to have turned a slightly different colour. Obviously this test is quite unscientific, however it shows that you CAN modify the phosphor from outside of the glass and there is a chance that you could get a bit more life out of an ageing EM8x valve (and probably others). If I were to investigate this further I would probably attempt to build some sort of laser scanning device to accurately draw lines back and forward across the whole face of the valve such that it all receives the same treatment and doesn't look patchy, but at this point I would have to beg the questions, is it worth the hastle when Russian Equivalents are still available? How long will the effect last? are there simpler ways of heating up/burning the phosphor? (Microwave or RF coil etc?). Dave.

17th Nov 2015, 11:09 am	#2
GMB Dekatron Join Date: Aug 2003 Location: near Reading (and sometimes Torquay) Posts: 3,100	Re: EM81 - Reactivation with High power Laser That's a really interesting result. It's something I had intended to try one day but never got a round tuit. Before wasting time on scanning, I would try to determine how long it took to achieve the effect. If you could pulse your laser with a square wave and sweep it across the target you might see a dashed line and by sweeping at different speeds you might get an idea for the minimum time it takes, then if you can determine the spot size you could get the important figure of the power density needed. From that it should be possible to estimate how hot it got. If only a modest temperature then RF heating might work, but it might be so hot that the top layer was evaporated.