Magic eye/bar tester

Unfortunately indicator tubes loose brightness with time being used. The light emitting screen is made of zinc silicate or similar. 

This fact makes them quite expensive, especially old types, like EM34 and EM11 or even older ones like the AM1. Therefore I decided to build a simple tester, to see If the light emitting coating of the anode is still good and that the tube still works. Means, that it has a working filament and a variation in the control grid voltage translates to a change in the display.

You might come across the older tube types on ebay, flee markets or you might get some, while tearing down old unrepairable radios. In each case it's good to know, if the tube works. If you have to complain because the seller lied to you or if you should buy it in the first place. 

The circuit is really nothing fancy. A MC34063A generates 250V from 8V for the anodes and the zinc silicate screen. Another one makes -20V to generate a saw tooth signal that gets applied to the control grid to test the functionality of the tube. Any circuit is fine.

For the heater I'll just use some diode or resistor drops for 6.3V and 4V.

I'll try to support all the indicator tubes, but I'm limited in space on the case, since I want it to be fearly portable.


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