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Precise 111m tube tester restoration
#15

Problem solved! Icon_clap Defect in power transformer. I was studying that 50v ac feed for the G2 (position B), since all the others worked and discovered it is derived from the filament transformer 50V tap. Remember, the 111 uses separate power and filament transformers. The plate is fed AC 160v on the EM test from the power transformer, and the grid is fed 50-55v ac also.  See where I am going with this yet? It didn't make sense that the voltages were all present, yet it still wouldn't work. ONLY the 50v ac was giving trouble. All others were working, so that meant the rest of the circuit had to be ok. Then I got to thinking about 'tube theory'. Could the transformers be out of phase? Double checked the wiring - connected correctly. I reversed the primary of the power transformer only. Bingo! Setting B" come to life and things calibrated right in! The G2 source was actually shoving the tube into cutoff via the screen grid. There was a positive half cycle on the plate while a negative half cycle was on the screen grid. The tube was doing exactally what it was designed to do, so no meter reading on that setting.

   

In a nutshell, the primary of the power transformer was reversed internally. So I switched terminal 1 & 2. A factory defect, which explains why this was never used. A bear to figure out, simple to fix. A fringe benefit of all this is that I got to know this tester,  basically have a NOS instrument.

I still am chasing a minor problem in that is the meter jumps some when doing a test, esp the Gm test on the more sensitive settings. Cleaned switches, checked solder connections. Not sure what to make of this. Isn't much, maybe 200-500 umhos. Don't notice it on the Em much or the higher plate shunt settings. Don't notice it on the old 111, same tubes. It was doing this before the big resto, so it isn't anything new. May be chasing a ghost.

If I could find the place called "Somewhere", I could find "Anything" Icon_confused

Tim

Jesus cried out and said, "Whoever believes in me , believes not in me but in him who sent me" John 12:44


Messages In This Thread
Precise 111m tube tester restoration - by TV MAN - 05-27-2016, 11:00 AM
RE: Precise 111m tube tester restoration - by TV MAN - 07-24-2016, 08:38 PM



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