Thursday, March 16, 2017

More tube tester restoration work


With my solid state replacement for the 83 rectifier tube in place, I found that the tester delivered no plate voltage to the circuits - one of the two potentials that are handled by this tube. Time to carefully probe all the voltages present at the tube socket, to determine if I have something burned out in the main power transformer.

The rectifier tube (type 83) is actually a dual diode. Each of the two plates is connected to an opposite polarity AC winding on the power transformer delivering +170V. There is a filament winding where the rectified DC output is the center tap and each side delivers 2.5V AC at opposite phase to light the tube filament.

83 rectifier circuit
I discovered that the filament center tapped winding works fine. I see the 155+VDC at the center tap but it is not getting to the tube socket where I am testing for it. Incidentally, the solid state replacement for the 83 tube uses two rectifier diodes, two resistors and two zener diodes to sit in place of the real vacuum tube.
Solid state replacement for 83 tube - large pins are the filament
I went through the checkout procedures in the manual I currently have, which is for the immediately prior version, the TV-3U and so a few items don't match. In spite of that, I verified all the filament voltages, the screen voltage, grid bias voltage and the small AC signal voltage that drives the gm (transconductance) measurement to validate the tubes gain. The short light tests worked properly as well.

The only thing that didn't check out was the plate voltage on the tube socket when I pushed the gm test button P4 - it should jump to 150V but it did not do that. Having verified the voltage is produced by the rectifier tube, it is just a matter of tracing and checking each step in the circuit from there to the plate pin of the tube socket.

I could have bad contacts on the push buttons or rotary switches that connect the plate voltage to the appropriate socket pin, or it could be a problem with some components in the path. I am overjoyed that this is working so well already, particularly that I don't have a bad power transformer.

Unfortunately, the path from the point where the rectifier delivers 150+ DC is where one of the deviations exists between the TV-3/U and my TV-3A/U unit. As well, there is no "large/small" signal switch on my unit, but the predecessor allows selection of 1V or 5V AC signal on the grid when testing transconductance.

I also discovered that P4 is not the gm test button on my unit - there is a row of pushbuttons with only a couple marked. It think it is better that I wait until I know what is what before I decide whether the plate voltage is delivered or not, plus I need an accurate schematic. I will set this aside until I get the manual and new DMM. 

No comments:

Post a Comment