Quote:
Originally Posted by vts1134
I've got some readings on the electrolytics and a better picture of the schematic.
There are 5 multi stage electrolytic capacitors in the set. Towards the top of the picture, just above, and just to the right of the vertical centering control you can see two sections of C116. The negative on this cap reads shorted to ground which according to the schematic is correct. The other four caps are isolated from ground. Coming right off the center tap of the power transformer is the negative leads of C127, C128, and C131. Further along the line you have C132. Rather than give it away, how would I read the schematic and figure out what resistance reading those four caps should have to ground?
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Hmm.. so the power xfmr CT is the
negative voltage source*, supplying 110V negative for stuff elsewhere in the chassis. And the main filter for it is the 40mf.cap (square symbol). Can't read it too good, but i b'lieve it's C128A). The positive side of this cap should go to ground. Is this wired correctly? If it's not, the negative supply would have a large ripple on it. So check this first, and then check for ripple. And verify that there's 110V negative on it (+/- 10% or so).
To check resistance of the -110V source to ground, just look for a DC path to the nearest ground symbol in the schematic (hint: it goes thru the ion trap coils), and add up the values of the resistances in that path. And see if the sum agrees with the ohms reading you get on the meter (+/- 10% or so).
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*This technique is called "back biasing". You'll find it used in old equipment quite often, particularly in 30s radios. The CT of the power xfmr winding, instead of going directly to ground, goes through a chain of resistors or a large tapped resistor to form a voltage divider, supplying negative voltages for biasing other tubes in the set. A Google under "Back bias voltage" should turn up some good info on it.