11-13-2015, 03:10 PM
My first concern is to avoid frustrating my mentors to the point that they give up on me. Please bear with me.
My response is slow because just last night I figured out that emails from the Phorum were going into the spam box.
I do have a basic understanding of the vacuum tube, how it can function as an amplifier or a rectifier. I also understand how a capacitor acts as a filter following rectification and why it can work as a bypass for AC but not DC. And I understand how a coil lets DC pass easily but resists AC.
I know that the resistance reading across a capacitor or between anode and cathode is expected to be infinite.
Link to schematic:
https://goo.gl/photos/z4KG89Ze1LSp6PaY9
So, looking at the schematic and my resistance readings chart; with radio unplugged; connecting one lead from my ohmmeter to the negative side of capacitor C101-C:
From 50A8 pin 7 (cathode) the chart says to expect 130 ohms. I read 130 ohms. I can trace the electrical path on the schematic from pin 7 to C101 (negative), and there is a 130 ohm resistor in that path. That makes sense.
From 50A8 pin 2 (anode) the chart says to expect 90 Kohms. I read 12 Megohms and the resistance slowly increases as long as I have the meter connected. If I trace the electrical path from pin 7 on the schematic, each direction ends at C203; the positive side of C101 A, B, or C; or the 35Z5 cathode. I would expect infinite resistance (setting aside that the meter is charging a capacitor). But I can’t see how I could expect to read 90 Kohms.
I’ve restored about a dozen prewar radios and I’ve never run into this issue. What am I missing?
(Regarding the Bz-z-zt: The first thing I did was check filament continuity in the 3 most suspect tubes, but they were OK. Next I’ll check all tubes in a tester and look for a break in the continuity all the way back to the wall socket.)
My response is slow because just last night I figured out that emails from the Phorum were going into the spam box.
I do have a basic understanding of the vacuum tube, how it can function as an amplifier or a rectifier. I also understand how a capacitor acts as a filter following rectification and why it can work as a bypass for AC but not DC. And I understand how a coil lets DC pass easily but resists AC.
I know that the resistance reading across a capacitor or between anode and cathode is expected to be infinite.
Link to schematic:
https://goo.gl/photos/z4KG89Ze1LSp6PaY9
So, looking at the schematic and my resistance readings chart; with radio unplugged; connecting one lead from my ohmmeter to the negative side of capacitor C101-C:
From 50A8 pin 7 (cathode) the chart says to expect 130 ohms. I read 130 ohms. I can trace the electrical path on the schematic from pin 7 to C101 (negative), and there is a 130 ohm resistor in that path. That makes sense.
From 50A8 pin 2 (anode) the chart says to expect 90 Kohms. I read 12 Megohms and the resistance slowly increases as long as I have the meter connected. If I trace the electrical path from pin 7 on the schematic, each direction ends at C203; the positive side of C101 A, B, or C; or the 35Z5 cathode. I would expect infinite resistance (setting aside that the meter is charging a capacitor). But I can’t see how I could expect to read 90 Kohms.
I’ve restored about a dozen prewar radios and I’ve never run into this issue. What am I missing?
(Regarding the Bz-z-zt: The first thing I did was check filament continuity in the 3 most suspect tubes, but they were OK. Next I’ll check all tubes in a tester and look for a break in the continuity all the way back to the wall socket.)
