I'm restoring my first tube radio and am hoping to get some advice. I have a 46-350 which I picked up at a thrift store. I threw it on the variac and I'm seeing very high DC voltage on the cathode of the 117Z3 rectifier. From what I've read on this forum and on the schematic I should be seeing ~90V DC from pin 6 (cathode pin) to B-. When I check the voltage I'm seeing about 163V DC between pin 6 and B-. Also, I noticed I get the same voltage results if I test from the B- terminal or the metal chassis as the ground. From my understanding, the B- should be an isolated ground.
As a test, I disconnected the wire from pin 6 to R100A, just to isolate the circuit. When I do this I'm getting about 70V DC from pin 6 on the 117Z3. If I reconnect the wire the voltage jumps back up to 163V DC. I have two rectifier tubes and I've tried both tubes, they both showed similar results. Just as a side note both tubes tested good on my tube tester.
The only other change I've made is replacing the filter caps. I did double-check polarity and size/voltage and everything looks good.
Does anyone have any ideas on why the voltage would be so high at pin 6?
- Do you apply 120VAC from the variac or 110V?
- Do you have the speaker plugged in and all the tubes hot and conducting like they should? Is the output tube 3Q5 conducting with the voltage on the plate?
People who do not drink, do not smoke, do not eat red meat will one day feel really stupid lying there and dying from nothing.
(This post was last modified: 03-11-2021, 05:40 PM by morzh.)
-I'm applying 110V from my variac.
-I didn't have the speaker plugged in when I was testing but I tried it and nothing seemed to change. None of the tubes besides the rectifier tube is heating. I testing the 3Q5 plate voltage (Pin 3) and it's showing the same voltage as the rectifier, 163V DC.
No tubes heating means no conduction means no load means high voltage. Here's a youtube video on your radio that might help. I didn't watch it. https://www.youtube.com/watch?v=G5B5Odql5tw
!!!!!!!!!!!!!!!!!!!RESISTANCE IS FUTILE!!!!!!!!!!!!!!!!!!!!!!
/\/\/\/\/\/\/\/\/\
Check R-100 a,b, and c for proper resistances. If those are bad the filaments won't light.
When my pals were reading comic books
I was down in the basement in my dad's
workshop. Perusing his Sam's Photofoacts
Vol 1-50 admiring the old set and trying to
figure out what all those squiggly meant.
Circa 1966
Now I think I've got!
Thank you for the suggestion. I did some more testing and found I don't have filament voltage. It looks like part of the candohm resistor has gone bad. I'm not getting any voltage on the last post after the R100C resistor and nothing on the filament pin on 3Q5. I just placed an order for a 900 Ohm 5W replacement. I'll keep everyone posted once I solder in the replacement resistor. Fingers crossed I can at least get the tubes warming.
It's important that the 8.5v at the end of R-100C ends up being 8.5v after you get done replacing that resistor. Too high and it shortens the tube's life. Too lo and the mixer/osc won't work so no reception. The rest of the voltages are not particularly critical within reason.
When my pals were reading comic books
I was down in the basement in my dad's
workshop. Perusing his Sam's Photofoacts
Vol 1-50 admiring the old set and trying to
figure out what all those squiggly meant.
Circa 1966
Now I think I've got!
Hey guys, remember these are 1.4 volt tubes. They won't create a lot of heat or produce a warm glow (maybe the 3Q5). I install a 9 volt zener diode across the filament voltage as a safety net.
When you run 110V AC through a half wave rectifier then there should be no more than 120V DC no load output. Sounds like something is connected wrong in the power supply filter.
An ac voltage is dynamic it's constantly changing. Because it's constantly changing it's voltage is not a single value. The peak value is the highest value seen. If I remember correctly to find peak value you multiply by the square root of two.
The voltage reading from a typical outlet is said to be approximately 117vac. The ac value assigned is from the power that's equivalent to 117VDC when either are measured into same resistor or resistive load.
!!!!!!!!!!!!!!!!!!!RESISTANCE IS FUTILE!!!!!!!!!!!!!!!!!!!!!!
/\/\/\/\/\/\/\/\/\
