05-25-2009, 11:25 AM
My father was am electonics technician before becoming a full-time electrician since that job wasn't as much of a political football. He worked on unmanned spacecraft at Honeywell during the 60's space boom. A lot of that was solid-state but radios and TVs were still tube operated. If memory serves me correct, he assembled a kit TV. I showed him the schematic of the 40-201 and he knew it was built in 1940. The first thing he focused on was the second audio push-pull stage. Thought they can be a bit tricky (what ever that means) and the went on the let me know that the plate can bite pretty hard and that working on televisions could be an even more enlightening experience. At 86, the conversation started to meander on to other relevant topics. He agreed that it could be possible that the mica caps were bad but thought I should start using my oscilloscope and signal generator to track the noise. I think that a good idea but I'm not sure if I can tell nosie from a good signal. We'll see.
The website All About Circuits has a nice review about resonance in series and parallel. The plate off the RF tube goes through a seires resonator with a resistor in parallel with that coil. That opens up the high frequency end but crops off the lows.
I'm getting negative voltage at my 1st audio grid. That seems like it would really limit the plate voltage. The negative voltage to the grid comes through a 10 Mohm resistor thats connected to the voltage divider that taps the center-tap, high voltage lead of the power supply. This is a fixed-bias power supply per Marcus and Levy. Now I know were the slight negative voltage is coming from the RF coil. I'll have to do some checking to see what the correct voltage should be.
Note to self: Since the original line capacitors were replaced I need to check my connections of the input and output electrolytics are indeed correct. The input capacitor does not ground to the chassis. I'm going to assume that the standard ground triangle symbol is a chassis ground.
I walking in the dark now so I could be missing something very obvious or just see one more bit of the puzzle.
The website All About Circuits has a nice review about resonance in series and parallel. The plate off the RF tube goes through a seires resonator with a resistor in parallel with that coil. That opens up the high frequency end but crops off the lows.
I'm getting negative voltage at my 1st audio grid. That seems like it would really limit the plate voltage. The negative voltage to the grid comes through a 10 Mohm resistor thats connected to the voltage divider that taps the center-tap, high voltage lead of the power supply. This is a fixed-bias power supply per Marcus and Levy. Now I know were the slight negative voltage is coming from the RF coil. I'll have to do some checking to see what the correct voltage should be.
Note to self: Since the original line capacitors were replaced I need to check my connections of the input and output electrolytics are indeed correct. The input capacitor does not ground to the chassis. I'm going to assume that the standard ground triangle symbol is a chassis ground.
I walking in the dark now so I could be missing something very obvious or just see one more bit of the puzzle.
