05-23-2020, 12:16 AM
Managed to sort that pesky band 4/5 oscillator issue. It was driving me nutz!
After chatting with some folks here who have '40-'42 sets with failing oscillators I got thinking about the circuit. The hv is fed the the plate thru a 50 or 25k resistor. Perhaps this resistor is a compromise between having a proper plate load vs enough voltage to power the plate. Originally the design used a 50k and then it was lowered to 25k for "more reliable operation". More voltage but lower load resistance. Maybe a better component would be a rf choke. This would offer a high impedance load for the plate and very little voltage drop. Understanding that Philco was pinching every penny for a low price and profit. Choke might be another 20c over a resistor.
Grab a 2.5Mh choke and installed it and it worked!!! For about 2 minutes and started back to no signal below 6.8mc. GRRRRRR this set is such a tease.
Started back looking at the diagram and I noticed something curious. It has two separate grid leak resistors, one for the bc/pb with a 1000mmfd cap in parallel with it. The second one is for the three sw bands. Both are 10,000 ohms. All of the parts around the 76 osc tube I've check or replaced. I looked over the chassis and #18 is nowhere to be found! Solder in a new #18 and it's working dandy! I suspect that this set may have been built on a Fri or Mon. There is no alignment point at the low end of band 4 or 5 it could have easily gone unnoticed. Now the really curious thing is the 2nd 16B doesn't have that resistor in it either!
Time to put my feet up on the ottoman and relax. Enjoy the pleasant sounds of the 16B.
After chatting with some folks here who have '40-'42 sets with failing oscillators I got thinking about the circuit. The hv is fed the the plate thru a 50 or 25k resistor. Perhaps this resistor is a compromise between having a proper plate load vs enough voltage to power the plate. Originally the design used a 50k and then it was lowered to 25k for "more reliable operation". More voltage but lower load resistance. Maybe a better component would be a rf choke. This would offer a high impedance load for the plate and very little voltage drop. Understanding that Philco was pinching every penny for a low price and profit. Choke might be another 20c over a resistor.
Grab a 2.5Mh choke and installed it and it worked!!! For about 2 minutes and started back to no signal below 6.8mc. GRRRRRR this set is such a tease.
Started back looking at the diagram and I noticed something curious. It has two separate grid leak resistors, one for the bc/pb with a 1000mmfd cap in parallel with it. The second one is for the three sw bands. Both are 10,000 ohms. All of the parts around the 76 osc tube I've check or replaced. I looked over the chassis and #18 is nowhere to be found! Solder in a new #18 and it's working dandy! I suspect that this set may have been built on a Fri or Mon. There is no alignment point at the low end of band 4 or 5 it could have easily gone unnoticed. Now the really curious thing is the 2nd 16B doesn't have that resistor in it either!
Time to put my feet up on the ottoman and relax. Enjoy the pleasant sounds of the 16B.
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!
Terry
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