04-18-2024, 05:46 PM
Hey folks, need some help please.
Working on an unmodified model 20 (all except the power switch that someone changed). And resistor 12…
For R12, detector cathode bias resistor, the schematics found on Philcoradio site for model 20 show this to be 50K. Service Bulletin 36, table 3 lists resistor 12 as 50K also. Someone replaced it with a 1K resistor. It is the only non-dog-bone in the unit. I have a 2nd model 20 that was modified but it has a 50K dog bone resistor in this spot.
Then to confuse me more, I looked in the Radio-Troubleshooter’s-Handbook (I like to look at the case-histories on models I am working on). In RTH for model 20 it referenced checking the detector tube cathode bias resistor to ensure it is a 200 ohm, 2W resistor and isn’t open.
How can I determine if there was some change done to reduce this resistor value from 50K down to the 1K installed or to the 200 ohm noted in the RTH? Anyone run across this and have any insight? I can replace it with a 50K and see what happens or leave the 1K. (I’m still preparing for re-cap, I discovered this during all my initial checks prior to power on)
Thanks,
George
Working on an unmodified model 20 (all except the power switch that someone changed). And resistor 12…
For R12, detector cathode bias resistor, the schematics found on Philcoradio site for model 20 show this to be 50K. Service Bulletin 36, table 3 lists resistor 12 as 50K also. Someone replaced it with a 1K resistor. It is the only non-dog-bone in the unit. I have a 2nd model 20 that was modified but it has a 50K dog bone resistor in this spot.
Then to confuse me more, I looked in the Radio-Troubleshooter’s-Handbook (I like to look at the case-histories on models I am working on). In RTH for model 20 it referenced checking the detector tube cathode bias resistor to ensure it is a 200 ohm, 2W resistor and isn’t open.
How can I determine if there was some change done to reduce this resistor value from 50K down to the 1K installed or to the 200 ohm noted in the RTH? Anyone run across this and have any insight? I can replace it with a 50K and see what happens or leave the 1K. (I’m still preparing for re-cap, I discovered this during all my initial checks prior to power on)
Thanks,
George
George - Berthoud, Colorado. Learned tube receiver repair and troubleshooting in the Navy way back when, came back around to it as an old guy. Re-learning all I can. Doing my part to restore a few of these old gems.
