Model 20 Cathode Bias Resistor Value - Printable Version
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Model 20 Cathode Bias Resistor Value - SRv2 - 04-18-2024
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
RE: Model 20 Cathode Bias Resistor Value - morzh - 04-18-2024
It would help to show photographs.
The resistor, as i remember, is 50K.
Maybe they referred to the part of the dual volume pot, which is also a cathode bias, which is 250 Ohm.
RE: Model 20 Cathode Bias Resistor Value - SRv2 - 04-18-2024
Attached is a pic of the resistor installed. Brown-black-red, assuming 1K?
Also attached is a pic from the RTH page, note 7.
Hope that helps. Thanks for the support.
George
RE: Model 20 Cathode Bias Resistor Value - David - 04-18-2024
Link to some info that might bring back memories. https://philcoradio.com/phorum/showthread.php?tid=22425&pid=177816#pid177816
RE: Model 20 Cathode Bias Resistor Value - SRv2 - 04-18-2024
The resistor in spot 12 is marked as 1K and measures 1.2K. But schematic says 50K. I can easily replace it, just wondered if anyone knows of a reason it would have this value, particularly with the notes in the RTH. Thanks.
RE: Model 20 Cathode Bias Resistor Value - morzh - 04-18-2024
As you could see, the resistor there is Orange, and the document will say "12 50,000 Orange"
RE: Model 20 Cathode Bias Resistor Value - SRv2 - 04-19-2024
Good deal, thanks @morzh. I’ll replace it with a 50K. Appreciate all the input.
George