07-19-2024, 10:35 PM
Hi Folks,
I have modified this post several times since originally posting it, so read it again. Sorry for the multiple mods.
You may have more than one issue causing excess current draw, which is common when one must do much work to restore a radio this old to functionality.
I've used the P-T156 transformer on more than one occasion, but not on a Push Pull (although it is designed just for that purpose). Most of the time I use them on old battery radios like AK20s, etc. Remember that the primary current for this transformer should not exceed 10 mA. I believe that the DC resistance for this transformer is a lot lower than many transformers that it replaces. Although this transformer is not supplied by the R11 Voltage Divider resistor, it could be contributing to excess current if the DC resistance of the transformer is too low.
Feel the interstage transformer. If it is hot, then it is drawing too much current. The transformer that you are using does have a 10KOhm primary impedance, but I don't know what the DC resistance is. These transformers were primarily used with a B+ voltage of about 90V. This radio has 250V on the 1st audio plate, which may be too much for this transformer (and also more than what is traditionally done with a type 27 tube, especially if it is driving a class A or AB push pull amplifier). OOps, this is not the 1st audio amp, it is the biased detector that I spoke about before. Verify the cathode resistor and bypass cap. Verify that between the cathode of the 27 and ground, there should be a rather high voltage. You can also pull the 27 to see if the current draw of the radio drops. The wrong bias on the detector will also lower the volume or even prevent detection. Per the RCA Receiving Tube Manual Version RC12, current draw in the biased detector circuit should be only about 0.2mA. The voltage drop across the cathode resistor should be about 30V. Sorry, I had Philco Model 20 on my mind, which uses both a biased detector and a first audio amp. However, coupling the transformer by an RC network may help. In addition, if the primary of the original transformer is gone but the secondary is good, you can still use it by "bridging" the transformer. Check out the following article:
https://philcoradio.com/library/index.ph...nsformers/
I think that the 10W resistors should be sufficient for R11. Are the voltages close to the diagram and what I listed in my last post? Oops, BAD JOHN. I did not see your answer. it does seem that the voltage at terminal 5 is rather high, DMM or not, considering the voltage at terminal 4. Is the speaker original? Is the field coil original? Your result was stated as 1000kOhm. Did you mean 1KOhm (1,000 Ohm)?
Is the 2200 Ohm resistor replacing a burned out section of R11? If so, how did you arrive at that value. Is the second leg of that resistor (between terminal 4 and ground) close to 2K Ohm?
Using Ohm's Law, Current = Voltage divided by resistance, or I = E/R
Measure the voltage dropped across the resistor in question Divide that voltage by the value of the resistor. This will give you the current flowing through the resistor. Now, multiply the current by the voltage drop across the resistor. This will give you the power being dissipated by the resistor.
Sorry, this post is getting complicated.
Good luck and keep us posted.
I have modified this post several times since originally posting it, so read it again. Sorry for the multiple mods.
You may have more than one issue causing excess current draw, which is common when one must do much work to restore a radio this old to functionality.
I've used the P-T156 transformer on more than one occasion, but not on a Push Pull (although it is designed just for that purpose). Most of the time I use them on old battery radios like AK20s, etc. Remember that the primary current for this transformer should not exceed 10 mA. I believe that the DC resistance for this transformer is a lot lower than many transformers that it replaces. Although this transformer is not supplied by the R11 Voltage Divider resistor, it could be contributing to excess current if the DC resistance of the transformer is too low.
Feel the interstage transformer. If it is hot, then it is drawing too much current. The transformer that you are using does have a 10KOhm primary impedance, but I don't know what the DC resistance is. These transformers were primarily used with a B+ voltage of about 90V. This radio has 250V on the 1st audio plate, which may be too much for this transformer (and also more than what is traditionally done with a type 27 tube, especially if it is driving a class A or AB push pull amplifier). OOps, this is not the 1st audio amp, it is the biased detector that I spoke about before. Verify the cathode resistor and bypass cap. Verify that between the cathode of the 27 and ground, there should be a rather high voltage. You can also pull the 27 to see if the current draw of the radio drops. The wrong bias on the detector will also lower the volume or even prevent detection. Per the RCA Receiving Tube Manual Version RC12, current draw in the biased detector circuit should be only about 0.2mA. The voltage drop across the cathode resistor should be about 30V. Sorry, I had Philco Model 20 on my mind, which uses both a biased detector and a first audio amp. However, coupling the transformer by an RC network may help. In addition, if the primary of the original transformer is gone but the secondary is good, you can still use it by "bridging" the transformer. Check out the following article:
https://philcoradio.com/library/index.ph...nsformers/
I think that the 10W resistors should be sufficient for R11. Are the voltages close to the diagram and what I listed in my last post? Oops, BAD JOHN. I did not see your answer. it does seem that the voltage at terminal 5 is rather high, DMM or not, considering the voltage at terminal 4. Is the speaker original? Is the field coil original? Your result was stated as 1000kOhm. Did you mean 1KOhm (1,000 Ohm)?
Is the 2200 Ohm resistor replacing a burned out section of R11? If so, how did you arrive at that value. Is the second leg of that resistor (between terminal 4 and ground) close to 2K Ohm?
Using Ohm's Law, Current = Voltage divided by resistance, or I = E/R
Measure the voltage dropped across the resistor in question Divide that voltage by the value of the resistor. This will give you the current flowing through the resistor. Now, multiply the current by the voltage drop across the resistor. This will give you the power being dissipated by the resistor.
Sorry, this post is getting complicated.
Good luck and keep us posted.
"Do Justly, love Mercy and walk humbly with your God"- Micah 6:8
Best Regards,
MrFixr55