Terry N.
I would double check resistor 11. Sometimes a DVM will show a resistor good with the power off but the resistor may increase in value with current flowing thru it.
Clifton
:Good Day All,
:Can anyone give me and ideas on how to find out why I only have 60 volts on pin 3 of the 6A7 an 78 tube when I should have 90 volts on this Philco 38-12. All the tubes check good and I replaced all the caps. All the other voltages are close to where they should be. Resistors all read a little higher than they should. That's about all I know how to do.
:Thanks for any help.
:
:Terry N.
:
I will replace that resistor it measured 48K and it should be 40K.
I did not know if that could cause a 30V loss.
:Terry,
:
:I would double check resistor 11. Sometimes a DVM will show a resistor good with the power off but the resistor may increase in value with current flowing thru it.
:
:Clifton
:
:
::Good Day All,
::Can anyone give me and ideas on how to find out why I only have 60 volts on pin 3 of the 6A7 an 78 tube when I should have 90 volts on this Philco 38-12. All the tubes check good and I replaced all the caps. All the other voltages are close to where they should be. Resistors all read a little higher than they should. That's about all I know how to do.
::Thanks for any help.
::
::Terry N.
::
:
:
The 40K resistor is supposed to drop 120V across it (210 V - 90 V)= 120 V. So the current through the resistor should be I= 120 V/40,000 ohms = 3 mA. This is the current draw of the two tube screens on the node fed by the 40K resistor.
The tubes will tend to draw the same current semi-independently of the voltage. So, if you multiply the tube screen current by the MEASURED resistance, you get (3ma)*(48K) = 144 V. This would mean that the screen voltage is 210 V- 144 V = 66 V or approximately the voltage that you measured. Replacing the 40K resistor should restore the voltage to around 90 V.
Thanks for the help. I will let you know.
::
::I will replace that resistor it measured 48K and it should be 40K.
::
::I did not know if that could cause a 30V loss.
::
:
:
:The 40K resistor is supposed to drop 120V across it (210 V - 90 V)= 120 V. So the current through the resistor should be I= 120 V/40,000 ohms = 3 mA. This is the current draw of the two tube screens on the node fed by the 40K resistor.
:
:The tubes will tend to draw the same current semi-independently of the voltage. So, if you multiply the tube screen current by the MEASURED resistance, you get (3ma)*(48K) = 144 V. This would mean that the screen voltage is 210 V- 144 V = 66 V or approximately the voltage that you measured. Replacing the 40K resistor should restore the voltage to around 90 V.
:
Could be items mentioned but there are other things that cause low voltage. Cap C10, .03 mf, may be leaky.
Does the radio operate? If not maybe the oscillator isn't operating. If it's not operating 6A7 will draw more current causing voltage to be lower.
Voltage will also be higher when the radio is receiving a strong station. This causes AVC to be more negative reducing current drain of 6A7 and 78 tubes.
Norm
:: Wow that's a little over my head but I copied that information to a word doc for future reference . That way I can study it a bit. It might sink in.
:
:Thanks for the help. I will let you know.
:::
:::I will replace that resistor it measured 48K and it should be 40K.
:::
:::I did not know if that could cause a 30V loss.
:::
::
::
::The 40K resistor is supposed to drop 120V across it (210 V - 90 V)= 120 V. So the current through the resistor should be I= 120 V/40,000 ohms = 3 mA. This is the current draw of the two tube screens on the node fed by the 40K resistor.
::
::The tubes will tend to draw the same current semi-independently of the voltage. So, if you multiply the tube screen current by the MEASURED resistance, you get (3ma)*(48K) = 144 V. This would mean that the screen voltage is 210 V- 144 V = 66 V or approximately the voltage that you measured. Replacing the 40K resistor should restore the voltage to around 90 V.
::
:
: