Thought I knew a little about electronics but this one's got me. Hope someone can help.
Working on a 1957 Ford car radio with four tubes and one transistor. Radio dead but the dial light works.The 12 volt power lead and the ground lead both go to the radio chassis, soldered on a copper plate with a wire from the ground lead going to the switch. Need to know the theory on how this can possibly work.
Thanks Tony

Hi Tony,
Those copper plates are called "Spark Plates", and form a small capacitance with the radio chassis. Unless they have been damaged, they are not grounded to the radio chassis. If your radio is a genuine FOMOCO radio (either Bendix or Motorola), there should be two spark plates, one for the radio power, and one for the dial light. Also there should be two power wires (no ground wire), one for 12 volt power, and one that connects to the dash light dimmer.

The radio power switch should be a 2-pole, one side for the radio, one side for the dial light. Therefore, the dial light will work although the radio has a problem because it is on a different circuit. The radio side of the on/off switch is often burned open or sometimes just dirty. Try squirting a little contact cleaner into the switch and exercising the switch--worth a try although not always successful.

Often you will find that the 2 poles are jumpered together in an attempt to use the dial light portion to substitute for the burned out radio portion.

Send me an email if you need more information--click on my user name at the top of the message block.

Meade

:Thought I knew a little about electronics but this one's got me. Hope someone can help.
:Working on a 1957 Ford car radio with four tubes and one transistor. Radio dead but the dial light works.The 12 volt power lead and the ground lead both go to the radio chassis, soldered on a copper plate with a wire from the ground lead going to the switch. Need to know the theory on how this can possibly work.
: Thanks Tony

:Hi Tony,
:Those copper plates are called "Spark Plates", and form a small capacitance with the radio chassis. Unless they have been damaged, they are not grounded to the radio chassis. If your radio is a genuine FOMOCO radio (either Bendix or Motorola), there should be two spark plates, one for the radio power, and one for the dial light. Also there should be two power wires (no ground wire), one for 12 volt power, and one that connects to the dash light dimmer.
:
: The radio power switch should be a 2-pole, one side for the radio, one side for the dial light. Therefore, the dial light will work although the radio has a problem because it is on a different circuit. The radio side of the on/off switch is often burned open or sometimes just dirty. Try squirting a little contact cleaner into the switch and exercising the switch--worth a try although not always successful.
:
:Often you will find that the 2 poles are jumpered together in an attempt to use the dial light portion to substitute for the burned out radio portion.
:
:Send me an email if you need more information--click on my user name at the top of the message block.
:
:Meade
:
:
:
::Thought I knew a little about electronics but this one's got me. Hope someone can help.
::Working on a 1957 Ford car radio with four tubes and one transistor. Radio dead but the dial light works.The 12 volt power lead and the ground lead both go to the radio chassis, soldered on a copper plate with a wire from the ground lead going to the switch. Need to know the theory on how this can possibly work.
:: Thanks Tony

MR. TONY,
I used to play around with old car radios when I was a kid. The radio you have is what is called a hybrid radio i.e. a mix of tube and transistor. The big transistor is a germanium power transistor and is used for audio output. The transistor, I believe, is most likely a PNP type (how is that for a definative statement). The tubes were specialy designed for 12 volt plate voltage. This eliminated the need for a vibrator power supply to generate a high B+ voltage for the tube plates. Make sure you have an antenna plugged in otherwise you won't get any reception. Be careful about the polarity of the power you are applying. Some vehicles had positive ground.
I hope you find this helpfull.

MRO

::Hi Tony,
::Those copper plates are called "Spark Plates", and form a small capacitance with the radio chassis. Unless they have been damaged, they are not grounded to the radio chassis. If your radio is a genuine FOMOCO radio (either Bendix or Motorola), there should be two spark plates, one for the radio power, and one for the dial light. Also there should be two power wires (no ground wire), one for 12 volt power, and one that connects to the dash light dimmer.
::
:: The radio power switch should be a 2-pole, one side for the radio, one side for the dial light. Therefore, the dial light will work although the radio has a problem because it is on a different circuit. The radio side of the on/off switch is often burned open or sometimes just dirty. Try squirting a little contact cleaner into the switch and exercising the switch--worth a try although not always successful.
::
::Often you will find that the 2 poles are jumpered together in an attempt to use the dial light portion to substitute for the burned out radio portion.
::
::Send me an email if you need more information--click on my user name at the top of the message block.
::
::Meade
::
::
::
:::Thought I knew a little about electronics but this one's got me. Hope someone can help.
:::Working on a 1957 Ford car radio with four tubes and one transistor. Radio dead but the dial light works.The 12 volt power lead and the ground lead both go to the radio chassis, soldered on a copper plate with a wire from the ground lead going to the switch. Need to know the theory on how this can possibly work.
::: Thanks Tony

Mark,
Thanks for the info. and I think I'm on the right track now. Worked on a few tube tv's and radios years ago but not much with cars. I do remember having one with the old vibrator output.

Tony

Meade,
Thanks much for your reply and you really saved me on this one. Yes, it's a FOMOCO and the two leads go to the two "spark plates". The dash light dimmer wire is the thing that had me going in every direction but the right one. Finally got a 12v power supply to it, the correct way, and looks like I have filament voltage on two tubes. Hope I can get it working.
Really appreciate your help.
Tony