[fenbach]

i'm working on a [Radiola] 7070 radio from a 1947 ford [ https://www.radiomuseum.org/r/ford_7070_...05_b2.html ]. basically the same as a Zenith 6MF780.
not a transitone obviously, but i searched for "spark plate" and came up with one other thread. which led me to think that philco did use them.

the radio works perfectly, but one of the spark plates measures 4-5 ohms to ground yet reads 6 volts when the radio is on, and the radio only draws 4 amps with or without the spark plate in the circuit. no short thru the spark plate that is, in spite of the low resistance. the resistance of the other spark plate is infinite, which it should be, i think. it measures 600pF.
i see no damage to the one with the low resistance. but worry that it could short out. am i missing something here??

could i just replace the spark plate with a 600 pF cap, say rather than try to rebuild it [it's riveted on.]?
or just eliminate it entirely?

PS
Antiqueradios had several threads but not a definitive answer that i could find.

[morzh]

What's spark plate?

[fenbach]

from antiqueradios:
(′spärk ′plāt)
(electricity) A metal plate insulated from the chassis of an auto radio by a thin sheet of mica, and connected to the battery lead to bypass noise signals picked up by battery wiring in the engine compartment. in other words, a cap to ground on the power line.

more than anything, i don't understand why it doesn't just short the 6 volts to ground with only a few ohms resistance. almost like the resistance increases [substantially] once the capacitor has charged up.

here's a picture https://www.antiqueradios.com/forums/vie...6&t=354480

[wannchev]

Dont take a chance with the low reading sparkplate. If it shorts in use it could damage the on/off switch which only has light contacts. Some of the older sets had silver contacts in the on/off switch which melted like a fuse on a short.

[Antipodal]

Hi there, When you measured the resistance to ground of the spark plate was it completely disconnected from the rest of the radio?Sometimes spark plates are " in series " with other bits of the circuit and that 5 ohms may have been the vibrator resistance or one side of the vibrator transformer primary to earth.

[fenbach]

the spark plate was completely disconnected when i measured the resistance. in the end, i replaced the spark plate with a 600pf, 2K volt cap, tested the radio multiple times and sent it back only to learn that it would barely pick up one station with noise otherwise. has me stumped.

[Tim Tress]

That could be an antenna problem; try substituting another one. The antenna trimmer must be peaked at the high end of the dial, with the radio installed in the car.