Author Topic: Vacuum tube filaments wont heat up  (Read 372 times)

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Offline CoopedUp

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Vacuum tube filaments wont heat up
« on: August 02, 2019, 07:37:36 pm »
I have a motorola a16w tube radio and I have replaced all of the capacitors and when I power it the filaments don't do anything. I know that the tubes are good I just dont know whats wrong. I measured from pin three of the 35w4 to b- and I got 120 volts. Any help would be appreciated, Thanks.
 

Offline james_s

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Re: Vacuum tube filaments wont heat up
« Reply #1 on: August 02, 2019, 08:47:17 pm »
The obvious place to start is the filament supply. Is this a radio with a power transformer or is it one of the ones that has all the tubes in a series string? If it's the latter then it only takes one bad connection in the string to make none of the filaments work. Either way start at the source and work your way down the line until you locate the fault.
 

Offline chris_leyson

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Re: Vacuum tube filaments wont heat up
« Reply #2 on: August 02, 2019, 09:34:05 pm »
It's a 5 tube AC/DC 120V radio so the heaters are in series. Downloaded schematic from https://www.radiomuseum.org.
The 35W4 heater wiring seemed a little strange at first but it's a tapped filament optionally used to drive a pilot lamp. You should have 7.5V across pins 4-6 and 27.5V across pins 3-6 assuming the series heater string isn't open circuit. As the voltage on pin 3 of the 35W4 is 120V it would seem to suggest that the heater string is open somewhere. I would check the heater voltage on 50C5, 12BA6, 12BE6 and finally the 12AV6. Either there is an open circuit heater or a dry joint on the PCB. Neat little radio and good luck.
« Last Edit: August 02, 2019, 09:37:42 pm by chris_leyson »
 

Offline CoopedUp

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Re: Vacuum tube filaments wont heat up
« Reply #3 on: August 02, 2019, 10:14:38 pm »
Thanks I'll check those things in a bit and then make an update!
 


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