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 Newbie ISO help!!

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murphy2k
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Join date : 2018-09-14

PostSubject: Newbie ISO help!!   September 14th 2018, 11:09 am

Hello, and thanks in advance for any help.  This is me and my sons 1st attempt at repairing an old radio.  This radio powers up and plays audio via audio input from record player input but no sound comes out in AM\FM mode.  Almost all voltages check out per the schematic (linked) with the exception of 3 places.  The RF Amplifier G2 shows 50V (which I think is a typo) and reads close to 100V.  The 1st IF AM/FM tube on G2 shows 90V but shows a steady decline to about 60V and lastly, per the schematic, the FM Detector plate voltage shows 330v and I get 0V reading on it. 

I appreciate any and all help.

well, it wouldn't let me post the schematic link...
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wildcat445
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Join date : 2014-08-16

PostSubject: Re: Newbie ISO help!!   September 14th 2018, 12:15 pm

What are you working on?  Make and model?

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murphy2k
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PostSubject: Re: Newbie ISO help!!   September 14th 2018, 12:17 pm

It's a Montgomery Ward Airline 15WG-2765B.  sorry, thought I had that in the original post.
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wildcat445
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PostSubject: Re: Newbie ISO help!!   September 14th 2018, 12:46 pm

Radio phono combo from the late 1940's?  Wells-Gardner built chassis.  I would find out why you have no voltage on a tube element.  Check components in the B+ stream from power supply to where you have no voltage.  Check the function switch as well.

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murphy2k
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PostSubject: Re: Newbie ISO help!!   September 14th 2018, 1:02 pm

that is the one!  I can't see how the 330 B+ Voltage gets to pin 7 via schematic.  I do have B+ at all other tubes.  the selector switch schematic is hard for me to comprehend
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wildcat445
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PostSubject: Re: Newbie ISO help!!   September 14th 2018, 2:37 pm

Function/selector switches are hard for me, as well.  I find a "starting point" some place I can identify on the switch and work from there.  I never take a function switch out or apart.  I stay away as far as I can.  Safer that way, and I don't mess anything up.  I clean them with contact cleaner and a spritz of WD-40 and move on.  We really need to see the schematic.  (I reckon I could look it up!)  Perhaps it was too big to post.

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murphy2k
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PostSubject: Re: Newbie ISO help!!   September 14th 2018, 2:44 pm

I tried posting the link but being new, the board wouldn't let me.  It is easily searchable by the model # I listed.  I just can't see how B+ gets to the FM Detector with the schematics.
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wildcat445
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PostSubject: Re: Newbie ISO help!!   September 14th 2018, 2:56 pm

I'm not sure the "330 volts" is that much.  Sounds like a lot of voltage on a detector.  3.30 volts might be more like it.  There is a 1 megohm resistor in B+ to the detector.  If you follow the schematic, the first place it goes to after that resistor is the function switch.  I blew the schematic up to 400% but will not swear to what the correct voltage is.  Detectors typically don't run a lot of voltage.  They are diodes rather than amplifiers.  You should refer to the diagram and follow that circuit to where it ultimately ends.  The schematic is the road map for the electrons in your radio.  First blush tells me the detector is not your problem.  A signal generator or some way of introducing noise into the radio would be the first thing I'd try.  That will give you a clue as to where the signal is being lost.

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murphy2k
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PostSubject: Re: Newbie ISO help!!   September 14th 2018, 3:17 pm

that circuit closes at the switch to switch 11 right next to it and then traces to pin 1 of the RF AMP on the far left and Pin 1 of the 1st IF AM-FM tube, none of which show a high voltage reading. 

I don't have a signal generator just a plain ol multimeter.
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wildcat445
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PostSubject: Re: Newbie ISO help!!   September 14th 2018, 9:14 pm

A signal generator and a signal tracer are indispensable items of electronic test gear.  You really can't do a lot of trouble shooting without one or both.  You need some way of injecting a signal at the intermediate frequency (IF) so you can follow it thru the radio and find out where the signal is being lost. 

Those pins you are following are in the B+ (high voltage) circuit.  Have you tested the tubes yet?  Since both bands are dead, your problem is something common to both bands.  You have proven the audio works, so you problem is ahead of the volume control most likely.  There are separate converters for AM and FM, so it is between the converter and the volume control.  If one converter were dead, it would not kill the other band.  That leave the IF stages.  Sub the IF amplifier tubes and the detector tube.  Make sure you have the function switch in the correct position.  Make sure the antenna has not become disconnected.  Consider the simple faults first.

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