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 Post subject: GE F-74 replace original speaker with PM speaker
PostPosted: Aug Fri 06, 2021 9:42 pm 
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Joined: Feb Mon 08, 2016 6:25 pm
Posts: 26
Location: Wichita Falls, Texas 76305
I purchased a GE F-74 without a speaker, voice coil or audio transformer I would like to replace the speaker with a PM speaker. I tested the vacuum tubes, replaced paper and electrolytic caps and out-of-tolerance resistors. Several wires were also replaced.

I have:
Output Transformer 5K/7K: 4/8/16 Ohm 8VA 45mADC
820 ohm 10 W resistor for L14 on the schematic.

Four wires remain that went to the original voice coil and audio transformer. I attempted to connected them to the L14 resistor and purchased output transformer with no luck. The wires originate from Pin 2 (plate) of tube 41, Pin 3 (screen grid) of tube 41, Pin 6 (ground) of tube 41 and Pin 4 of tube 80.

I thought I may have heard a faint station when I attempted, but I'm not sure.

Any help on how to connect these to the L14 resistor replacement and audio transformer would be appreciated.

Rider Manual G.E. 9-15
Attachment:
File comment: Rider Manual, G.E. 9-15
GE F-74, schematic.pdf [343.18 KiB]
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 Post subject: Re: GE F-74 replace original speaker with PM speaker
PostPosted: Aug Fri 06, 2021 11:11 pm 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 28344
Location: Detroit, MI USA
As I see it, you have wires also running from the secondary of the output transformer back to the chassis in the tone control circuit. Let's ignore that for a moment though.

One end of your new resistor goes to pin 4 of the 80 tube, the other end of it to pin 3 of the 41 tube. The primary side of the output transformer goes from pin 3 of the 41 tube to pin 2 of the 41 tube. With those connections in place and the secondary of your output transformer connected to the voice coil of the new speaker the radio should work, if everything else in the set is good.

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Dennis

Experience is what you gain when the results aren't what you were expecting.


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 Post subject: Re: GE F-74 replace original speaker with PM speaker
PostPosted: Aug Sat 07, 2021 9:01 pm 
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Joined: Feb Mon 08, 2016 6:25 pm
Posts: 26
Location: Wichita Falls, Texas 76305
You stated that two leads off pin 3 of tube 41 go two different locations. I only had one wire off pin 3. Did you mean pin 2 tube 41 for one of them?
Thanks.


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 Post subject: Re: GE F-74 replace original speaker with PM speaker
PostPosted: Aug Sat 07, 2021 9:09 pm 
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Joined: Feb Mon 08, 2016 6:25 pm
Posts: 26
Location: Wichita Falls, Texas 76305
Never mind the previous. I connected as you suggested and I got reception. Thanks.


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 Post subject: Re: GE F-74 replace original speaker with PM speaker
PostPosted: Aug Sat 07, 2021 10:25 pm 
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Joined: Feb Mon 08, 2016 6:25 pm
Posts: 26
Location: Wichita Falls, Texas 76305
When I connect the secondary of the audio output transformer to the 1 terminal in the tone control (S2) circuit, it results in a very loud screeching from the speaker.

Any suggestions?
Thanks.


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 Post subject: Re: GE F-74 replace original speaker with PM speaker
PostPosted: Aug Mon 09, 2021 9:28 pm 
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Joined: Dec Sat 24, 2011 9:17 pm
Posts: 8811
Location: Vancouver Island, Canada
It sounds like, not that I actually heard it, :x like a feedback loop from something miswired.

Here's the easy to view schematic.
Go over it again, maybe try a jumper from the other side of the secondary or check that you have all connections correct.

http://www.nostalgiaair.org/pagesbymode ... 007798.pdf

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 Post subject: Re: GE F-74 replace original speaker with PM speaker
PostPosted: Aug Mon 09, 2021 9:33 pm 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 28344
Location: Detroit, MI USA
One side of the output transformer secondary needs to be grounded, the other side goes to the tone control. If you get unwanted feedback, simply reverse the two secondary wires to change the phase, and it should work properly.

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Dennis

Experience is what you gain when the results aren't what you were expecting.


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 Post subject: Re: GE F-74 replace original speaker with PM speaker
PostPosted: Aug Mon 09, 2021 9:52 pm 
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Location: Vancouver Island, Canada
Also, the connection is to the wiper, not to pin 1, altho the print isn't clear if pin one has a jumper to it.

_________________
Watch the doughnut, not the hole.
Burl Ives, RIP, oldtimer.
[:l>)


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 Post subject: Re: GE F-74 replace original speaker with PM speaker
PostPosted: Aug Thu 12, 2021 8:09 pm 
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Joined: Feb Mon 08, 2016 6:25 pm
Posts: 26
Location: Wichita Falls, Texas 76305
Thanks. The ground on the secondary worked.


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