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 Post subject: Philco PT-25 Speaker challenge.
PostPosted: Dec Tue 05, 2017 3:28 pm 
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Joined: Nov Sun 29, 2015 1:22 am
Posts: 568
Location: Parkland, Florida
Hi all.
I am working on this half rusted set just for educational purposes.
The set needs total repair, caps, resistors, wires etc.
The speaker does not make any noise to a 9V battery. There is a rip on the cone.
I did not test the speaker's functionality with the radio as it is not working.
I did Ohm the primary and secondary of the output transformer(182, 3.1 Ohms) as well as the speaker coil (447, 2.6 Ohms) and there are no open coils.
The two wires that go to the cone measured 4.8 ohms.
Why won't the speaker make any noise to the 9V battery? Is there a way to fix this?
Thanks,
Carlos


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 Post subject: Re: Philco PT-25 Speaker challenge.
PostPosted: Dec Tue 05, 2017 4:12 pm 
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Location: Halfway between Possum Trot and Monkey's Eyebrow KY
It might should make a slight noise, but that is an electrodynamic speaker Which requires power through the field coil on the speaker to produce sound.

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 Post subject: Re: Philco PT-25 Speaker challenge.
PostPosted: Dec Tue 05, 2017 5:08 pm 
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Joined: Nov Sun 29, 2015 1:22 am
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Location: Parkland, Florida
Thanks for the reply, simplex1040.
I'm still a newbie, the reason for my basic question.
But I thought that regardless it should make a noise.

Carlos


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 Post subject: Re: Philco PT-25 Speaker challenge.
PostPosted: Dec Tue 05, 2017 7:13 pm 
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Location: Halfway between Possum Trot and Monkey's Eyebrow KY
test between the points I have circled with your battery


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 Post subject: Re: Philco PT-25 Speaker challenge.
PostPosted: Dec Tue 05, 2017 8:32 pm 
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Joined: Nov Sun 29, 2015 1:22 am
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Location: Parkland, Florida
I just tried that and nothing.
Is it possible that where those leads connect to the speaker cone are loose?
If so, how can I reconnect them?
Thanks,
Carlos


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 Post subject: Re: Philco PT-25 Speaker challenge.
PostPosted: Dec Tue 05, 2017 9:10 pm 
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Location: Halfway between Possum Trot and Monkey's Eyebrow KY
They go to the cone and then from there to the voice coil. I suspect you have either a broken wire to the VC or a bad VC.
them going to the cone is just a "holder" for the wires

Measure with the ohm meter across those two points.

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 Post subject: Re: Philco PT-25 Speaker challenge.
PostPosted: Dec Tue 05, 2017 9:43 pm 
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Joined: Nov Sun 29, 2015 1:22 am
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Location: Parkland, Florida
The two wires that connect to the cone measured 4.8 Ohms.
Thanks for your help.
Carlos


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 Post subject: Re: Philco PT-25 Speaker challenge.
PostPosted: Dec Tue 05, 2017 9:56 pm 
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CLC wrote:
The two wires that connect to the cone measured 4.8 Ohms.
Thanks for your help.
Carlos
The tinsel leads from the terminals to the cone should be zero ohms, not 4 ohms, not infinity.


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 Post subject: Re: Philco PT-25 Speaker challenge.
PostPosted: Dec Tue 05, 2017 11:35 pm 
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Joined: Nov Sun 29, 2015 1:22 am
Posts: 568
Location: Parkland, Florida
I removed the felt dust cover, uncovered the two fine wires on the cone, removed the enamel cover on them and got a reading of 4.9 Ohms.
I noticed rust around the periphery, I guess of the voice coil area.
Maybe nothing is moving or vibrating in there because of the rust.

Carlos


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 Post subject: Re: Philco PT-25 Speaker challenge.
PostPosted: Dec Wed 06, 2017 4:00 am 
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Location: S. Dartmouth MA 02748-1225 USA
Four inch field coil speakers are very hard to find...

The speaker could be repaired by un-gluing to cone and spider perimeter using toluene and a plastic eyedropper to dispense the toluene. Repeat the wetting of the cone perimeter may take some 30 minutes of repeated application of solvent. Do the perimeter of the spider too, difficult to get at but plastic eyedropper snout can be stretched small, curved and thin to apply the solvent. Be sure tinsel wire is unsoldered at terminals. Cone should come out with a bit of pressure, held in by rust. Use sand paper to clean pole pieces. Threat the raw metal with cold gun bluing to retard further rust or thin film of lacquer/shellac. Make any other repairs to cone/voice coil. Test cone for drop in fit. Use snug fitting shims in four places in voice coil gap/polepiece and re-install cone. Use G-C speaker cement and same eyedropper to apply thin bead of cement. When dry, pull shims cone should be free and work correctly. The cement is solvent removable if a mistake is made.

Alternates: Professional rebuild $$. PM speaker, sometimes the field coil is across the B+ so replacement with a PM is easy. A resistor can be used to replace the field coil in the power supply if the coil is in series, add some 20% to the second filter cap mf value to compensate for the hum. Usually a 1 - 1.8k 5 watt resistor should suffice for a 5 tube radio. May have to search for a suitable speaker with the proper frame.

YMMV

Chas


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 Post subject: Re: Philco PT-25 Speaker challenge.
PostPosted: Dec Wed 06, 2017 5:03 am 
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Joined: Nov Sun 29, 2015 1:22 am
Posts: 568
Location: Parkland, Florida
Thanks, Chas.
This repair will test my patience for sure.
The cone is very frail. It almost feels like onion skin. I just patched up a crack in it.
Is there any type of coating I could paint or spray on it to make it stiffer?
Thanks.

Carlos


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