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 Post subject: Airmaster 6B
PostPosted: Jun Sun 09, 2019 9:46 pm 
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Joined: Jun Wed 28, 2017 5:32 pm
Posts: 86
Location: Hampton Roads, VA
Just wanted to make sure that I was installing these electrolytics correctly. Negative is tied together, and one positive goes to ground and the other goes to the center tap. Or is it both positives joined and the negatives separate?
Here's the link to the schematic
http://www.nostalgiaair.org/pagesbymode ... 003003.pdf

Does someone mind explaining this circuit?


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 Post subject: Re: Airmaster 6B
PostPosted: Jun Sun 09, 2019 9:56 pm 
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Joined: Jan Mon 16, 2012 4:15 pm
Posts: 5926
Location: Near Brandon, Iowa
Unusual dual electrolytic cap in that both + terminals are tied together. The common + terminal goes to the B+ rail; one of the - terminals goes to the power transformer high voltage center tap and the other cap's - terminal goes to chassis ground.

By routing the high voltage center tap to ground via a set of series resistors, the center tap DC potential is depressed from ground voltage (zero) by several volts. This negative voltage is used to provide grid bias for various tubes in the set.


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 Post subject: Re: Airmaster 6B
PostPosted: Jun Mon 10, 2019 2:10 pm 
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Joined: Jun Wed 28, 2017 5:32 pm
Posts: 86
Location: Hampton Roads, VA
Well, now I have a new issue, the field coil is open, and because of outer winding on the coil, I cannot get to the leads. I've posted a picture of the speaker, what's the best way to get the coil out? The cone looks almost brand new.


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 Post subject: Re: Airmaster 6B
PostPosted: Jun Mon 10, 2019 2:52 pm 
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Joined: Jan Mon 16, 2012 4:15 pm
Posts: 5926
Location: Near Brandon, Iowa
In that style of speaker, the coil is inserted in the field bar metal frame and the core piece is pressed in, trapping it. This requires a hydraulic press and jigs. Unless you have a hydraulic press you will not be able to press out the core piece.

Some people when confronted with this scenario try to "weld-bridge" the break by discharging a large capacitor that has been charged to a high voltage across the coil end wires. This may or may not restore continuity; in my experience, it doesn't work often enough for me to recommend trying it. But it's an inexpensive thing to try.

Other options are (1) try to find a good used speaker; or (2) replace the existing field-coil speaker with a permanent-magnet one. Both of these options require that you understand exactly how the original speaker was used in your radio, since the speaker field coil often does double-duty as both field magnetism source and a high voltage supply filter reactor.


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 Post subject: Re: Airmaster 6B
PostPosted: Jun Fri 14, 2019 7:42 pm 
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Joined: Jun Wed 28, 2017 5:32 pm
Posts: 86
Location: Hampton Roads, VA
Alright, made purchase of a 6" field coil speaker, not sure of the resistance on the field coil until I get it, but fingers crossed. Hopefully it's in the range of the original. I can always swap output transformers if needed. It's interesting that the original output trans was replaced with a generic multi tap unit as well.


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 Post subject: Re: Airmaster 6B
PostPosted: Jun Fri 14, 2019 8:00 pm 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 33333
Location: SoCal, 91387
I don't see why you just didn't trace the FC connections in the circuit, and bridge it with a two watt resistor of approximately 1K Ω. That would have been the easiest and cheapest method.

_________________
\\\\\\\\\\\\\\\\\\\\\\\\He Who Dies With The Most Radios Wins//////////////////


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 Post subject: Re: Airmaster 6B
PostPosted: Jun Mon 17, 2019 2:04 am 
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Joined: Jun Wed 28, 2017 5:32 pm
Posts: 86
Location: Hampton Roads, VA
Either way I would have had to replace the speaker. It was almost the same price for the Field Coil speaker and permanent magnet speaker. It was simple enough, that's the reason I did it.


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 Post subject: Re: Airmaster 6B
PostPosted: Jun Mon 17, 2019 7:42 am 
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Joined: Jan Mon 16, 2012 4:15 pm
Posts: 5926
Location: Near Brandon, Iowa
Zsuttle wrote:
Alright, made purchase of a 6" field coil speaker, not sure of the resistance on the field coil until I get it, but fingers crossed. Hopefully it's in the range of the original. I can always swap output transformers if needed. It's interesting that the original output trans was replaced with a generic multi tap unit as well.

The original field coil was rated at 1700 ohms. If your replacement speaker deviates very far from this figure, it may mess up the negative DC grid bias voltage circuit; or power dissipation in the field coil may be excessive and cause it to burn out. You may need to use additional power resistors in series or parallel (or both) with the field coil in order to make its apparent resistance 1700 ohms.

How far is too far? I'd say that if your replacement speaker field coil resistance is within a 1400 to 2000 ohm window, it should be OK with no additional resistor "padding" necessary.


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 Post subject: Re: Airmaster 6B
PostPosted: Jun Mon 17, 2019 4:39 pm 
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Joined: Jun Wed 28, 2017 5:32 pm
Posts: 86
Location: Hampton Roads, VA
lorenz200w wrote:
Zsuttle wrote:
Alright, made purchase of a 6" field coil speaker, not sure of the resistance on the field coil until I get it, but fingers crossed. Hopefully it's in the range of the original. I can always swap output transformers if needed. It's interesting that the original output trans was replaced with a generic multi tap unit as well.

The original field coil was rated at 1700 ohms. If your replacement speaker deviates very far from this figure, it may mess up the negative DC grid bias voltage circuit; or power dissipation in the field coil may be excessive and cause it to burn out. You may need to use additional power resistors in series or parallel (or both) with the field coil in order to make its apparent resistance 1700 ohms.

How far is too far? I'd say that if your replacement speaker field coil resistance is within a 1400 to 2000 ohm window, it should be OK with no additional resistor "padding" necessary.


Just got the speaker today, Field coil resistance is a little under 1800ohms, and the output transformer is about 350ohms. Hopefully it should be a good replacement, we shall see once I finish recapping it.


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