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Philco 46-480 Replacing Field Coil w/ Perm Mag (I know I know) - Printable Version

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Philco 46-480 Replacing Field Coil w/ Perm Mag (I know I know) - RetroRev - 09-10-2022

Hey yall and thanks for having me on the forum!

I've got a Philco 46-480 which I got a few years ago and am finally getting around to working on. It's been recapped and seems to all be in good working order as well as cosmetically, though it is missing the entire speaker setup as well as the plug for said speaker. 

So, as I'm sure is common knowledge here, when replacing a field coil speaker with a permanent magnet speaker you have to put a resistor in line to take the place of the voice coil. 

My problem is, I've seen three different versions of the schematics that all have different values for it. 

One says 510 ohm, another says 1k and a third I found says 3ohm

I put in a 1k 50w and got nothing.

Before I go any further, is there a step I'm missing?

Does anyone know the proper resistance value I need?

RE: Philco 46-480 Replacing Field Coil w/ Perm Mag (I know I know) - morzh - 09-11-2022

You are looking at different parts.
510 Ohm - is the primary of the audiotransformer.
3 Ohm is probably Voice coil.

1000 ohm is the field coil and this is the value you need.

RE: Philco 46-480 Replacing Field Coil w/ Perm Mag (I know I know) - RetroRev - 09-11-2022

I knew it was going to be something stupid on my part like that!  Icon_e_biggrin

Thanks very much!

RE: Philco 46-480 Replacing Field Coil w/ Perm Mag (I know I know) - morzh - 09-11-2022

When sizing up a resistor, keep in mind it has to be large enough to dissipate some serious heat.

RE: Philco 46-480 Replacing Field Coil w/ Perm Mag (I know I know) - RetroRev - 09-11-2022

These are the Resistors I got for the project. I figured 50 watts should be good for it, yeah?

RE: Philco 46-480 Replacing Field Coil w/ Perm Mag (I know I know) - morzh - 09-11-2022

Yes, 50W is enough

RE: Philco 46-480 Replacing Field Coil w/ Perm Mag (I know I know) - RetroRev - 09-12-2022

Alright so after installing the resistor, I'm getting no sound, short of a little crackling when I twist the volume knob.

Just to be sure: Since there's no male connector, I've put the resistor in between the orange and red (pin 1 and 5) and the speaker between lines on the female side, and the speaker between the white and green (pin 2 and 4)

RE: Philco 46-480 Replacing Field Coil w/ Perm Mag (I know I know) - radiorich - 09-12-2022

hello RetroRev ,
Yes that is Correct

RE: Philco 46-480 Replacing Field Coil w/ Perm Mag (I know I know) - MrFixr55 - 09-13-2022

Hi Retrorev,

Sounds like your wiring is correct. make some voltage readings. All of the Radio's current goes through that resistor. This is a somewhat strange setup in that the field coil was in the B Power Supply return. That wsa done years ago by RCA to keep the field coil voltage above (or below) chassis ground as low as possible. To troubleshoot:
- With all tubes in place, carefully check the voltage between the Audio Output transformer red wire B+ and Chassis Ground. DANGER!!! This is high voltage. Voltage should be around 250V. Check across the 100 0Ohm Resistor and note the voltage. - Check across C59 (Between Rectifier Cathode and center tap of HV Secondary). The sum of the B+ Voltage and the voltage across the resistor should equal the voltage across C59, which will likely be between 300 and 350 Volts DC.

If this is all good, inject an audio signal from a CD, MPE, etc. Headphone jack through a 47K Resistor between the junction of C47, r26, C42 and R20, and chassis ground. Clean audio should be heard from the speaker. If so, the power supply and Audio Section are good. troubleshoot the RF, IF and Detector section.

Hope this helps.

Best Regards,

John, MrFixr55

RE: Philco 46-480 Replacing Field Coil w/ Perm Mag (I know I know) - radio1 - 09-17-2022

'Just to be sure: Since there's no male connector, I've put the resistor in between the orange and red (pin 1 and 5)'

What is the voltage between 5 and ground?

RE: Philco 46-480 Replacing Field Coil w/ Perm Mag (I know I know) - RetroRev - 09-18-2022

Just letting yall known I haven't abandoned this thread. I will be back at the shop tomorrow and will be testing the points as instructed.

RE: Philco 46-480 Replacing Field Coil w/ Perm Mag (I know I know) - RetroRev - 09-18-2022

Quote:Radio1: What is the voltage between 5 and ground?

I'm getting about 65v

RE: Philco 46-480 Replacing Field Coil w/ Perm Mag (I know I know) - RetroRev - 09-18-2022

Mr. Fixr55:

Voltage at the transformer is about 220v
297v at c59
And 62.5v at the resistor

RE: Philco 46-480 Replacing Field Coil w/ Perm Mag (I know I know) - radio1 - 09-18-2022

The schematic I see says -75v through 1kohm to ground, is your connection to ground. I am looking at this https://philcoradio.com/library/download/service%20info/yearbooks/1946-1947/split/Part16-Philco%20Home%20Radio%20Yearbook%201946-1947.pdf

What is your source schematic?

RE: Philco 46-480 Replacing Field Coil w/ Perm Mag (I know I know) - RetroRev - 09-18-2022

I have both that schematic and the one from radiomuseum.com