Field Coil speaker

1/15/2010 1:00:10 PMKY

How is ever body? I have replaced a field coil speaker with a p-m speaker. added a 1K 10w resistor to replace the field coil. now all I get is a hum from the speaker with no volume control.
KY

1/15/2010 1:59:19 PMWarren

What is the make and model ? Did you remember that the primary of the output transformer is connected to the new added resistor ?

1/15/2010 2:36:26 PMKY

:What is the make and model ? Did you remember that the primary of the output transformer is connected to the new added resistor ?
:I added the resistor in line to what was the field coil Connection and took the output transformer off the old speaker and mounted it to the new speaker. (this resistor 1k 10w starts smoking after about 2 minutes.

the mod. is Coronado 908
thanks, KY

1/15/2010 3:56:45 PMDoug Criner

Schematic here: http://www.nostalgiaair.org/PagesByModel/174/M0007174.pdf

The schematic has currents, but I can't quite read the number for the current through the field coil. But, I would say that a 10-W resistor is too small. The max DC output of a 5W4 is 100mA, which at 1000 ohms equates to 10W. But double that rating for reasonable heat dissipation and safety.

I would up-rate the resistor wattage and then troubleshoot for the hum. Did you replace the filter caps?

1/15/2010 4:24:50 PMKY

:Schematic here: http://www.nostalgiaair.org/PagesByModel/174/M0007174.pdf
:
:The schematic has currents, but I can't quite read the number for the current through the field coil. But, I would say that a 10-W resistor is too small. The max DC output of a 5W4 is 100mA, which at 1000 ohms equates to 10W. But double that rating for reasonable heat dissipation and safety.
:
:I would up-rate the resistor wattage and then troubleshoot for the hum. Did you replace the filter caps?
:Doug, Thanks for the advice I did replace all caps 2 @ a time the test until my cone fell apart I will install a 1k 20w resistor. So am I on he right track as far as installing a resistor on the field coil and not the output transformer?
Thanks. KY

1/15/2010 3:57:33 PMWarren

Not able to make out what the original field coil resistance was. However it's more likely there is a short somewhere. Check the primary of the output transformer. Check it to ground also. Backward capacitor or shorting one ? Is the original field coil open ?

1/15/2010 5:59:09 PMEdd

Sir KY. . . . .

In further clarifying what Sir Warren had specified , somehow in the conversion process, any chance of the loss of the [YELLOW] referenced line connection between the AF output xfmr and the B+ past the new power resistor.

(Probably not . . . since the resistor IS complaining to you now. )

I also tend to feel a 10W would be a borderline marginal power sizing for that units power selection.

Now for help in further analysis, how's about finding us 3 tidbits of info ?

How about unplugging the AF output tube and wait that prescribed 2-3 mins of powered up time and
see if the resistor then even hints of warming up.

That will let us know that there is no overload present on the RF front end's B+ via the [E] feed line.
And that the problem seems to be AF output stage oriented instead of any ancillary shorted capacitors to ground.

Plug the AF out back in and just after the AF comes up on its filaments temp, check the [J] [K] test points to see how much negative voltage is being read out there, as referenced to ground.

AT that "HOT" resistor onset condition, measure across [A] to [B] so we can get an idea of the B+ current being consumed by the set as it is .

The last two measurements might require a bit of cool down in case you are slow in getting the
measurements completed.

If still not overtaxing the unit, give us B+ measurements at [C] and [D] as referenced to gnd.

Also, just now read this add on :

So am I on he right track as far as installing a resistor on the field coil and not the output transformer?

The power resistor is no way associated with the output transformer, nor does it end up shunting across it.

The power resistor is to fill in for a field coil winding in case it is open, has a bunch of shorted turns inside of itself or possibly has leakage to ground.(Thus loading down your B+.)

In which case, its 2 old wire connections out from it would be disconnected and thereby be left TOTALLY isolated from circuitry that it was tied in to previously.

Standing by . . . .

(Figured out your LCD projo " lite 'bub" quandry yet ?)

73's de Edd