[Hallicrafters] Field Coil Speaker heat

[Rich Oliver]

John,

Time to get out your milliammeter.  The coil will dissipate:

Power (Watts) = [Current (Amperes) squared] X [resistance (Ohms)]

It is possible that something is causing the set to draw excessive 
current (leaky capacitor, failed bias resistor, shorted transformer, or 
bad tube are the "usual suspects"), and since the field coil dissipates 
power as the square of the current it is worth checking.  Otherwise you 
may be facing a burned out speaker in addition to whatever is causing 
excessive current draw.

Hopefully you can find some documentation on nominal current draw (or 
voltage across the field coil) but if you can't there are still a couple 
of things you can check.  Look up the current rating of the rectifier 
tube.  If the set was properly designed the supply current will be less 
than that.  Most of the current will go to the audio output tube so 
check its specs for plate current (I am assuming a single tube biassed 
for Class A).  Total supply current should not be a great deal higher 
than that.

Hope this helps.

73, Rich, KC9GQ

John Brooks wrote:
> Sorry for the non-HC content, but you folks seem to
> know so much about old electronics.  I am helping a
> friend repair his RCA console 78 player and the magnet
> structure on the field coil speaker is getting pretty
> warm, maybe 120F.  Resistance of the field coil is
> in-spec at 1100 ohms.  It is, of course, the choke for
> the power supply and does carry a fair amount of
> current, but my question is how warm are these things
> supposed to get?
> 
> Thanks,
> John
> KG4JZT
> 
> 
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